12.9.5 Foreign stream encoding

IOSTREAM has a field encoding that is managed at initialization from SIO_TEXT. The available encodings are defined as a C enum as below.

typedef enum
{ ENC_UNKNOWN = 0,                      /* invalid/unknown */
  ENC_OCTET,                            /* raw 8 bit input */
  ENC_ASCII,                            /* US-ASCII (0..127) */
  ENC_ISO_LATIN_1,                      /* ISO Latin-1 (0..256) */
  ENC_ANSI,                             /* default (multibyte) codepage */
  ENC_UTF8,
  ENC_UNICODE_BE,                       /* big endian unicode file */
  ENC_UNICODE_LE,                       /* little endian unicode file */
  ENC_WCHAR                             /* wchar_t */
} IOENC;

Binary streams always have the encoding ENC_OCTET. The default encoding of a text stream depends on the Prolog flag encoding. The encoding is used by all functions that perform text I/O on a stream. The encoding can be changed at any moment using Ssetenc() which is available from Prolog using the set_stream/2 encoding(Encoding) property. Functions that explicitly manage the encoding are:

int Ssetenc(IOSTREAM *s, IOENC new_enc, IOENC *old_enc)

Set the encoding for s to new_enc and, if old_enc is not NULL, return the old encoding. This function may fail, returning -1 if the Scontrol_function() of the stream returns -1 on the SIO_SETENCODING request. On succcess it returns 0. If new_enc is ENC_OCTET the stream is switched to binary mode. Otherwise text mode is enabled.

int ScheckBOM(IOSTREAM *s)

This function may be called on a buffered input stream immediately after opening the stream. If the stream starts with a known Byte Order Mark (BOM) the encoding is set accordingly and the flag SIO_BOM is set on the stream. Possibly resulting encodings are ENC_UTF8, ENC_UNICODE_BE and ENC_UNICODE_LE.

int SwriteBOM(IOSTREAM *s)

This function writes a Byte Order Mark (BOM) to s and should be called immediately after opening a stream for writing. If the encoding is one of ENC_UTF8, ENC_UNICODE_BE or ENC_UNICODE_LE it writes the code point \ufeff (a zero-width white space) to the stream in the current encoding and sets the SIO_BOM flag on the stream.

int Scanrepresent(int c, IOSTREAM *s)

Returns 0 if the encoding of s can represent the code point c and -1 otherwise.