Table of Contents

1 Introduction

Many useful facilities offered by one or more of the operating systems supported by SWI-Prolog are not supported by the SWI-Prolog kernel distribution. Including these would enlarge the footprint and complicate portability matters while supporting only a limited part of the user-community.

This document describes library(unix) to deal with the Unix process API, library(socket) to deal with inet-domain TCP and UDP sockets, library(cgi) to deal with getting CGI form-data if SWI-Prolog is used as a CGI scripting language, library(crypt) to provide password encryption and verification, library(sha) providing cryptographic hash functions and library(memfile) providing in-memorty pseudo files.

2 library(process): Create processes and redirect I/O

Compatibility

SICStus 4

To be done

Implement detached option in process_create/3

The module library(process) implements interaction with child processes and unifies older interfaces such as shell/[1,2], open(pipe(command), ...) etc. This library is modelled after SICStus 4.

The main interface is formed by process_create/3. If the process id is requested the process must be waited for using process_wait/2. Otherwise the process resources are reclaimed automatically.

In addition to the predicates, this module defines a file search path (see user:file_search_path/2 and absolute_file_name/3) named path that locates files on the system's search path for executables. E.g. the following finds the executable for ls:

?- absolute_file_name(path(ls), Path, [access(execute)]).

Incompatibilities and current limitations

• Where SICStus distinguishes between an internal process id and the OS process id, this implementation does not make this distinction. This implies that is_process/1 is incomplete and unreliable.
• It is unclear what the detached(true) option is supposed to do. Disable signals in the child? Use setsid() to detach from the session? The current implementation uses setsid() on Unix systems.
• An extra option env([Name=Value, ...]) is added to process_create/3. As of version 4.1 SICStus added environment(List) which modifies the environment. A compatible option was added to SWI-Prolog 7.7.23.

[det]process_create(+Exe, +Args:list, +Options)

Create a new process running the file Exe and using arguments from the given list. Exe is a file specification as handed to absolute_file_name/3. Typically one use the path file alias to specify an executable file on the current PATH. Args is a list of arguments that are handed to the new process. On Unix systems, each element in the list becomes a separate argument in the new process. In Windows, the arguments are simply concatenated to form the commandline. Each argument itself is either a primitive or a list of primitives. A primitive is either atomic or a term file(Spec). Using file(Spec), the system inserts a filename using the OS filename conventions which is properly quoted if needed.

Options:

stdin(Spec)

stdout(Spec)

stderr(Spec)

Bind the standard streams of the new process. Spec is one of the terms below. If pipe(Pipe) is used, the Prolog stream is a stream in text-mode using the encoding of the default locale. The encoding can be changed using set_stream/2, or by using the two-argument form of pipe, which accepts an encoding(Encoding) option. The options stdout and stderr may use the same stream, in which case both output streams are connected to the same Prolog stream.

std

Just share with the Prolog I/O streams. On Unix, if the user_input, etc. are bound to a file handle but not to 0,1,2 the process I/O is bound to the file handles of these streams.

null

Bind to a null stream. Reading from such a stream returns end-of-file, writing produces no output

pipe(-Stream)

pipe(-Stream, +StreamOptions)

Attach input and/or output to a Prolog stream. The optional StreamOptions argument is a list of options that affect the stream. Currently only the options type(+Type) and encoding(+Encoding) are supported, which have the same meaning as the stream properties of the same name (see stream_property/2). StreamOptions is provided mainly for SICStus compatibility - the SWI-Prolog predicate set_stream/2 can be used for the same purpose.

stream(+Stream)

Attach input or output to an existing Prolog stream. This stream must be associated with an OS file handle (see stream_property/2, property file_no). This option is not provided by the SICStus implementation.

cwd(+Directory)

Run the new process in Directory. Directory can be a compound specification, which is converted using absolute_file_name/3. See also process_set_method/1.

env(+List)

As environment(List), but only the specified variables are passed, i.e., no variables are inherited.

environment(+List)

Specify additional environment variables for the new process. List is a list of Name=Value terms, where Value is expanded the same way as the Args argument. If neither env nor environment is passed the environment is inherited from the Prolog process. At most one env(List) or environment(List) term may appear in the options. If multiple appear a permission_error is raised for the second option.

process(-PID)

Unify PID with the process id of the created process.

detached(+Bool)

In Unix: If true, detach the process from the terminal Currently mapped to setsid(); Also creates a new process group for the child In Windows: If true, detach the process from the current job via the CREATE_BREAKAWAY_FROM_JOB flag. In Vista and beyond, processes launched from the shell directly have the’compatibility assistant' attached to them automatically unless they have a UAC manifest embedded in them. This means that you will get a permission denied error if you try and assign the newly-created PID to a job you create yourself.

window(+Bool)

If true, create a window for the process (Windows only)

priority(+Priority)

In Unix: specifies the process priority for the newly created process. Priority must be an integer between -20 and 19. Positive values are nicer to others, and negative values are less so. The default is zero. Users are free to lower their own priority. Only the super-user may raise it to less-than zero.

If the user specifies the process(-PID) option, he must call process_wait/2 to reclaim the process. Without this option, the system will wait for completion of the process after the last pipe stream is closed.

If the process is not waited for, it must succeed with status 0. If not, an process_error is raised.

Windows notes

On Windows this call is an interface to the CreateProcess() API. The commandline consists of the basename of Exe and the arguments formed from Args. Arguments are separated by a single space. If all characters satisfy iswalnum() it is unquoted. If the argument contains a double-quote it is quoted using single quotes. If both single and double quotes appear a domain_error is raised, otherwise double-quote are used.

The CreateProcess() API has many options. Currently only the CREATE_NO_WINDOW options is supported through the window(+Bool) option. If omitted, the default is to use this option if the application has no console. Future versions are likely to support more window specific options and replace win_exec/2.

Examples

First, a very simple example that behaves the same as shell('ls -l'), except for error handling:

?- process_create(path(ls), ['-l'], []).

The following example uses grep to find all matching lines in a file.

grep(File, Pattern, Lines) :-
        setup_call_cleanup(
            process_create(path(grep), [ Pattern, file(File) ],
                           [ stdout(pipe(Out))
                           ]),
            read_lines(Out, Lines),
            close(Out)).

read_lines(Out, Lines) :-
        read_line_to_codes(Out, Line1),
        read_lines(Line1, Out, Lines).

read_lines(end_of_file, _, []) :- !.
read_lines(Codes, Out, [Line|Lines]) :-
        atom_codes(Line, Codes),
        read_line_to_codes(Out, Line2),
        read_lines(Line2, Out, Lines).

Errors

process_error(Exe, Status) where Status is one of exit(Code) or killed(Signal). Raised if the process is waited for (i.e., Options does not include process(-PID)), and does not exit with status 0.

bug

On Windows, environment(List) is handled as env(List), i.e., the environment is not inherited.

[semidet]process_which(+Exe, -Path)

True when Path is an absolute file name for the specification Exe. This deals with the search path as well as extensions used by the OS.

[det]process_id(-PID)

True if PID is the process id of the running Prolog process.

deprecated

Use current_prolog_flag(pid, PID)

[det]process_id(+Process, -PID)

PID is the process id of Process. Given that they are united in SWI-Prolog, this is a simple unify.

[semidet]is_process(+PID)

True if PID might be a process. Succeeds for any positive integer.

process_release(+PID)

Release process handle. In this implementation this is the same as process_wait(PID, _).

[det]process_wait(+PID, -Status)

[det]process_wait(+PID, -Status, +Options)

True if PID completed with Status. This call normally blocks until the process is finished. Options:

timeout(+Timeout)

Default: infinite. If this option is a number, the waits for a maximum of Timeout seconds and unifies Status with timeout if the process does not terminate within Timeout. In this case PID is not invalidated. On Unix systems only timeout 0 and infinite are supported. A 0-value can be used to poll the status of the process.

release(+Bool)

Do/do not release the process. We do not support this flag and a domain_error is raised if release(false) is provided.

Status

is one of exit(Code) or killed(Signal), where Code and Signal are integers. If the timeout option is used Status is unified with timeout after the wait timed out.

[det]process_kill(+PID)

[det]process_kill(+PID, +Signal)

Send signal to process PID. Default is term. Signal is an integer, Unix signal name (e.g. SIGSTOP) or the more Prolog friendly variation one gets after removing SIG and downcase the result: stop. On Windows systems, Signal is ignored and the process is terminated using the TerminateProcess() API. On Windows systems PID must be obtained from process_create/3, while any PID is allowed on Unix systems.

Compatibility

SICStus does not accept the prolog friendly version. We choose to do so for compatibility with on_signal/3.

[det]process_group_kill(+PID)

[det]process_group_kill(+PID, +Signal)

Send signal to the group containing process PID. Default is term. See process_wait/1 for a description of signal handling. In Windows, the same restriction on PID applies: it must have been created from process_create/3, and the the group is terminated via the TerminateJobObject API.

[det]process_set_method(+Method)

Determine how the process is created on Unix systems. Method is one of spawn (default), fork or vfork. If the method is spawn but this cannot be used because it is either not supported by the OS or the cwd(Dir) option is given fork is used.

The problem is to be understood as follows. The official portable and safe method to create a process is using the fork() system call. This call however copies the process page tables and get seriously slow as the (Prolog) process is multiple giga bytes large. Alternatively, we may use vfork() which avoids copying the process space. But, the safe usage as guaranteed by the POSIX standard of vfork() is insufficient for our purposes. On practical systems your mileage may vary. Modern posix systems also provide posix_spawn(), which provides a safe and portable alternative for the fork() and exec() sequence that may be implemented using fork() or may use a fast but safe alternative. Unfortunately posix_spawn() doesn't support the option to specify the working directory for the child and we cannot use working_directory/2 as the working directory is shared between threads.

Summarizing, the default is safe and tries to be as fast as possible. On some scenarios and on some OSes it is possible to do better. It is generally a good idea to avoid using the cwd(Dir) option of process_create/3 as without we can use posix_spawn().

3 library(filesex): Extended operations on files

This module provides additional operations on files. This covers both more obscure and possible non-portable low-level operations and high-level utilities.

Using these Prolog primitives is typically to be preferred over using operating system primitives through shell/1 or process_create/3 because (1) there are no potential file name quoting issues, (2) there is no dependency on operating system commands and (3) using the implementations from this library is usually faster.

[det]set_time_file(+File, -OldTimes, +NewTimes)

Query and set POSIX time attributes of a file. Both OldTimes and NewTimes are lists of option-terms. Times are represented in SWI-Prolog's standard floating point numbers. New times may be specified as now to indicate the current time. Defined options are:

access(Time)

Describes the time of last access of the file. This value can be read and written.

modified(Time)

Describes the time the contents of the file was last modified. This value can be read and written.

changed(Time)

Describes the time the file-structure itself was changed by adding (link()) or removing (unlink()) names.

Below are some example queries. The first retrieves the access-time, while the second sets the last-modified time to the current time.

?- set_time_file(foo, [access(Access)], []).
?- set_time_file(foo, [], [modified(now)]).

[det]link_file(+OldPath, +NewPath, +Type)

Create a link in the filesystem from NewPath to OldPath. Type defines the type of link and is one of hard or symbolic.

With some limitations, these functions also work on Windows. First of all, the underlying filesystem must support links. This requires NTFS. Second, symbolic links are only supported in Vista and later.

Errors

domain_error(link_type, Type) if the requested link-type is unknown or not supported on the target OS.

[det]relative_file_name(+Path:atom, +RelToFile:atom, -RelPath:atom)

[det]relative_file_name(-Path:atom, +RelToFile:atom, +RelPath:atom)

True when RelPath is Path, relative to the file RelToFile. Path and RelTo are first handed to absolute_file_name/2, which makes the absolute and canonical. Below are two examples:

?- relative_file_name('/home/janw/nice',
                      '/home/janw/deep/dir/file', Path).
Path = '../../nice'.

?- relative_file_name(Path, '/home/janw/deep/dir/file', '../../nice').
Path = '/home/janw/nice'.

Add a terminating / to get a path relative to a directory, e.g.

?- relative_file_name('/home/janw/deep/dir/file', './', Path).
Path = 'deep/dir/file'.

All	paths must be in canonical POSIX notation, i.e., using / to separate segments in the path. See prolog_to_os_filename/2.

bug

It would probably have been cleaner to use a directory as second argument. We can not do such dynamically as this predicate is defined as a syntactical operation, which implies it may be used for non-existing paths and URLs.

[det]directory_file_path(+Directory, +File, -Path)

[det]directory_file_path(?Directory, ?File, +Path)

True when Path is the full path-name for File in Dir. This is comparable to atom_concat(Directory, File, Path), but it ensures there is exactly one / between the two parts. Notes:

• In mode (+,+,-), if File is given and absolute, Path is unified to File.
• Mode (-,-,+) uses file_directory_name/2 and file_base_name/2

[nondet]directory_member(+Directory, -Member, +Options)

True when Member is a path inside Directory. Options defined are:

recursive(+Boolean)

If true (default false), recurse into subdirectories

follow_links(+Boolean)

If true (default), follow symbolic links.

file_type(+Type)

See absolute_file_name/3.

extensions(+List)

Only return entries whose extension appears in List.

file_errors(+Errors)

How to handle errors. One of fail, warning or error. Default is warning. Errors notably happen if a directory is unreadable or a link points nowhere.

access(+Access)

Only return entries with Access

matches(+GlobPattern)

Only return files that match GlobPattern.

exclude(+GlobPattern)

Exclude files matching GlobPattern.

exclude_directory(+GlobPattern)

Do not recurse into directories matching GlobPattern.

hidden(+Boolean)

If true (default), also return hidden files.

This predicate is safe against cycles introduced by symbolic links to directories.

The idea for a non-deterministic file search predicate comes from Nicos Angelopoulos.

[det]copy_file(+From, +To)

Copy a file into a new file or directory. The data is copied as binary data.

[det]make_directory_path(+Dir)

Create Dir and all required components (like mkdir -p). Can raise various file-specific exceptions.

[det]ensure_directory(+Dir)

Ensure the directory Dir exists. Similar to make_directory_path/1, but creates at most one new directory, i.e., the directory or its direct parent must exist.

[det]copy_directory(+From, +To)

Copy the contents of the directory From to To (recursively). If To is the name of an existing directory, the contents of From are copied into To. I.e., no subdirectory using the basename of From is created.

[det]delete_directory_and_contents(+Dir)

Recursively remove the directory Dir and its contents. If Dir is a symbolic link or symbolic links inside Dir are encountered, the links are removed rather than their content. Use with care!

[det]delete_directory_contents(+Dir)

Remove all content from directory Dir, without removing Dir itself. Similar to delete_directory_and_contents/2, if symbolic links are encountered in Dir, the links are removed rather than their content.

[det]chmod(+File, +Spec)

Set the mode of the target file. Spec is one of +Mode, -Mode or a plain Mode, which adds new permissions, revokes permissions or sets the exact permissions. Mode itself is an integer, a POSIX mode name or a list of POSIX mode names. Defines names are suid, sgid, svtx and all names defined by the regular expression [ugo]*[rwx]*. Specifying none of "ugo" is the same as specifying all of them. For example, to make a file executable for the owner (user) and group, we can use:

?- chmod(myfile, +ugx).

4 library(uid): User and group management on Unix systems

See also

Please check the documentation of your OS for details on the semantics of this predicates.

This module provides and interface to user and group information on Posix systems. In addition, it allows for changing user and group ids. When changing user and group settings for the calling process, bear in mind that:

• Changing user and groups of the calling process requires permission.
• The functions setgroups() and initgroups() are not part of the POSIX standard and therefore the derived predicates may not be present.

[det]getuid(-UID)

UID is the real user ID of the calling process.

[det]getgid(-GID)

GID is the real group ID of the calling process.

[det]geteuid(-UID)

UID is the effective user ID of the calling process.

[det]getegid(-GID)

GID is the effective group ID of the calling process.

[det]getgroups(-GroupsIDs:list(integer))

GroupsIDs is the set of supplementary group IDs of the calling process. Note that these are numeric identifiers. Use group_info/2 to obtain details on the returned group identifiers.

[det]user_info(+User, -UserData)

UserData represent the passwd information for User. User is either a numeric UID or a user name. The predicate user_data/3 can be used to extract information from UserData.

user_data(?Field, ?UserData, ?Value)

Value is the value for Field in UserData. Defined fields are:

name

Name of the user

password

Password hash of the user (or x if this is not accessible)

uid

Numeric user id of the user

gid

Numeric primary group id of the user

comment

The gecos field

home

Home directory of the user

shell

Default (login) shell of the user.

[det]group_info(+Group, -GroupData)

GroupData represent the group information for Group. Group is either a numeric GID or a group name. The predicate group_data/3 can be used to extract information from GroupData.

group_data(?Field, ?GroupData, ?Value)

Value is the value for Field GroupData. Defined fields are:

name

Name of the user

password

Password hash of the user (or x if this is not accessible)

gid

Numeric group id of the group

members

List of user-names that are member of this group.

setuid(+UID)

Set the user id of the calling process.

seteuid(+UID)

Set the effective user id of the calling process.

setgid(+GID)

Set the group id of the calling process.

setegid(+GID)

Set the effective group id of the calling process.

[det]initgroups(+User, +Group)

Initialise the group access list of the calling process to the registered groups for User and the group Group. This predicate is only available if the underlying OS provides it.

[det]setgroups(+Groups:list(integer))

Set the group access list of the caling process to the indicated groups. This predicate is only available if the underlying OS provides it.

[det]set_user_and_group(+User)

[det]set_user_and_group(+User, +Group)

Set the UID and GID to the User. User is either a UID or a user name. If Group is not specified, the primary group of User is used. If initgroups/2 is available, the resulting group access list of the calling process consists of the registered groups for User and the specified Group.

5 library(syslog): Unix syslog interface

See also

- detach_IO/1 to detach normal I/O of the process and remove it from the process group.
- fork/1 to create a daemon process.
- library(uid) to manage user identifiers (e.g., drop root privileges).

This library provides an interface to the Unix syslog() facility. The interface is an almost direct translation of the POSIX syslog API, with two additions:

• syslog/3 exploits format/3 to format syslog messages
• The library integrates into library(debug) using prolog:debug_print_hook/3, where debug topics are mapped to syslog priorities and remaining debug topics are mapped to the syslog priority debug.

Note that this interface makes no attempt to abstract over logging facilities of operating systems. We expect that such abstractions will be implemented at the Prolog level using multiple integrations into library(debug).

[det]openlog(+Ident:atom, +Options:list(atom), +Facility:atom)

Open system log. This predicate provides a direct interface into the openlog() library call. If the library call is successful, it runs at_halt(closelog) to ensure closing the system log on clean exit.

Ident	prepended to every message, and is typically set to the program name.
Options	is a list of options. Values are corresponding C options, after removing =LOG_= and translation to lower case: cons, ndelay, nowait, odelay, perror, pid.
Facility	is one of auth, authpriv, cron, daemon, ftp, kern, local0 ... local7, lpr, mail, news, syslog, user or uucp.

[det]syslog(+Priority, +Message)

Send a message to the system log. Note that syslog/2 implicitly opens a connection to the system log if such a connection has not been opened explicitly using openlog/3.

Priority

is one of emerg, alert, crit, err, warning, notice, info or debug.

[det]syslog(+Priority, +Format, +Args)

Send a formatted message to the system log if system logging is opened using openlog/3. This predicate combined format/3 with syslog/2. If there is no open syslog connection, syslog/3 calls print_message/2.

[det]closelog

Close the system log.

[semidet,multifile]prolog:debug_print_hook(+Topic, +Format, +Args)

Integration of debug/3 with the syslog facility. If syslog is enabled, debug/3 is re-routed to use the syslog facilities. If the topic of the debug message matches one of the sylog priority values (see syslog/2), the message is sent with the corresponding syslog priority. Otherwise it it sent with the debug priority.

6 library(socket): Network socket (TCP and UDP) library

The library(socket) provides TCP and UDP inet-domain sockets from SWI-Prolog, both client and server-side communication. The interface of this library is very close to the Unix socket interface, also supported by the MS-Windows winsock API. SWI-Prolog applications that wish to communicate with multiple sources have two options:

• Use I/O multiplexing based on wait_for_input/3. On Windows systems this can only be used for sockets, not for general (device-) file handles.
• Use multiple threads, handling either a single blocking socket or a pool using I/O multiplexing as above.

6.1 Client applications

Using this library to establish a TCP connection to a server is as simple as opening a file. See also http_open/3.

dump_swi_homepage :-
    setup_call_cleanup(
        tcp_connect('www.swi-prolog.org':http, Stream, []),
        ( format(Stream,
                 'GET / HTTP/1.1~n\c
                  Host: www.swi-prolog.org~n\c
                  Connection: close~n~n', []),
          flush_output(Stream),
          copy_stream_data(Stream, current_output)
        ),
        close(Stream)).

To deal with timeouts and multiple connections, threads, wait_for_input/3 and/or non-blocking streams (see tcp_fcntl/3) can be used.

6.2 Server applications

The typical sequence for generating a server application is given below. To close the server, use close/1 on AcceptFd.

create_server(Port) :-
      tcp_socket(Socket),
      tcp_bind(Socket, Port),
      tcp_listen(Socket, 5),
      tcp_open_socket(Socket, AcceptFd, _),
      <dispatch>

There are various options for <dispatch>. The most commonly used option is to start a Prolog thread to handle the connection. Alternatively, input from multiple clients can be handled in a single thread by listening to these clients using wait_for_input/3. Finally, on Unix systems, we can use fork/1 to handle the connection in a new process. Note that fork/1 and threads do not cooperate well. Combinations can be realised but require good understanding of POSIX thread and fork-semantics.

Below is the typical example using a thread. Note the use of setup_call_cleanup/3 to guarantee that all resources are reclaimed, also in case of failure or exceptions.

dispatch(AcceptFd) :-
        tcp_accept(AcceptFd, Socket, Peer),
        thread_create(process_client(Socket, Peer), _,
                      [ detached(true)
                      ]),
        dispatch(AcceptFd).

process_client(Socket, Peer) :-
        setup_call_cleanup(
            tcp_open_socket(Socket, StreamPair),
            handle_service(StreamPair),
            close(StreamPair)).

handle_service(StreamPair) :-
        ...

6.3 Socket exceptions

Errors that are trapped by the low-level library are mapped to an exception of the shape below. In this term, Code is a lower case atom that corresponds to the C macro name, e.g., epipe for a broken pipe. Message is the human readable string for the error code returned by the OS or the same as Code if the OS does not provide this functionality. Note that Code is derived from a static set of macros that may or may not be defines for the target OS. If the macro name is not known, Code is ERROR_nnn, where nnn is an integer.

error(socket_error(Code, Message), _)

Note that on Windows Code is a wsa* code which makes it hard to write portable code that handles specific socket errors. Even on POSIX systems the exact set of errors produced by the network stack is not defined.

6.4 Socket addresses (families)

The library supports both IP4 and IP6 addresses. On Unix systems it also supports Unix domain sockets (AF_UNIX). The address of a Unix domain sockets is a file name. Unix domain sockets are created using socket_create/2 or unix_domain_socket/1.

IP4 or IP6 sockets can be created using socket_create/2 or tcp_connect/3 with the inet (default, IP3) or inet6 domain option. Some of the predicates produce or consume IP addresses as a Prolog term. The format of this term is one of:

ip(A, B, C, D)

Represents an IP4 address. Each field is an integer in the range 0..255 (8 bit).

ip(A, B, C, D, E, F, G, H)

Represents an IP6 address. Each field is an integer in the range 0..65535 (16 bit).

The predicate ip_name/2 translates between the canonical textual representation and the above defined address terms.

6.5 Socket predicate reference

[det]socket_create(-SocketId, +Options)

Create a socket according to Options. Supported Options are:

domain(+Domain)

One of inet (default), inet6, unix or local (same as unix)

type(+Type)

One of stream (default) to create a TCP connection or dgram to create a UDP socket.

This predicate subsumes tcp_socket/1, udp_socket/1 and unix_domain_socket/1.

[det]tcp_socket(-SocketId)

Equivalent to socket_create(SocketId, []) or, explicit, socket_create(SocketId, [domain(inet), type(stream)]).

[det]unix_domain_socket(-SocketId)

Equivalent to socket_create(SocketId, [domain(unix)]) or, explicit, socket_create(SocketId, [domain(unix), type(stream)])

Unix domain socket affect tcp_connect/2 (for clients) and tcp_bind/2 and tcp_accept/3 (for servers). The address is an atom or string that is handled as a file name. On most systems the length of this file name is limited to 128 bytes (including null terminator), but according to the Linux documentation (unix(7)), portable applications must keep the address below 92 bytes. Note that these lengths are in bytes. Non-ascii characters may be represented as multiple bytes. If the length limit is exceeded a representation_error(af_unix_name) exception is raised.

[det]tcp_close_socket(+SocketId)

Closes the indicated socket, making SocketId invalid. Normally, sockets are closed by closing both stream handles returned by open_socket/3. There are two cases where tcp_close_socket/1 is used because there are no stream-handles:

• If, after tcp_accept/3, the server uses fork/1 to handle the client in a sub-process. In this case the accepted socket is not longer needed from the main server and must be discarded using tcp_close_socket/1.
• If, after discovering the connecting client with tcp_accept/3, the server does not want to accept the connection, it should discard the accepted socket immediately using tcp_close_socket/1.

[det]tcp_open_socket(+SocketId, -StreamPair)

Create streams to communicate to SocketId. If SocketId is a master socket (see tcp_bind/2), StreamPair should be used for tcp_accept/3. If SocketId is a connected (see tcp_connect/2) or accepted socket (see tcp_accept/3), StreamPair is unified to a stream pair (see stream_pair/3) that can be used for reading and writing. The stream or pair must be closed with close/1, which also closes SocketId.

[det]tcp_open_socket(+SocketId, -InStream, -OutStream)

Similar to tcp_open_socket/2, but creates two separate sockets where tcp_open_socket/2 would have created a stream pair.

deprecated

New code should use tcp_open_socket/2 because closing a stream pair is much easier to perform safely.

[det]tcp_bind(SocketId, ?Address)

Bind the socket to Address on the current machine. This operation, together with tcp_listen/2 and tcp_accept/3 implement the server-side of the socket interface. Address is either an plain Port or a term HostPort. The first form binds the socket to the given port on all interfaces, while the second only binds to the matching interface. A typical example is below, causing the socket to listen only on port 8080 on the local machine's network.

  tcp_bind(Socket, localhost:8080)

If Port is unbound, the system picks an arbitrary free port and unifies Port with the selected port number. Port is either an integer or the name of a registered service. See also tcp_connect/4.

[det]tcp_listen(+SocketId, +BackLog)

Tells, after tcp_bind/2, the socket to listen for incoming requests for connections. Backlog indicates how many pending connection requests are allowed. Pending requests are requests that are not yet acknowledged using tcp_accept/3. If the indicated number is exceeded, the requesting client will be signalled that the service is currently not available. A commonly used default value for Backlog is 5.

[det]tcp_accept(+Socket, -Slave, -Peer)

This predicate waits on a server socket for a connection request by a client. On success, it creates a new socket for the client and binds the identifier to Slave. Peer is bound to the IP-address of the client or the atom af_unix if Socket is an AF_UNIX socket (see unix_domain_socket/1).

[det]tcp_connect(+SocketId, +Address)

Connect SocketId. After successful completion, tcp_open_socket/3 can be used to create I/O-Streams to the remote socket. This predicate is part of the low level client API. A connection to a particular host and port is realised using these steps:

    tcp_socket(Socket),
    tcp_connect(Socket, Host:Port),
    tcp_open_socket(Socket, StreamPair)

Typical client applications should use the high level interface provided by tcp_connect/3 which avoids resource leaking if a step in the process fails, and can be hooked to support proxies. For example:

    setup_call_cleanup(
        tcp_connect(Host:Port, StreamPair, []),
        talk(StreamPair),
        close(StreamPair))

If SocketId is an AF_UNIX socket (see unix_domain_socket/1), Address is an atom or string denoting a file name.

[nondet,multifile]rewrite_host(+HostIn, -HostOut, +Socket)

Allow rewriting the host for tcp_connect/2 and therefore all other predicates to connect a socket.

This hook is currently defined in Windows to map localhost to ip(127,0,0,1) as resolving localhost on Windows is often very slow. Note that we do not want to do that in general as a system may prefer to map localhost to‘::1`, i.e., the IPv6 loopback address.

[det]tcp_connect(+Socket, +Address, -Read, -Write)

Connect a (client) socket to Address and return a bi-directional connection through the stream-handles Read and Write. This predicate may be hooked by defining socket:tcp_connect_hook/4 with the same signature. Hooking can be used to deal with proxy connections. E.g.,

:- multifile socket:tcp_connect_hook/4.

socket:tcp_connect_hook(Socket, Address, Read, Write) :-
    proxy(ProxyAdress),
    tcp_connect(Socket, ProxyAdress),
    tcp_open_socket(Socket, Read, Write),
    proxy_connect(Address, Read, Write).

deprecated

New code should use tcp_connect/3 called as tcp_connect(+Address, -StreamPair, +Options).

[det]tcp_connect(+Address, -StreamPair, +Options)

[det]tcp_connect(+Socket, +Address, -StreamPair)

Establish a TCP communication as a client. The +,-,+ mode is the preferred way for a client to establish a connection. This predicate can be hooked to support network proxies. To use a proxy, the hook proxy_for_url/3 must be defined. Permitted options are:

bypass_proxy(+Boolean)

Defaults to false. If true, do not attempt to use any proxies to obtain the connection

nodelay(+Boolean)

Defaults to false. If true, set nodelay on the resulting socket using tcp_setopt(Socket, nodelay)

domain(+Domain)

One of‘inet' or inet6. When omitted we use host_address/2 with type(stream) and try the returned addresses in order.

The +,+,- mode is deprecated and does not support proxies. It behaves like tcp_connect/4, but creates a stream pair (see stream_pair/3).

Address

is either a Host:Port term or a file name (atom or string). The latter connects to an AF_UNIX socket and requires unix_domain_socket/1.

Errors

proxy_error(tried(ResultList)) is raised by mode (+,-,+) if proxies are defines by proxy_for_url/3 but no proxy can establsh the connection. ResultList contains one or more terms of the form false(Proxy) for a hook that simply failed or error(Proxy, ErrorTerm) for a hook that raised an exception.

See also

library(http/http_proxy) defines a hook that allows to connect through HTTP proxies that support the CONNECT method.

tcp_select(+ListOfStreams, -ReadyList, +TimeOut)

Same as the built-in wait_for_input/3. Used to allow for interrupts and timeouts on Windows. A redesign of the Windows socket interface makes it impossible to do better than Windows select() call underlying wait_for_input/3. As input multiplexing typically happens in a background thread anyway we accept the loss of timeouts and interrupts.

deprecated

Use wait_for_input/3

[semidet,multifile]try_proxy(+Proxy, +TargetAddress, -Socket, -StreamPair)

Attempt a socket-level connection via the given proxy to TargetAddress. The Proxy argument must match the output argument of proxy_for_url/3. The predicate tcp_connect/3 (and http_open/3 from the library(http/http_open)) collect the results of failed proxies and raise an exception no proxy is capable of realizing the connection.

The default implementation recognises the values for Proxy described below. The library(http/http_proxy) adds proxy(Host,Port) which allows for HTTP proxies using the CONNECT method.

direct

Do not use any proxy

socks(Host, Port)

Use a SOCKS5 proxy

[nondet,multifile]proxy_for_url(+URL, +Hostname, -Proxy)

This hook can be implemented to return a proxy to try when connecting to URL. Returned proxies are tried in the order in which they are returned by the multifile hook try_proxy/4. Pre-defined proxy methods are:

direct

connect directly to the resource

proxy(Host, Port)

Connect to the resource using an HTTP proxy. If the resource is not an HTTP URL, then try to connect using the CONNECT verb, otherwise, use the GET verb.

socks(Host, Port)

Connect to the resource via a SOCKS5 proxy

These correspond to the proxy methods defined by PAC Proxy auto-config. Additional methods can be returned if suitable clauses for http:http_connection_over_proxy/6 or try_proxy/4 are defined.

[det]udp_socket(-SocketId)

Equivalent to socket_create(SocketId, [type(dgram)]) or, explicit, socket_create(SocketId, [domain(inet), type(dgram)]).

[det]udp_receive(+Socket, -Data, -From, +Options)

Wait for and return the next datagram. The Data is returned as a Prolog term depending on Options. From is a term of the format Ip:Port indicating the sender of the message. Here, Ip is either an ip4 or ip6 structure. Socket can be waited for using wait_for_input/3. Defined Options:

as(+Type)

Defines the type for Data. Possible values are atom, codes, string (default) or term (parse as Prolog term).

encoding(+Encoding)

Specify the encoding used to interpret the message. It is one of octet. iso_latin_1, text or utf8.

max_message_size(+Size)

Specify the maximum number of bytes to read from a UDP datagram. Size must be within the range 0-65535. If unspecified, a maximum of 4096 bytes will be read.

For example:

receive(Port) :-
    udp_socket(Socket),
    tcp_bind(Socket, Port),
    repeat,
        udp_receive(Socket, Data, From, [as(atom)]),
        format('Got ~q from ~q~n', [Data, From]),
        fail.

[det]udp_send(+Socket, +Data, +To, +Options)

Send a UDP message. Data is a string, atom or code-list providing the data. To is an address of the form Host:Port where Host is either the hostname or an IP address. Defined Options are:

encoding(+Encoding)

Specifies the encoding to use for the string. See udp_receive/4 for details

as(+Type)

This uses the same values for Type as the as(Type) option of udp_receive/4. The are interpreted differently though. No Type corresponds to CVT_ALL of PL_get_chars(). Using atom corresponds to CVT_ATOM and any of string or codes is mapped to CVT_STRING|CVT_LIST, allowing for a SWI-Prolog string object, list of character codes or list of characters. Finally, term maps to CVT_WRITE_CANONICAL. This implies that arbitrary Prolog terms can be sent reliably using the option list‘[as(term),encoding(utf8)])`, using the same option list for udp_receive/4.

For example

send(Host, Port, Message) :-
    udp_socket(S),
    udp_send(S, Message, Host:Port, []),
    tcp_close_socket(S).

A broadcast is achieved by using tcp_setopt(Socket, broadcast) prior to sending the datagram and using the local network broadcast address as a ip/4 term.

[det]tcp_setopt(+SocketId, +Option)

Set options on the socket. Defined options are:

reuseaddr

Allow servers to reuse a port without the system being completely sure the port is no longer in use.

bindtodevice(+Device)

Bind the socket to Device (an atom). For example, the code below binds the socket to the loopback device that is typically used to realise the localhost. See the manual pages for setsockopt() and the socket interface (e.g., socket(7) on Linux) for details.

tcp_socket(Socket),
tcp_setopt(Socket, bindtodevice(lo))

nodelay

nodelay(true)

If true, disable the Nagle optimization on this socket, which is enabled by default on almost all modern TCP/IP stacks. The Nagle optimization joins small packages, which is generally desirable, but sometimes not. Please note that the underlying TCP_NODELAY setting to setsockopt() is not available on all platforms and systems may require additional privileges to change this option. If the option is not supported, tcp_setopt/2 raises a domain_error exception. See Wikipedia for details.

broadcast

UDP sockets only: broadcast the package to all addresses matching the address. The address is normally the address of the local subnet (i.e. 192.168.1.255). See udp_send/4.

ip_add_membership(+MultiCastGroup)

ip_add_membership(+MultiCastGroup, +LocalInterface)

ip_add_membership(+MultiCastGroup, +LocalInterface, +InterfaceIndex)

ip_drop_membership(+MultiCastGroup)

ip_drop_membership(+MultiCastGroup, +LocalInterface)

ip_drop_membership(+MultiCastGroup, +LocalInterface, +InterfaceIndex)

Join/leave a multicast group. Calls setsockopt() with the corresponding arguments.

dispatch(+Boolean)

In GUI environments (using XPCE or the Windows swipl-win.exe executable) this flags defines whether or not any events are dispatched on behalf of the user interface. Default is true. Only very specific situations require setting this to false.

sndbuf(+Integer)

Sets the send buffer size to Integer (bytes). On Windows this defaults (now) to 64kb. Higher latency links may benefit from increasing this further since the maximum theoretical throughput on a link is given by buffer-size / latency. See https://support.microsoft.com/en-gb/help/823764/slow-performance-occurs-when-you-copy-data-to-a-tcp-server-by-using-a for Microsoft's discussion

[det]tcp_fcntl(+Stream, +Action, ?Argument)

Interface to the fcntl() call. Currently only suitable to deal switch stream to non-blocking mode using:

  tcp_fcntl(Stream, setfl, nonblock),

An attempt to read from a non-blocking stream while there is no data available returns -1 (or end_of_file for read/1), but at_end_of_stream/1 fails. On actual end-of-input, at_end_of_stream/1 succeeds.

[semidet]tcp_getopt(+Socket, ?Option)

Get information about Socket. Defined properties are below. Requesting an unknown option results in a domain_error exception.

file_no(-File)

Get the OS file handle as an integer. This may be used for debugging and integration.

[nondet]host_address(+HostName, -Address, +Options)

[det]host_address(-HostName, +Address, +Options)

Translate between a machines host-name and it's (IP-)address. Supported options:

domain(+Domain)

One of inet or inet6 to limit the results to the given family.

type(+Type)

One of stream or dgram.

canonname(+Boolean)

If true (default false), return the canonical host name in the frist answer

In mode (+,-,+) Address is unified to a dict with the following keys:

address

A Prolog terms describing the ip address.

domain

One of inet or inet6. The underlying getaddrinfo() calls this family. We use domain for consistency with socket_create/2.

type

Currently one of stream or dgram.

host

Available if canonname(true) is specified on the first returned address. Holds the official canonical host name.

[det]tcp_host_to_address(?HostName, ?Address)

Translate between a machines host-name and it's (IP-)address. If HostName is an atom, it is resolved using getaddrinfo() and the IP-number is unified to Address using a term of the format ip(Byte1,Byte2,Byte3,Byte4). Otherwise, if Address is bound to an ip(Byte1,Byte2,Byte3,Byte4) term, it is resolved by gethostbyaddr() and the canonical hostname is unified with HostName.

deprecated

New code should use host_address/3. This version is bootstrapped from host_address/3 and only searches for IP4 addresses that support TCP connections.

[det]gethostname(-Hostname)

Return the canonical fully qualified name of this host. This is achieved by calling gethostname() and return the canonical name returned by getaddrinfo().

[det]ip_name(?IP, ?Name)

Translate between the textual representation of an IP address and the Prolog data structure. Prolog represents ip4 addresses as ip(A,B,C,D) and ip6 addresses as ip(A,B,C,D,E,F,H). For example:

?- ip_name(ip(1,2,3,4), Name)
Name = '1.2.3.4'.
?- ip_name(IP, '::').
IP = ip(0,0,0,0,0,0,0,0).
?- ip_name(IP, '1:2::3').
IP = ip(1,2,0,0,0,0,0,3).

[det]negotiate_socks_connection(+DesiredEndpoint, +StreamPair)

Negotiate a connection to DesiredEndpoint over StreamPair. DesiredEndpoint should be in the form of either:

• hostname : port
• ip(A,B,C,D) : port

Errors

socks_error(Details) if the SOCKS negotiation failed.

7 The stream_pool library

The library(streampool) library dispatches input from multiple streams based on wait_for_input/3. It is part of the clib package as it is used most of the time together with the library(socket) library. On non-Unix systems it often can only be used with socket streams.

With SWI-Prolog 5.1.x, multi-threading often provides a good alternative to using this library. In this schema one thread watches the listening socket waiting for connections and either creates a thread per connection or processes the accepted connections with a pool of worker threads. The library library(http/thread_httpd) provides an example realising a mult-threaded HTTP server.

add_stream_to_pool(+Stream, :Goal)

Add Stream, which must be an input stream and ---on non-unix systems--- connected to a socket to the pool. If input is available on Stream, Goal is called.

delete_stream_from_pool(+Stream)

Delete the given stream from the pool. Succeeds, even if Stream is no member of the pool. If Stream is unbound the entire pool is emtied but unlike close_stream_pool/0 the streams are not closed.

close_stream_pool

Empty the pool, closing all streams that are part of it.

dispatch_stream_pool(+TimeOut)

Wait for maximum of TimeOut for input on any of the streams in the pool. If there is input, call the Goal associated with add_stream_to_pool/2. If Goal fails or raises an exception a message is printed. TimeOut is described with wait_for_input/3.

If Goal is called, there is some input on the associated stream. Goal must be careful not to block as this will block the entire pool.^{1This is hard to achieve at the moment as none of the Prolog read-commands provide for a timeout.}

stream_pool_main_loop

Calls dispatch_stream_pool/1 in a loop until the pool is empty.

Below is a very simple example that reads the first line of input and echos it back.

:- use_module(library(streampool)).

server(Port) :-
        tcp_socket(Socket),
        tcp_bind(Socket, Port),
        tcp_listen(Socket, 5),
        tcp_open_socket(Socket, In, _Out),
        add_stream_to_pool(In, accept(Socket)),
        stream_pool_main_loop.

accept(Socket) :-
        tcp_accept(Socket, Slave, Peer),
        tcp_open_socket(Slave, In, Out),
        add_stream_to_pool(In, client(In, Out, Peer)).

client(In, Out, _Peer) :-
        read_line_to_codes(In, Command),
        close(In),
        format(Out, 'Please to meet you: ~s~n', [Command]),
        close(Out),
        delete_stream_from_pool(In).

8 library(uri): Process URIs

This library provides high-performance C-based primitives for manipulating URIs. We decided for a C-based implementation for the much better performance on raw character manipulation. Notably, URI handling primitives are used in time-critical parts of RDF processing. This implementation is based on RFC-3986:

http://labs.apache.org/webarch/uri/rfc/rfc3986.html

The URI processing in this library is rather liberal. That is, we break URIs according to the rules, but we do not validate that the components are valid. Also, percent-decoding for IRIs is liberal. It first tries UTF-8; then ISO-Latin-1 and finally accepts %-characters verbatim.

Earlier experience has shown that strict enforcement of the URI syntax results in many errors that are accepted by many other web-document processing tools.

[det]uri_components(+URI, -Components)

[det]uri_components(-URI, +Components)

Break a URI into its 5 basic components according to the RFC-3986 regular expression:

^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
 12            3  4          5       6  7        8 9

Components

is a term uri_components(Scheme, Authority, Path, Search, Fragment). If a URI is parsed, i.e., using mode (+,-), components that are not found are left uninstantiated (variable). See uri_data/3 for accessing this structure.

[semidet]uri_data(?Field, +Components, ?Data)

Provide access the uri_component structure. Defined field-names are: scheme, authority, path, search and fragment

[semidet]uri_data(+Field, +Components, +Data, -NewComponents)

NewComponents is the same as Components with Field set to Data.

[det]uri_normalized(+URI, -NormalizedURI)

NormalizedURI is the normalized form of URI. Normalization is syntactic and involves the following steps:

• 6.2.2.1. Case Normalization
• 6.2.2.2. Percent-Encoding Normalization
• 6.2.2.3. Path Segment Normalization

[det]iri_normalized(+IRI, -NormalizedIRI)

NormalizedIRI is the normalized form of IRI. Normalization is syntactic and involves the following steps:

• 6.2.2.1. Case Normalization
• 6.2.2.3. Path Segment Normalization

See also

This is similar to uri_normalized/2, but does not do normalization of %-escapes.

[det]uri_normalized_iri(+URI, -NormalizedIRI)

As uri_normalized/2, but percent-encoding is translated into IRI Unicode characters. The translation is liberal: valid UTF-8 sequences of %-encoded bytes are mapped to the Unicode character. Other %XX-sequences are mapped to the corresponding ISO-Latin-1 character and sole % characters are left untouched.

See also

uri_iri/2.

[semidet]uri_is_global(+URI)

True if URI has a scheme. The semantics is the same as the code below, but the implementation is more efficient as it does not need to parse the other components, nor needs to bind the scheme. The condition to demand a scheme of more than one character is added to avoid confusion with DOS path names.

uri_is_global(URI) :-
        uri_components(URI, Components),
        uri_data(scheme, Components, Scheme),
        nonvar(Scheme),
        atom_length(Scheme, Len),
        Len > 1.

[det]uri_resolve(+URI, +Base, -GlobalURI)

Resolve a possibly local URI relative to Base. This implements http://labs.apache.org/webarch/uri/rfc/rfc3986.html\#relative-transform

[det]uri_normalized(+URI, +Base, -NormalizedGlobalURI)

NormalizedGlobalURI is the normalized global version of URI. Behaves as if defined by:

uri_normalized(URI, Base, NormalizedGlobalURI) :-
        uri_resolve(URI, Base, GlobalURI),
        uri_normalized(GlobalURI, NormalizedGlobalURI).

[det]iri_normalized(+IRI, +Base, -NormalizedGlobalIRI)

NormalizedGlobalIRI is the normalized global version of IRI. This is similar to uri_normalized/3, but does not do %-escape normalization.

[det]uri_normalized_iri(+URI, +Base, -NormalizedGlobalIRI)

NormalizedGlobalIRI is the normalized global IRI of URI. Behaves as if defined by:

uri_normalized(URI, Base, NormalizedGlobalIRI) :-
        uri_resolve(URI, Base, GlobalURI),
        uri_normalized_iri(GlobalURI, NormalizedGlobalIRI).

[det]uri_query_components(+String, -Query)

[det]uri_query_components(-String, +Query)

Perform encoding and decoding of an URI query string. Query is a list of fully decoded (Unicode) Name=Value pairs. In mode (-,+), query elements of the forms Name(Value) and Name-Value are also accepted to enhance interoperability with the option and pairs libraries. E.g.

?- uri_query_components(QS, [a=b, c('d+w'), n-'VU Amsterdam']).
QS = 'a=b&c=d%2Bw&n=VU%20Amsterdam'.

?- uri_query_components('a=b&c=d%2Bw&n=VU%20Amsterdam', Q).
Q = [a=b, c='d+w', n='VU Amsterdam'].

[det]uri_authority_components(+Authority, -Components)

[det]uri_authority_components(-Authority, +Components)

Break-down the authority component of a URI. The fields of the structure Components can be accessed using uri_authority_data/3. This predicate deals with IPv6 addresses written as [ip], returning the ip as host, without the enclosing []. When constructing an authority string and the host contains :, the host is embraced in []. If [] is not used correctly, the behavior should be considered poorly defined. If there is no balancing‘]` or the host part does not end with‘]`, these characters are considered normal characters and part of the (invalid) host name.

[semidet]uri_authority_data(+Field, ?Components, ?Data)

Provide access the uri_authority structure. Defined field-names are: user, password, host and port

[det]uri_encoded(+Component, +Value, -Encoded)

[det]uri_encoded(+Component, -Value, +Encoded)

Encoded is the URI encoding for Value. When encoding (Value->Encoded), Component specifies the URI component where the value is used. It is one of query_value, fragment, path or segment. Besides alphanumerical characters, the following characters are passed verbatim (the set is split in logical groups according to RFC3986).

query_value, fragment

"-._~" | "!$’()*,;" | "@" | "/?"

path

"-._~" | "!$&’()*,;=" | "@" | "/"

segment

"-._~" | "!$&’()*,;=" | "@"

[det]uri_iri(+URI, -IRI)

[det]uri_iri(-URI, +IRI)

Convert between a URI, encoded in US-ASCII and an IRI. An IRI is a fully expanded Unicode string. Unicode strings are first encoded into UTF-8, after which %-encoding takes place.

Errors

syntax_error(Culprit) in mode (+,-) if URI is not a legally percent-encoded UTF-8 string.

[semidet]uri_file_name(+URI, -FileName)

[det]uri_file_name(-URI, +FileName)

Convert between a URI and a local file_name. This protocol is covered by RFC 1738. Please note that file-URIs use absolute paths. The mode (-, +) translates a possible relative path into an absolute one.

[det]uri_edit(+Actions, +URI0, -URI)

Modify a URI according to Actions. Actions is either a single action or a (nested) list of actions. Defined primitive actions are:

scheme(+Scheme)

Set the Scheme of the URI (typically http, https, etc.)

user(+User)

Add/set the user of the authority component.

password(+Password)

Add/set the password of the authority component.

host(+Host)

Add/set the host (or ip address) of the authority component.

port(+Port)

Add/set the port of the authority component.

path(+Path)

Set/extend the path component. If Path is not absolute it is taken relative to the path of URI0.

search(+KeyValues)

Extend the Key=Value pairs of the current search (query) component. New values replace existing values. If KeyValues is written as =(KeyValues) the current search component is ignored. KeyValues is a list, whose elements are one of Key=Value, Key-Value or‘Key(Value)`.

fragment(+Fragment)

Set the Fragment of the uri.

Components can be removed by using a variable as value, except from path which can be reset using path(/) and query which can be dropped using query(=([])).

URI0	is either a valid uri or a variable to start fresh.

9 CGI Support library

This is currently a very simple library, providing support for obtaining the form-data for a CGI script:

cgi_get_form(-Form)

Decodes standard input and the environment variables to obtain a list of arguments passed to the CGI script. This predicate both deals with the CGI GET method as well as the POST method. If the data cannot be obtained, an existence_error exception is raised.

Below is a very simple CGI script that prints the passed parameters. To test it, compile this program using the command below, copy it to your cgi-bin directory (or make it otherwise known as a CGI-script) and make the query http://myhost.mydomain/cgi-bin/cgidemo?hello=world

% pl -o cgidemo --goal=main --toplevel=halt -c cgidemo.pl

:- use_module(library(cgi)).

main :-
        set_stream(current_output, encoding(utf8)),
        cgi_get_form(Arguments),
        format('Content-type: text/html; charset=UTF-8~n~n', []),
        format('<html>~n', []),
        format('<head>~n', []),
        format('<title>Simple SWI-Prolog CGI script</title>~n', []),
        format('</head>~n~n', []),
        format('<body>~n', []),
        format('<p>', []),
        print_args(Arguments),
        format('</body>~n</html>~n', []).

print_args([]).
print_args([A0|T]) :-
        A0 =.. [Name, Value],
        format('<b>~w</b>=<em>~w</em><br>~n', [Name, Value]),
        print_args(T).

9.1 Some considerations

Printing an HTML document using format/2 is not a neat way of producing HTML because it is vulnerable to required escape sequences. A high-level alternative is provided by library(http/html_write) from the HTTP library.

The startup-time of Prolog is relatively long, in particular if the program is large. In many cases it is much better to use the SWI-Prolog HTTP server library and make the main web-server relay requests to the SWI-Prolog webserver. See the SWI-Prolog HTTP package for details.

The CGI standard is unclear about handling Unicode data. The above two declarations ensure the CGI script will send all data in UTF-8 and thus provide full support of Unicode. It is assumed that browsers generally send form-data using the same encoding as the page in which the form appears, UTF-8 or ISO Latin-1. The current version of cgi_get_form/1 assumes the CGI data is in UTF-8.

10 Password encryption library

The library(crypt) library defines crypt/2 for encrypting and testing passwords. The clib package also provides crytographic hashes as described in section 12

crypt(+Plain, ?Encrypted)

This predicate can be used in three modes. To test whether a password matches an encrypted version thereof, simply run with both arguments fully instantiated. To generate a default encrypted version of Plain, run with unbound Encrypted and this argument is unified to a list of character codes holding an encrypted version.

The library supports two encryption formats: traditional Unix DES-hashes^{2On non-Unix systems, crypt() is provided by the NetBSD library. The license header is added at the end of this document.} and FreeBSD compatible MD5 hashes (all platforms). MD5 hashes start with the magic sequence $1$, followed by an up to 8 character salt. DES hashes start with a 2 character salt. Note that a DES hash considers only the first 8 characters. The MD5 considers the whole string.

Salt and algorithm can be forced by instantiating the start of Encrypted with it. This is typically used to force MD5 hashes:

?- phrase("$1$", E, _),
   crypt("My password", E),
   format('~s~n', [E]).

$1$qdaDeDZn$ZUxSQEESEHIDCHPNc3fxZ1

Encrypted is always a list of ASCII character codes. Plain only supports ISO-Latin-1 passwords in the current implementation.

Plain is either an atom, SWI-Prolog string, list of characters or list of character-codes. It is not advised to use atoms, as this implies the password will be available from the Prolog heap as a defined atom.

NOTE: crypt/2 provides an interface to the Unix password hashing API. Above we already introduced support for classical DES and MD5 hashes, both hashes that are considered insecure by today's standards.^{3Insecure means that the password can realistically be derived from the password hash using a brute-force attack. This implies that leaking the password database is an immediate security risk.} The crypt() API of modern Unix systems typically support more secure hashes. Using crypt/2 is suitable if compatibility with OS passwords is required. If strong hashes and platform independence are important to you, use crypto_password_hash/2 provided by library library(crypto) from the ssl package.

11 library(uuid): Universally Unique Identifier (UUID) Library

See also

- http://www.ossp.org/pkg/lib/uuid/
- https://en.wikipedia.org/wiki/Universally_unique_identifier

To be done

Compare UUIDs, extract time and version from UUIDs

The library provides operations on UUIDs. Please consult other sources for understanding UUIDs and the implications of the different UUID versions. Some typical calls are given below:

?- uuid(X).
X = 'ea6589fa-19dd-11e2-8a49-001d92e1879d'.

?- uuid(X, [url('http://www.swi-prolog.org')]).
X = '73a07870-6a90-3f2e-ae2b-ffa538dc7c2c'.

[det]uuid(-UUID)

UUID is an atom representing a new UUID. This is the same as calling uuid(UUID, []). See uuid/2 for options.

[det]uuid(-UUID, +Options)

Create a new UUID according to Options. The following options are defined:

version(+Versions)

Integer in the range 1..5, which specifies the UUID version that is created. Default is 1.

dns(DNS)

url(URL)

oid(OID)

x500(X500)

Provide additional context information for UUIDs using version 3 or 5. If there is no explicit version option, UUID version 3 is used.

format(+Format)

Representation of the UUID. Default is atom, yielding atoms such as 8304efdd-bd6e-5b7c-a27f-83f3f05c64e0. The alternative is integer, returning a large integer that represents the 128 bits of the UUID.

If SWI-Prolog was not built with the OSSP UUID dependency library a simple Prolog alternative that only implements version 4 random UUIDs is provided. In this case the default version is 4 and the only admissible options are version(4) and format(Format).

uuid_property(+UUID, ?Property)

True when UUID is a property of the given UUID. Supported properties are:

version(V)

Return the version of the UUID (1..5)

time(-Stamp)

Time using SWI-Prolog's time stamp (float with seconds since January 1, 1970, UTC). Only for version 1 and 2 UUIDs

To be done

Implement more properties.

12 SHA* Secure Hash Algorithms

The library library(sha) provides Secure Hash Algorihms approved by FIPS (Federal Information Processing Standard). Quoting Wikipedia: “The SHA (Secure Hash Algorithm) hash functions refer to five FIPS-approved algorithms for computing a condensed digital representation (known as a message digest) that is, to a high degree of probability, unique for a given input data sequence (the message). These algorithms are called‘secure' because (in the words of the standard), “for a given algorithm, it is computationally infeasible 1) to find a message that corresponds to a given message digest, or 2) to find two different messages that produce the same message digest. Any change to a message will, with a very high probability, result in a different message digest.''

The current library supports all 5 approved algorithms, both computing the hash-key from data and the hash Message Authentication Code (HMAC).

A general secure hash interface is provided by library(crypto), part of the ssl package.

Input is text, represented as an atom, packed string object or code-list. Note that these functions operate on byte-sequences and therefore are not meaningful on Unicode text. The result is returned as a list of byte-values. This is the most general format that is comfortable supported by standard Prolog and can easily be transformed in other formats. Commonly used text formats are ASCII created by encoding each byte as two hexadecimal digits and ASCII created using base64 encoding. Representation as a large integer can be desirable for computational processing.

sha_hash(+Data, -Hash, +Options)

Hash is the SHA hash of Data. Data is either an atom, packed string or list of character codes. Hash is unified with a list of bytes (integers in the range 0..255) representing the hash. See hash_atom/2 to convert this into the more commonly seen hexadecimal representation. The conversion is controlled by Options:

algorithm(+Algorithm)

One of sha1 (default), sha224, sha256, sha384 or sha512

encoding(+Encoding)

This option defines the mapping from Prolog (Unicode) text to bytes on which the SHA algorithm is performed. It has two values. The defualt is utf8, which implies that Unicode text is encoded as UTF-8 bytes. This option can deal with any atom. The alternative is octet, which implies that the text is considered as a sequence of bytes. This is suitable for e.g., atoms that represent binary data. An error is raised if the text contains code-points outside the range 0..255.

hmac_sha(+Key, +Data, -HMAC, +Options)

Quoting Wikipedia: “A keyed-hash message authentication code, or HMAC, is a type of message authentication code (MAC) calculated using a cryptographic hash function in combination with a secret key. As with any MAC, it may be used to simultaneously verify both the data integrity and the authenticity of a message. Any iterative cryptographic hash function, such as MD5 or SHA-1, may be used in the calculation of an HMAC; the resulting MAC algorithm is termed HMAC-MD5 or HMAC-SHA-1 accordingly. The cryptographic strength of the HMAC depends upon the cryptographic strength of the underlying hash function, on the size and quality of the key and the size of the hash output length in bits.''

Key and Data are either an atom, packed string or list of character codes. HMAC is unified with a list of integers representing the authentication code. Options is the same as for sha_hash/3, but currently only sha1 and sha256 are supported.

hash_atom(+Hash, -HexAtom)

True when HexAtom is the commonly used hexadecimal encoding of the hash code. E.g.,

?- sha_hash('SWI-Prolog', Hash, []),
   hash_atom(Hash, Hex).
Hash = [61, 128, 252, 38, 121, 69, 229, 85, 199|...],
Hex = '3d80fc267945e555c730403bd0ab0716e2a68c68'.

12.1 License terms

The underlying SHA-2 library is an unmodified copy created by Dr Brian Gladman, Worcester, UK. It is distributed under the license conditions below.

The free distribution and use of this software in both source and binary form is allowed (with or without changes) provided that:

1. distributions of this source code include the above copyright notice, this list of conditions and the following disclaimer;

2. distributions in binary form include the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other associated materials;

3. the copyright holder's name is not used to endorse products built using this software without specific written permission.

ALTERNATIVELY, provided that this notice is retained in full, this product may be distributed under the terms of the GNU General Public License (GPL), in which case the provisions of the GPL apply INSTEAD OF those given above.

13 library(md5): MD5 hashes

See also

library(sha), library(hash_stream) and library(crypto).

Compute MD5 hashes from a Prolog string. This library provides a lightweight alternative to the general secure hash interface provided by library(crypto) from the ssl package.

[det]md5_hash(+Data, -Hash, +Options)

Hash is the MD5 hash of Data, The conversion is controlled by Options:

encoding(+Encoding)

If Data is a sequence of character codes, this must be translated into a sequence of bytes, because that is what the hashing requires. The default encoding is utf8. The other meaningful value is octet, claiming that Data contains raw bytes.

Data	is either an atom, string, code-list or char-list.
Hash	is an atom holding 32 characters, representing the hash in hexadecimal notation

14 library(hash_stream): Maintain a hash on a stream

See also

In addition to this hash library, SWI-Prolog provides library(md5), library(sha) and hash functions through library(crypto), part of the ssl package.

This library defines a filter stream that maintains a hash of the data that passes through the stream. It can be used to compute the hash of input data while it is being processed. This is notably interesting if data is processed from a socket as it avoids the need for collecting the data first in a temporary file.

A typical processing sequence is illustrated below, where process/2 somehow processed the data and save_result/3 records the result as obtained from URL with content digest SHA256 its Result.

    ...,
    http_open(URL, In0, []),
    open_hash_stream(In0, In, [algorithm(sha256)]),
    process(In, Result),
    stream_hash(In, SHA256),
    close(In),
    save_result(URL, SHA256, Result)

This library can also be used to compute the hash for the content of a file. The advantage is that this code doesn't rely on external tools. It is considerably faster for short files, but considerably slower on large files because Prolog I/O is based on character streams rather than blocks.

file_hash(Algorithm, File, Hash) :-
    setup_call_cleanup(
        open(File, read, In0, [type(binary)]),
        setup_call_cleanup(
            open_hash_stream(In0, In,
                             [ algorithm(Algorithm),
                               close_parent(false)
                             ]),
            ( setup_call_cleanup(
                  open_null_stream(Null),
                  copy_stream_data(In, Null),
                  close(Null)),
              stream_hash(In, Hash)
            ),
            close(In)),
        close(In0)).

[det]open_hash_stream(+OrgStream, -HashStream, +Options)

Open a filter stream on OrgStream that maintains a hash. The hash can be retrieved at any time using stream_hash/2. Provided options:

algorithm(+Algorithm)

One of md5, sha1, sha224, sha256, sha384 or sha512. Default is sha1.

close_parent(+Bool)

If true (default), closing the filter stream also closes the original (parent) stream.

[det]stream_hash(+HashStream, -Digest:atom)

Unify Digest with a hash for the bytes send to or read from HashStream. Note that the hash is computed on the stream buffers. If the stream is an output stream, it is first flushed and the Digest represents the hash at the current location. If the stream is an input stream the Digest represents the hash of the processed input including the already buffered data.

15 Memory files

The library(memfile) provides an alternative to temporary files, intended for temporary buffering of data. Memory files in general are faster than temporary files and do not suffer from security risks or naming conflicts associated with temporary-file management.

There is no limit to the number of memory streams, nor the size of them. However, a single memory file cannot have multiple streams at the same time, i.e., a memory file cannot be opened multiple times, not even for reading. Memory files are thread-safe and subject to (atom) garbage collection.

These predicates are first of all intended for building higher-level primitives such as open_codes_stream/3. See also format/3, atom_to_term/3, term_to_atom/2, term_string/2, etc.

new_memory_file(-Handle)

Create a new memory file and return a unique opaque handle to it.

free_memory_file(+Handle)

Discard the memory file and its contents. If the file is open it is first closed.

open_memory_file(+Handle, +Mode, -Stream)

Open the memory-file. Mode is one of read, write, append, update or insert. The resulting Stream must be closed using close/1. When opened for update or insert, the current location is initialized at the start of the data and can be modified using seek/4 or set_stream_position/2. In update mode, existing content is replaced, while the size is enlarged after hitting the end of the data. In insert mode, the new data is inserted at the current point.

open_memory_file(+Handle, +Mode, -Stream, +Options)

Open a memory-file as open_memory_file/3. Options:

encoding(+Encoding)

Set the encoding for a memory file and the created stream. Encoding names are the same as used with open/4. By default, memoryfiles represent UTF-8 streams, making them capable of storing arbitrary Unicode text. In practice the only alternative is octet, turning the memoryfile into binary mode. Please study SWI-Prolog Unicode and encoding issues before using this option.

free_on_close(+Bool)

If true (default false) and the memory file is opened for reading, discard the file (see free_memory_file/1) if the input is closed. This is used to realise open_chars_stream/2 in library(charsio).

size_memory_file(+Handle, -Size)

Return the content-length of the memory-file in characters in the current encoding of the memory file. The file should be closed and contain data.

size_memory_file(+Handle, -Size, +Encoding)

Return the content-length of the memory-file in characters in the given Encoding. The file should be closed and contain data.

atom_to_memory_file(+Atom, -Handle)

Turn an atom into a read-only memory-file containing the (shared) characters of the atom. Opening this memory-file in mode write yields a permission error.

insert_memory_file(+Handle, +Offset, +Data)

Insert Data into the memory file at location Offset. The offset is specified in characters. Data can be an atom, string, code or character list. Other terms are first serialized using writeq/1. This predicate raises a domain_error exception if Offset is out of range and a permission_error if the memory file is read-only or opened.

delete_memory_file(+Handle, +Offset, +Length)

Delete a Length characters from the memory file, starting at Offset. This predicate raises a domain_error exception if Offset or Offset+Length is out of range and a permission_error if the memory file is read-only or opened.

memory_file_to_atom(+Handle, -Atom)

Return the content of the memory-file in Atom.

memory_file_to_atom(+Handle, -Atom, +Encoding)

Return the content of the memory-file in Atom, pretending the data is in the given Encoding. This can be used to convert from one encoding into another, typically from/to bytes. For example, if we must convert a set of bytes that contain text in UTF-8, open the memory file as octet stream, fill it, and get the result using Encoding is utf8. Currently only supported if Encoding is one of iso_latin_1, octed (the same as iso_latin_1), wchar or utf8. Use with another encoding raises a domain error.

memory_file_to_codes(+Handle, -Codes)

Return the content of the memory-file as a list of character-codes in Codes.

memory_file_to_codes(+Handle, -Codes, +Encoding)

Return the content of the memory-file as a list of character-codes in Codes, pretending the data is in the given Encoding.

memory_file_to_string(+Handle, -String)

Return the content of the memory-file as a string in -String.

memory_file_to_string(+Handle, -String, +Encoding)

Return the content of the memory-file as a string in String, pretending the data is in the given Encoding.

memory_file_substring(+Handle, ?Before, ?Length, ?After, -SubString)

SubString is a substring of the memory file. There are Before characters in the memory file before SubString, SubString contains Length character and is followed by After characters in the memory file. The signature is the same as sub_string/5 and sub_atom/5, but currently at least two of the 3 position arguments must be specified. Future versions might implement the full functionality of sub_string/5.

memory_file_line_position(+MF, ?Line, ?LinePos, ?Offset)

True if the character offset Offset corresponds with the LinePos character on line Line. Lines are counted from one (1). Note that LinePos is not the column as each character counts for one, including backspace and tab.

16 library(time): Time and alarm library

The library(time) provides timing and alarm functions. Alarms are thread-specific, i.e., creating an alarm causes the alarm goal to be called in the thread that created it. The predicate current_alarm/4 only reports alarms that are related to the calling thread. If a thread terminates, all remaining alarms are silently removed. Most applications use call_with_time_limit/2.

[det]alarm(+Time, :Callable, -Id)

[det]alarm(+Time, :Callable, -Id, +Options)

Set up an alarm to be signaled Time seconds from now. If the alarm expires, Callable is called asynchronously. Callable can be used to raise an exception using throw/1 to abort some execution.

Options is a list of Name(Value) options. Currently defined options are:

remove(Bool)

If true (default false), remove the alarm-event (as remove_alarm/1) after it has been fired.

install(Bool)

If false (default true) do not install the alarm. It must be installed separately using install_alarm/1.

[det]alarm_at(+Time, :Callable, -Id)

[det]alarm_at(+Time, :Callable, -Id, +Options)

As alarm/3 and alarm/4, but schedule the alarm at an absolute point in time.

See also

date_time_stamp/2.

[det]install_alarm(+Id)

[det]install_alarm(+Id, +RelTime)

Install an alarm allocated using alarm/4 with the install(false) option or de-activated using uninstall_alarm/1. With a given RelTime, the alarm is scheduled at the RelTime from now. Otherwise it is scheduled on the same (absolute) time on which is was created.

[det]uninstall_alarm(+Id)

De-activate an alarm. This does not invalidate Id, but ensures that the alarm will not fire. The alarm can be rescheduled to the original time using install_alarm/1 or to a new time using install_alarm/2.

[det]remove_alarm(+Id)

Remove an alarm. If it has not yet been fired, it never will.

[nondet]current_alarm(?Time, :Goal, ?Id, ?Status)

Enumerate the alarms in the schedule. Time is the absolute time the event is scheduled for (see also get_time/1). Goal is the goal to execute, Id is the identifier and Status is the scheduling status. It takes the value done if the alarm has been fired, next if the event is the next to be executed and scheduled otherwise.

[det]call_with_time_limit(+Time, :Goal)

[det]call_with_time_limit(+Time, :Goal, +Context)

Call Goal, while watching out for a (wall-time) limit. If this limit is exceeded, the exception time_limit_exceeded is raised. call_with_time_limit/3 throws time_limit_exceeded(Context). Goal is called as in once/1.

throws

time_limit_exceeded (call_with_time_limit/2) or time_limit_exceeded(Context) (call_with_time_limit/3).

17 library(unix): Unix specific operations

See also

library(process) provides a portable high level interface to create and manage processes.

The library(unix) library provides the commonly used Unix primitives to deal with process management. These primitives are useful for many tasks, including server management, parallel computation, exploiting and controlling other processes, etc.

The predicates in this library are modelled closely after their native Unix counterparts.

[det]fork(-Pid)

Clone the current process into two branches. In the child, Pid is unified to child. In the original process, Pid is unified to the process identifier of the created child. Both parent and child are fully functional Prolog processes running the same program. The processes share open I/O streams that refer to Unix native streams, such as files, sockets and pipes. Data is not shared, though on most Unix systems data is initially shared and duplicated only if one of the programs attempts to modify the data.

Unix fork() is the only way to create new processes and fork/1 is a simple direct interface to it.

Errors

permission_error(fork, process, main) is raised if the calling thread is not the only thread in the process. Forking a Prolog process with threads will typically deadlock because only the calling thread is cloned in the fork, while all thread synchronization are cloned.

[det]fork_exec(+Command)

Fork (as fork/1) and exec (using exec/1) the child immediately. This behaves as the code below, but bypasses the check for the existence of other threads because this is a safe scenario.

fork_exec(Command) :-
      (   fork(child)
      ->  exec(Command)
      ;   true
      ).

exec(+Command)

Replace the running program by starting Command. Command is a callable term. The functor is the command and the arguments provide the command-line arguments for the command. Each command-line argument must be atomic and is converted to a string before passed to the Unix call execvp(). Here are some examples:

• exec(ls('-l'))
• exec('/bin/ls'('-l', '/home/jan'))

Unix exec() is the only way to start an executable file executing. It is commonly used together with fork/1. For example to start netscape on an URL in the background, do:

run_netscape(URL) :-
        (    fork(child),
             exec(netscape(URL))
        ;    true
        ).

Using this code, netscape remains part of the process-group of the invoking Prolog process and Prolog does not wait for netscape to terminate. The predicate wait/2 allows waiting for a child, while detach_IO/0 disconnects the child as a deamon process.

[det]wait(?Pid, -Status)

Wait for a child to change status. Then report the child that changed status as well as the reason. If Pid is bound on entry then the status of the specified child is reported. If not, then the status of any child is reported. Status is unified with exited(ExitCode) if the child with pid Pid was terminated by calling exit() (Prolog halt/1). ExitCode is the return status. Status is unified with signaled(Signal) if the child died due to a software interrupt (see kill/2). Signal contains the signal number. Finally, if the process suspended execution due to a signal, Status is unified with stopped(Signal).

[det]kill(+Pid, +Signal)

Deliver a software interrupt to the process with identifier Pid using software-interrupt number Signal. See also on_signal/2. Signals can be specified as an integer or signal name, where signal names are derived from the C constant by dropping the SIG prefix and mapping to lowercase. E.g. int is the same as SIGINT in C. The meaning of the signal numbers can be found in the Unix manual.

[det]pipe(-InSream, -OutStream)

Create a communication-pipe. This is normally used to make a child communicate to its parent. After pipe/2, the process is cloned and, depending on the desired direction, both processes close the end of the pipe they do not use. Then they use the remaining stream to communicate. Here is a simple example:

:- use_module(library(unix)).

fork_demo(Result) :-
        pipe(Read, Write),
        fork(Pid),
        (   Pid == child
        ->  close(Read),
            format(Write, '~q.~n',
                   [hello(world)]),
            flush_output(Write),
            halt
        ;   close(Write),
            read(Read, Result),
            close(Read)
        ).

[det]dup(+FromStream, +ToStream)

Interface to Unix dup2(), copying the underlying filedescriptor and thus making both streams point to the same underlying object. This is normally used together with fork/1 and pipe/2 to talk to an external program that is designed to communicate using standard I/O.

Both FromStream and ToStream either refer to a Prolog stream or an integer descriptor number to refer directly to OS descriptors. See also demo/pipe.pl in the source-distribution of this package.

[det]detach_IO(+Stream)

This predicate is intended to create Unix deamon processes. It performs two actions.

1. The I/O streams user_input, user_output and user_error are closed if they are connected to a terminal (see tty property in stream_property/2). Input streams are rebound to a dummy stream that returns EOF. Output streams are reboud to forward their output to Stream.
2. The process is detached from the current process-group and its controlling terminal. This is achieved using setsid() if provided or using ioctl() TIOCNOTTY on /dev/tty.

To ignore all output, it may be rebound to a null stream. For example:

      ...,
      open_null_stream(Out),
      detach_IO(Out).

The detach_IO/1 should be called only once per process. Subsequent calls silently succeed without any side effects.

See also

detach_IO/0 and library(syslog).

[det]detach_IO

Detach I/O similar to detach_IO/1. The output streams are bound to a file /tmp/pl-out.<pid>. Output is line buffered (see set_stream/2).

See also

library(syslog) allows for sending output to the Unix logging service.

Compatibility

Older versions of this predicate only created this file if there was output.

[det]prctl(+Option)

Access to Linux process control operations. Defines values for Option are:

set_dumpable(+Boolean)

Control whether the process is allowed to dump core. This right is dropped under several uid and gid conditions.

get_dumpable(-Boolean)

Get the value of the dumpable flag.

[semidet]sysconf(+Conf)

Access system configuration. See sysconf(1) for details. Conf is a term Config(Value), where Value is always an integer. Config is the sysconf() name after removing =_SC_= and conversion to lowercase. Currently support the following configuration info: arg_max, child_max, clk_tck, open_max, pagesize, phys_pages, avphys_pages, nprocessors_conf and nprocessors_onln. Note that not all values may be supported on all operating systems.

18 Limiting process resources

The library(rlimit) library provides an interface to the POSIX getrlimit()/setrlimit() API that control the maximum resource-usage of a process or group of processes. This call is especially useful for servers such as CGI scripts and inetd-controlled servers to avoid an uncontrolled script claiming too much resources.

rlimit(+Resource, -Old, +New)

Query and/or set the limit for Resource. Time-values are in seconds and size-values are counted in bytes. The following values are supported by this library. Please note that not all resources may be available and accessible on all platforms. This predicate can throw a variety of exceptions. In portable code this should be guarded with catch/3. The defined resources are:

as	Max address space
cpu	CPU time in seconds
fsize	Maximum filesize
data	max data size
stack	max stack size
core	max core file size
rss	max resident set size
nproc	max number of processes
nofile	max number of open files
memlock	max locked-in-memory address

When the process hits a limit POSIX systems normally send the process a signal that terminates it. These signals may be caught using SWI-Prolog's on_signal/3 primitive. The code below illustrates this behaviour. Please note that asynchronous signal handling is dangerous, especially when using threads. 100% fail-safe operation cannot be guaranteed, but this procedure will inform the user properly‘most of the time'.

rlimit_demo :-
        rlimit(cpu, _, 2),
        on_signal(xcpu, _, cpu_exceeded),
        ( repeat, fail ).

cpu_exceeded(_Sig) :-
        format(user_error, 'CPU time exceeded~n', []),
        halt(1).

19 library(udp_broadcast): A UDP broadcast proxy

author

Jeffrey Rosenwald (JeffRose@acm.org), Jan Wielemaker

See also

tipc.pl

license

BSD-2

SWI-Prolog's broadcast library provides a means that may be used to facilitate publish and subscribe communication regimes between anonymous members of a community of interest. The members of the community are however, necessarily limited to a single instance of Prolog. The UDP broadcast library removes that restriction. With this library loaded, any member on your local IP subnetwork that also has this library loaded may hear and respond to your broadcasts.

This library support three styles of networking as described below. Each of these networks have their own advantages and disadvantages. Please study the literature to understand the consequences.

broadcast

Broadcast messages are sent to the LAN subnet. The broadcast implementation uses two UDP ports: a public to address the whole group and a private one to address a specific node. Broadcasting is generally a good choice if the subnet is small and traffic is low.

unicast

Unicast sends copies of packages to known peers. Unicast networks can easily be routed. The unicast version uses a single UDP port per node. Unicast is generally a good choice for a small party, in particular if the peers are in different networks.

multicast

Multicast is like broadcast, but it can be configured to work accross networks and may work more efficiently on VLAN networks. Like the broadcast setup, two UDP ports are used. Multicasting can in general deliver the most efficient LAN and WAN networks, but requires properly configured routing between the peers.

After initialization and, in the case of a unicast network managing the set of peers, communication happens through broadcast/1, broadcast_request/1 and listen/1,2,3.

A broadcast/1 or broadcast_request/1 of the shape udp(Scope, Term) or udp(Scope, Term, TimeOut) is forwarded over the UDP network to all peers that joined the same Scope. To prevent the potential for feedback loops, only the plain Term is broadcasted locally. The timeout is optional. It specifies the amount to time to wait for replies to arrive in response to a broadcast_request/1. The default period is 0.250 seconds. The timeout is ignored for broadcasts.

An example of three separate processes cooperating in the same scope called peers:

Process A:

   ?- listen(number(X), between(1, 5, X)).
   true.

   ?-

Process B:

   ?- listen(number(X), between(7, 9, X)).
   true.

   ?-

Process C:

   ?- findall(X, broadcast_request(udp(peers, number(X))), Xs).
   Xs = [1, 2, 3, 4, 5, 7, 8, 9].

   ?-

It is also possible to carry on a private dialog with a single responder. To do this, you supply a compound of the form, Term:PortId, to a UDP scoped broadcast/1 or broadcast_request/1, where PortId is the ip-address and port-id of the intended listener. If you supply an unbound variable, PortId, to broadcast_request, it will be unified with the address of the listener that responds to Term. You may send a directed broadcast to a specific member by simply providing this address in a similarly structured compound to a UDP scoped broadcast/1. The message is sent via unicast to that member only by way of the member's broadcast listener. It is received by the listener just as any other broadcast would be. The listener does not know the difference.

For example, in order to discover who responded with a particular value:

Host B Process 1:

   ?- listen(number(X), between(1, 5, X)).
   true.

   ?-

Host A Process 1:


   ?- listen(number(X), between(7, 9, X)).
   true.

   ?-

Host A Process 2:

   ?- listen(number(X), between(1, 5, X)).
   true.

   ?- bagof(X, broadcast_request(udp(peers,number(X):From,1)), Xs).
   From = ip(192, 168, 1, 103):34855,
   Xs = [7, 8, 9] ;
   From = ip(192, 168, 1, 103):56331,
   Xs = [1, 2, 3, 4, 5] ;
   From = ip(192, 168, 1, 104):3217,
   Xs = [1, 2, 3, 4, 5].

All incomming trafic is handled by a single thread with the alias udp_inbound_proxy. This thread also performs the internal dispatching using broadcast/1 and broadcast_request/1. Future versions may provide for handling these requests in separate threads.

19.1 Caveats

While the implementation is mostly transparent, there are some important and subtle differences that must be taken into consideration:

• UDP broadcast requires an initialization step in order to launch the broadcast listener proxy. See udp_broadcast_initialize/2.
• Prolog's broadcast_request/1 is nondet. It sends the request, then evaluates the replies synchronously, backtracking as needed until a satisfactory reply is received. The remaining potential replies are not evaluated. With UDP, all peers will send all answers to the query. The receiver may however stop listening.
• A UDP broadcast/1 is completely asynchronous.
• A UDP broadcast_request/1 is partially synchronous. A broadcast_request/1 is sent, then the sender balks for a period of time (default: 250 ms) while the replies are collected. Any reply that is received after this period is silently discarded. A optional second argument is provided so that a sender may specify more (or less) time for replies.
• Replies are presented to the user as a choice point on arrival, until the broadcast request timer finally expires. This allows traffic to propagate through the system faster and provides the requestor with the opportunity to terminate a broadcast request early if desired, by simply cutting choice points.
• Please beware that broadcast request transactions remain active and resources consumed until broadcast_request finally fails on backtracking, an uncaught exception occurs, or until choice points are cut. Failure to properly manage this will likely result in chronic exhaustion of UDP sockets.
• If a listener is connected to a generator that always succeeds (e.g. a random number generator), then the broadcast request will never terminate and trouble is bound to ensue.
• broadcast_request/1 with udp_subnet scope is not reentrant. If a listener performs a broadcast_request/1 with UDP scope recursively, then disaster looms certain. This caveat does not apply to a UDP scoped broadcast/1, which can safely be performed from a listener context.
• UDP broadcast's capacity is not infinite. While it can tolerate substantial bursts of activity, it is designed for short bursts of small messages. Unlike TIPC, UDP is unreliable and has no QOS protections. Congestion is likely to cause trouble in the form of non-Byzantine failure. That is, late, lost (e.g. infinitely late), or duplicate datagrams. Caveat emptor.
• A UDP broadcast_request/1 term that is grounded is considered to be a broadcast only. No replies are collected unless the there is at least one unbound variable to unify.
• A UDP broadcast/1 always succeeds, even if there are no listeners.
• A UDP broadcast_request/1 that receives no replies will fail.
• Replies may be coming from many different places in the network (or none at all). No ordering of replies is implied.
• Prolog terms are sent to others after first converting them to atoms using term_string/3. Serialization does not deal with cycles, attributes or sharing. The hook udp_term_string_hook/3 may be defined to change the message serialization and support different message formats and/or encryption.
• The broadcast model is based on anonymity and a presumption of trust--a perfect recipe for compromise. UDP is an Internet protocol. A UDP broadcast listener exposes a public port, which is static and shared by all listeners, and a private port, which is semi-static and unique to the listener instance. Both can be seen from off-cluster nodes and networks. Usage of this module exposes the node and consequently, the cluster to significant security risks. So have a care when designing your application. You must talk only to those who share and contribute to your concerns using a carefully prescribed protocol.
• UDP broadcast categorically and silently ignores all message traffic originating from or terminating on nodes that are not members of the local subnet. This security measure only keeps honest people honest!

udp_broadcast_close(+Scope)

Close a UDP broadcast scope.

[semidet]udp_broadcast_initialize(+IPAddress, +Options)

Initialized UDP broadcast bridge. IPAddress is the IP address on the network we want to broadcast on. IP addresses are terms ip(A,B,C,D) or an atom or string of the format A.B.C.D. Options processed:

scope(+ScopeName)

Name of the scope. Default is subnet.

subnet_mask(+SubNet)

Subnet to broadcast on. This uses the same syntax as IPAddress. Default classifies the network as class A, B or C depending on the the first octet and applies the default mask.

port(+Port)

Public port to use. Default is 20005.

method(+Method)

Method to send a message to multiple peers. One of

broadcast

Use UDP broadcast messages to the LAN. This is the default

multicast

Use UDP multicast messages. This can be used on WAN networks, provided the intermediate routers understand multicast.

unicast

Send the messages individually to all registered peers.

For compatibility reasons Options may be the subnet mask.

[det]udp_peer_add(+Scope, +Address)

[det]udp_peer_del(+Scope, ?Address)

[nondet]udp_peer(?Scope, ?Address)

Manage and query the set of known peers for a unicast network. Address is either a term IP:Port or a plain IP address. In the latter case the default port registered with the scope is used.

Address

has canonical form ip(A,B,C,D):Port.

[det,multifile]udp_term_string_hook(+Scope, +Term, -String)

[semidet,multifile]udp_term_string_hook(+Scope, -Term, +String)

Hook for serializing the message Term. The default writes %prolog\n, followed by the Prolog term in quoted notation while ignoring operators. This hook may use alternative serialization such as fast_term_serialized/2, use library(ssl) to realise encrypted messages, etc.

Scope	is the scope for which the message is broadcasted. This can be used to use different serialization for different scopes.
Term	encapsulates the term broadcasted by the application as follows: send(ApplTerm) Is sent by broadcast(udp(Scope, ApplTerm)) request(Id, ApplTerm) Is sent by broadcast_request/1, where Id is a unique large (64 bit) integer. reply(Id, ApplTerm) Is sent to reply on a broadcast_request/1 request that has been received. Arguments are the same as above.

throws

The hook may throw udp(invalid_message) to stop processing the message.

[semidet,multifile]udp_unicast_join_hook(+Scope, +From, +Data)

This multifile hook is called if an UDP package is received on the port of the unicast network identified by Scope. From is the origin IP and port and Data is the message data that is deserialized as defined for the scope (see udp_term_string/3).

This hook is intended to initiate a new node joining the network of peers. We could in theory also omit the in-scope test and use a normal broadcast to join. Using a different channal however provides a basic level of security. A possibe implementation is below. The first fragment is a hook added to the server, the second is a predicate added to a client and the last initiates the request in the client. The excanged term (join(X)) can be used to exchange a welcome handshake.

:- multifile udp_broadcast:udp_unicast_join_hook/3.
udp_broadcast:udp_unicast_join_hook(Scope, From, join(welcome)) :-
    udp_peer_add(Scope, From),

join_request(Scope, Address, Reply) :-
    udp_peer_add(Scope, Address),
    broadcast_request(udp(Scope, join(X))).

?- join_request(myscope, "1.2.3.4":10001, Reply).
Reply = welcome.

20 library(prolog_stream): A stream with Prolog callbacks

This library defines a Prolog stream that realises its low-level I/O with callbacks to Prolog. The library was developed to bind normal Prolog I/O to Pengines I/O. This type of I/O redirection is probably the primary use case.

open_prolog_stream(+Module, +Mode, -Stream, +Options)

Create a new stream that implements its I/O by calling predicates in Module. The called predicates are:

Module:stream_write(+Stream, +String)

Called for a Mode = write stream if data is available. String contains the (textual) data that is written to Stream. The callback is called if the buffer of Stream overflows, the user calls flush_output(Stream) or Stream is closed and there is buffered data.

Module:stream_read(+Stream, -Term)

Called for a Mode == read stream to get new data. On success the stream extracts text from the provided Term. Term is typically a string, atom, code or character list. If term is not one of the above, it is handed to writeq/1. To signal end-of-file, unify stream with an empty text, e.g., stream_read(Stream, "").

Module:stream_close(+Stream)

Called when the stream is closed. This predicate must succeed. The callback can be used to cleanup associated resources.

The current implementation only deals with text streams. The stream uses the wchar_t encoding. The buffer size must be a multiple of wchar_t, i.e., a multiple of four for portability. The newline mode of the stream is posix on all platforms, disabling the translation "\n" --> "\r\n".

Options

is currently ignored.

bug

Futher versions might require additional callbacks. As we demand all callbacks to be defined, existing code needs to implement the new callbacks.

NetBSD Crypt license

 * Copyright (c) 1989, 1993
 *      The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Tom Truscott.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.

Index

?

add_stream_to_pool/2

7

alarm/3

alarm/4

alarm_at/3

alarm_at/4

atom_to_memory_file/2

atom_to_term/3

15

call_with_time_limit/2

call_with_time_limit/3

catch/3

18

cgi_get_form/1

9.1

chmod/2

close/1

15

close_stream_pool/0

7

closelog/0

copy_directory/2

copy_file/2

crypt/2

10 10 10

crypto_password_hash/2

10

current_alarm/4

delete_directory_and_contents/1

delete_directory_contents/1

delete_memory_file/3

delete_stream_from_pool/1

directory_file_path/3

directory_member/3

dispatch_stream_pool/1

7

dup/2

ensure_directory/1

exec/1

fork/1

fork_exec/1

format/2

9.1

format/3

15

free_memory_file/1

15

getegid/1

geteuid/1

getgid/1

getgroups/1

gethostname/1

getuid/1

group_data/3

group_info/2

hash_atom/2

12

hmac_sha/4

host_address/3

initgroups/2

insert_memory_file/3

install_alarm/1

install_alarm/2

ip_name/2

iri_normalized/2

iri_normalized/3

is_process/1

kill/2

link_file/3

make_directory_path/1

md5_hash/3

memory_file_line_position/4

memory_file_substring/5

memory_file_to_atom/2

memory_file_to_atom/3

memory_file_to_codes/2

memory_file_to_codes/3

memory_file_to_string/2

memory_file_to_string/3

negotiate_socks_connection/2

new_memory_file/1

on_signal/3

18

open/4

15

open_chars_stream/2

15

open_codes_stream/3

15

open_hash_stream/3

open_memory_file/3

15

open_memory_file/4

open_prolog_stream/4

openlog/3

pipe/2

prctl/1

process_create/3

process_group_kill/1

process_group_kill/2

process_id/1

process_id/2

process_kill/1

process_kill/2

process_release/1

process_set_method/1

process_wait/2

process_wait/3

process_which/2

prolog:debug_print_hook/3

proxy_for_url/3

relative_file_name/3

remove_alarm/1

rewrite_host/3

rlimit/3

seek/4

15

set_stream_position/2

15

set_time_file/3

set_user_and_group/1

set_user_and_group/2

setegid/1

seteuid/1

setgid/1

setgroups/1

setuid/1

sha_hash/3

12

size_memory_file/2

size_memory_file/3

socket_create/2

stream_hash/2

stream_pool_main_loop/0

sub_atom/5

15

sub_string/5

15 15

sysconf/1

syslog/2

syslog/3

tcp_accept/3

tcp_bind/2

tcp_close_socket/1

tcp_connect/2

tcp_connect/3

tcp_connect/4

tcp_fcntl/3

tcp_getopt/2

tcp_host_to_address/2

tcp_listen/2

tcp_open_socket/2

tcp_open_socket/3

tcp_select/3

tcp_setopt/2

tcp_socket/1

term_string/2

15

term_to_atom/2

15

try_proxy/4

udp_broadcast_close/1

udp_broadcast_initialize/2

udp_peer/2

udp_peer_add/2

udp_peer_del/2

udp_receive/4

udp_send/4

udp_socket/1

udp_term_string_hook/3

udp_unicast_join_hook/3

uninstall_alarm/1

unix_domain_socket/1

uri_authority_components/2

uri_authority_data/3

uri_components/2

uri_data/3

uri_data/4

uri_edit/3

uri_encoded/3

uri_file_name/2

uri_iri/2

uri_is_global/1

uri_normalized/2

uri_normalized/3

uri_normalized_iri/2

uri_normalized_iri/3

uri_query_components/2

uri_resolve/3

user_data/3

user_info/2

uuid/1

uuid/2

uuid_property/2

wait/2

wait_for_input/3

7 7

writeq/1

15

detach_IO/0

detach_IO/1