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|Input and output|
SWI-Prolog provides two different packages for input and output. The native I/O system is based on the ISO standard predicates open/3, close/1 and friends.96Actually based on Quintus Prolog, providing this interface before the ISO standard existed. Being more widely portable and equipped with a clearer and more robust specification, new code is encouraged to use these predicates for manipulation of I/O streams.
Section 4.17.3 describes tell/1, see/1 and friends, providing I/O in the spirit of the traditional Edinburgh standard. These predicates are layered on top of the ISO predicates. Both packages are fully integrated; the user may switch freely between them.
Each thread has five stream aliases:
current_output. Newly created threads inherit these stream
aliases from their parent. The
user_error aliases of the
main thread are
initially bound to the standard operating system I/O streams (stdin,
stdout and stderr, normally bound to the POSIX file
handles 0, 1 and 2). These aliases may be re-bound, for
example if standard I/O refers to a window such as in the swipl-win.exe
GUI executable for Windows. They can be re-bound by the user using
by setting the alias of a stream (e.g,
set_stream(S, alias(user_output))). An example of rebinding
can be found in library
library(prolog_server), providing a telnet
service. The aliases
define the source and destination for predicates that do not take a
stream argument (e.g., read/1, write/1, get_code/1,
... ). Initially, these are bound to the same stream as
user_error. They are re-bound by see/1, tell/1, set_input/1
current_output stream is also temporary re-bound by with_output_to/2
format(atom(A), .... Note that code which explicitly writes
to the streams
not be redirected by with_output_to/2.
Note that the ISO standard only defines the
streams. The‘current' streams can be accessed using current_input/1
For example, an ISO compatible implementation of
write(Term) :- current_output(Out), write_term(Out, Term).
while SWI-Prolog additionally allows for
write(Term) :- write(current_output, Term).
The predicates described in this section provide ISO compliant I/O, where streams are explicitly created using the predicate open/3. The resulting stream identifier is then passed as a parameter to the reading and writing predicates to specify the source or destination of the data.
This schema is not vulnerable to filename and stream ambiguities as well as changes to the working directory. On the other hand, using the notion of current-I/O simplifies reusability of code without the need to pass arguments around. E.g., see with_output_to/2.
SWI-Prolog streams are, compatible with the ISO standard, either input or output streams. To accommodate portability to other systems, a pair of streams can be packed into a stream-pair. See stream_pair/3 for details.
SWI-Prolog stream handles are unique symbols that have no syntactical
representation. They are written as
which is not valid input for read/1.
They are realised using a blob of type
stream (see blob/2
appendopens the file for writing, positioning the file pointer at the end. Mode
updateopens the file for writing, positioning the file pointer at the beginning of the file without truncating the file. Stream is either a variable, in which case it is bound to an integer identifying the stream, or an atom, in which case this atom will be the stream identifier.97New code should use the
alias(Alias)option for compatibility with the ISO standard.
SWI-Prolog also allows SrcDest to be a term
In this form, Command is started as a child process and if
write, output written to Stream
is sent to the standard input of Command. Vice versa, if Mode
read, data written by Command to the standard
output can be read from Stream. On Unix systems, Command
is handed to popen() which hands it to the Unix shell. On Windows, Command
is executed directly and therefore shell syntax such as redirecting
> file) does not work. Use of
pipe(Command) feature is deprecated. The predicate
library(process) provides a richer and more portable
alternative for interacting with processes including handling all three
If SrcDest is an IRI, i.e., starts with
://, where <scheme>
is a non-empty sequence of lowercase ASCII letters open/3,4
calls hooks registered by register_iri_scheme/3.
Currently the only predefined IRI scheme is
access to the resource database. See
The following Options are recognised by open/4:
?- open(data, read, Fd, [alias(input)]). ..., read(input, Term), ...
write. See also stream_property/2 and especially section 184.108.40.206 for a discussion of this feature.
full(default) defines full buffering,
linebuffering by line, and
falseimplies the stream is fully unbuffered. Smaller buffering is useful if another process or the user is waiting for the output as it is being produced. See also flush_output/[0,1]. This option is not an ISO option.
true(default), the stream is closed on an abort (see abort/0). If
false, the stream is not closed. If it is an output stream, however, it will be flushed. Useful for logfiles and if the stream is associated to a process (using the
updatemode. Currently, List is a list of atoms that describe the permissions of the created file.98Added after feedback from Joachim Shimpf and Per Mildner. Defined values are below. Not recognised values are silently ignored, allowing for adding platform specific extensions to this set.
Note that if List is empty, the created file has no
associated access permissions. The create options map to the POSIX mode
option of open(), where
read map to 0444,
to 0222 and
execute to 0111. On POSIX systems, the final
permission is defined as (mode &
textis derived from the Prolog flag encoding. For
binarystreams the default encoding is
octet. For details on encoding issues, see section 2.19.1.
eof_code, which makes get0/1 and friends return -1, and read/1 and friends return the atom
end_of_file. Repetitive reading keeps yielding the same result. Action
eof_code, but repetitive reading will raise an error. With action
reset, Prolog will examine the file again and return more data if the file has grown.
none, which does not lock the file. The value
sharedmeans other processes may read the file, but not write it. The value
exclusivemeans no other process may read or write the file.
Locks are acquired through the POSIX function fcntl() using the
F_SETLKW, which makes a blocked call wait for the lock to
be released. Please note that fcntl() locks are advisory and
therefore only other applications using the same advisory locks honour
your lock. As there are many issues around locking in Unix, especially
related to NFS (network file system), please study the fcntl() manual
page before trusting your locks!
lock option is a SWI-Prolog extension.
detect. This option is ignored for binary streams. Using
detecton an output stream raises an exception. See also set_stream/2.
true), drop the position tracking logic from the stream. This disables the use of stream_position/3 on this stream.
text(default), Prolog will write a text file in an operating system compatible way. Using type
binarythe bytes will be read or written without any translation. See also the option
true), the open call returns immediately with an exception if the file is locked. The exception has the format
permission_error(lock, source_sink, SrcDest).
/dev/null) or exploit the counting properties. The initial encoding of Stream is
utf8, enabling arbitrary Unicode output. The encoding can be changed to determine byte counts of the output in a particular encoding or validate if output is possible in a particular encoding. For example, the code below determines the number of characters emitted when writing Term.
write_length(Term, Len) :- open_null_stream(Out), write(Out, Term), character_count(Out, Len0), close(Out), Len = Len0.
If the closed stream is the current input, output or error stream, the stream alias is bound to the initial standard I/O streams of the process. Calling close/1 on the initial standard I/O streams of the process is a no-op for an input stream and flushes an output stream without closing it.99This behaviour was defined with purely interactive usage of Prolog in mind. Applications should not count on this behaviour. Future versions may allow for closing the initial standard I/O streams.
close(Stream, [force(true)])as the only option. Called this way, any resource errors (such as write errors while flushing the output buffer) are ignored.
false. See also open/4.
true, a BOM (Byte Order Mark) was detected while opening the file for reading, or a BOM was written while opening the stream. See section 220.127.116.11 for details.
F_SETFDusing the flag
FD_CLOEXECon Unix and (negated)
past. See also at_end_of_stream/[0,1].
error. See open/4 for details.
true, the stream is in an error state. Applies to both input and output streams.
appendand the SWI-Prolog extension
dos, text streams will emit
\rfrom input streams. Default depends on the operating system.
error(throw an I/O error exception),
\UXXXXXXXXescape sequences) or
&#...;XML character entity). The initial mode is
unicodefor the user streams and
errorfor all other streams. See also section 2.19.1 and set_stream/2.
trueif the stream is associated with a terminal. See also set_stream/2.
ignore. The latter is intended to deal with service processes for which the standard output handles are not connected to valid streams. In these cases write errors may be ignored on
if the stream refers to some other object. Mode is one of
This predicate is deprecated. New code should use the ISO predicate stream_property/2.
Stream-pairs can be used by all I/O operations on streams, where the operation selects the appropriate member of the pair. The predicate close/1 closes the still open streams of the pair.100As of version 7.1.19, it is allowed to close one of the members of the stream directly and close the pair later. The output stream is closed before the input stream. If closing the output stream results in an error, the input stream is still closed. Success is only returned if both streams were closed successfully.
position(Pos)property. See also seek/4.
position(Pos)property. Field is one of
byte_count. See also line_count/2, line_position/2, character_count/2 and byte_count/2.101Introduced in version 5.6.4 after extending the position term with a byte count. Compatible with SICStus Prolog.
eof, indicating positioning relative to the start, current point or end of the underlying object. NewLocation is unified with the new offset, relative to the start of the stream.
Positions are counted in‘units'. A unit is 1 byte, except for text files using 2-byte Unicode encoding (2 bytes) or wchar encoding (sizeof(wchar_t)). The latter guarantees comfortable interaction with wide-character text objects. Otherwise, the use of seek/4 on non-binary files (see open/4) is of limited use, especially when using multi-byte text encodings (e.g. UTF-8) or multi-byte newline files (e.g. DOS/Windows). On text files, SWI-Prolog offers reliable backup to an old position using stream_property/2 and set_stream_position/2. Skipping N character codes is achieved calling get_code/2 N times or using copy_stream_data/3, directing the output to a null stream (see open_null_stream/1). If the seek modifies the current location, the line number and character position in the line are set to 0.
If the stream cannot be repositioned, a
is raised. If applying the offset would result in a file position less
than zero, a
domain_error is raised. Behaviour when seeking
to positions beyond the size of the underlying object depend on the
object and possibly the operating system. The predicate seek/4
is compatible with Quintus Prolog, though the error conditions and
signalling is ISO compliant. See also stream_property/2
eof_actiononly applies to the read stream,
representation_errorsonly applies to the write stream and trying to set
line_positionon a pair results in a
permission_errorexception. See also stream_property/2 and open/4.
set_stream(S, current_input)is the same as set_input/1, and by setting the alias of a stream to
user_input, etc., all user terminal input is read from this stream. See also interactor/0.
close_on_execproperty. See stream_property/2.
bomcauses the stream to check whether the current character is a Unicode BOM marker. If a BOM marker is found, the encoding is set accordingly and the call succeeds. Otherwise the call fails.
detect. It will be set to
\rcharacter was removed.
timeout_error(read, Stream), _)
binary. See also open/4 and the
encodingproperty of streams. Switching to
binarysets the encoding to
octet. Switching to
textsets the encoding to the default text encoding.
set_stream(S, record_position(true))resets the position the start of line 1.
current_inputof the calling thread. Out becomes
current_output. If Error equals Out an unbuffered stream is associated to the same destination and linked to
user_error. Otherwise Error is used for
user_error. Output buffering for Out is set to
lineand buffering on Error is disabled. See also prolog/0 and set_stream/2. The clib package provides the library
library(prolog_server), creating a TCP/IP server for creating an interactive session to Prolog.
stdin. Out becomes
stdout. If Error equals Out an unbuffered stream is associated to the same destination and linked to
stderr. Otherwise Error is used for
stderr. Output buffering for Out is set to line and buffering on Error is disabled. The operating system I/O streams are shared across all threads. The three streams must be related to a file descriptor or a
file_streamis raised. See also stream_property/2, property
rebinds the Prolog streams
user_error for a specific
thread providing a private interactive session, set_system_IO/3
rebinds the shared console I/O and also captures Prolog kernel events
(e.g., low-level debug messages, unexpected events) as well as messages
from foreign libraries that are directly written to
This predicate is intended to capture all output in situations where standard I/O is normally lost, such as when Prolog is running as a service on Windows.
The package for implicit input and output destinations is (almost) compatible with Edinburgh DEC-10 and C-Prolog. The reading and writing predicates refer to, resp., the current input and output streams. Initially these streams are connected to the terminal. The current output stream is changed using tell/1 or append/1. The current input stream is changed using see/1. The stream's current value can be obtained using telling/1 for output and seeing/1 for input.
Source and destination are either a file,
user, or a
term‘pipe(Command)'. The reserved stream name
refers to the terminal.102The ISO
I/O layer uses
In the predicate descriptions below we will call the source/destination
argument‘SrcDest’. Below are some examples of
|% Start reading from file‘data'.|
|% Start writing to the terminal.|
|% Start writing to the printer.|
Another example of using the
pipe/1 construct is shown
below.103As of version 5.3.15, the
pipe construct is supported in the MS-Windows version, both for swipl.exe
and swipl-win.exe. The implementation uses code from the LUA
programming language (http://www.lua.org).
Note that the
pipe/1 construct is not part of Prolog's
standard I/O repertoire.
getwd(Wd) :- seeing(Old), see(pipe(pwd)), collect_wd(String), seen, see(Old), atom_codes(Wd, String). collect_wd([C|R]) :- get0(C), C \== -1, !, collect_wd(R). collect_wd().
The effect of tell/1 is not undone on backtracking, and since the stream handle is not specified explicitly in further I/O operations when using Edinburgh-style I/O, you may write to unintended streams more easily than when using ISO compliant I/O. For example, the following query writes both "a" and "b" into the file‘out' :
?- (tell(out), write(a), false ; write(b)), told.
Unlike Edinburgh Prolog systems, telling/1 and seeing/1 do not return the filename of the current input/output but rather the stream identifier, to ensure the design pattern below works under all circumstances:104Filenames can be ambiguous and SWI-Prolog streams can refer to much more than just files.
..., telling(Old), tell(x), ..., told, tell(Old), ...,
The predicates tell/1
first check for
pipe(command) and a stream handle. Otherwise, if the
argument is an atom it is first compared to open streams associated to a
file with exactly the same name. If such a stream exists,
output (input) is switched to the open stream. Otherwise a file with the
specified name is opened.
The behaviour is compatible with Edinburgh Prolog. This is not without problems. Changing directory, non-file streams, and multiple names referring to the same file easily lead to unexpected behaviour. New code, especially when managing multiple I/O channels, should consider using the ISO I/O predicates defined in section 4.17.2.
useris returned if the current input is the stream
user_inputto improve compatibility with traditional Edinburgh I/O. See the introduction of section 4.17.3 for details.
useris returned if the current output is the stream
user_outputto improve compatibility with traditional Edinburgh I/O. See the introduction of section 4.17.3 for details.
The predicates below can be used for switching between the implicit and the explicit stream-based I/O predicates.
open(file, read, Stream), set_input(Stream)is equivalent to
The file handling predicates may be hooked to deal with
IRIs. An IRI starts with <scheme>
where <scheme> is a non-empty sequence of lowercase
ASCII letters. After detecting the scheme the file manipulation
predicates call a hook that is registered using register_iri_scheme/3.
Hooking the file operations using extensible IRI schemas allows us to place any resource that is accessed through Prolog I/O predicates on arbitrary devices such as web servers or the ZIP archive used to store program resources (see section 14.2). This is typically combined with file_search_path/2 declarations to switch between accessing a set of resources from local files, from the program resource database, from a web-server, etc.
file) and exists_directory/1 (Mode is
directory). The result argument must be unified with a boolean.
For capturing other streams, see with_output_to/3.
Applications should generally avoid creating atoms by breaking and concatenating other atoms, as the creation of large numbers of intermediate atoms generally leads to poor performance, even more so in multithreaded applications. This predicate supports creating difference lists from character data efficiently. The example below defines the DCG rule term//1 to insert a term in the output:
term(Term, In, Tail) :- with_output_to(codes(In, Tail), write(Term)). ?- phrase(term(hello), X). X = [104, 101, 108, 108, 111]
Output takes one of the shapes below. Except for the
first, the system creates a temporary stream using the
internal encoding that points at a memory buffer. The encoding cannot be
changed and an attempt to call set_stream/2
encoding(Encoding) results in a
The predicates in this section provide fast binary I/O of arbitrary
Prolog terms, including cyclic terms and terms holding attributed
library(fastrw) is a SICSTus/Ciao
compatible library that extends the core primitives described below.