12.9.1 Get IO stream handles

There are several ways to get access to an IO Stream handle, basically get them from Prolog, get access to the standard streams and create a new stream. The standard streams are available as Sinput, Soutput and Serror. Note that these are thread specific. Creating a new stream is discussed with Snew(). Below are the functions to obtain a stream handle from a Prolog term, obtain and release ownership.

int PL_get_stream(term_t t, IOSTREAM **s, int flags)

Get a stream handle from the Prolog term t. Returns TRUE on success and FALSE on failure, by default generating an exception. The flags argument is a bitwise disjunction of these flags:

SIO_INPUT

Get an input stream. If t is a stream pair (see stream_pair/3), return the input channel. If t is an output stream the function fails.

SIO_OUTPUT

Get an output stream. See SIO_INPUT for details. If neither SIO_OUTPUT nor SIO_INPUT is given t may not be a pair.

SIO_NOERROR

If the function fails no exception is produced.

The returned stream is owned by the calling thread using PL_acquire_stream().

int PL_get_stream_from_blob(atom_t b, IOSTREAM **s, int flags)

Same as PL_get_stream(), but operates directly on the blob b. This allows for foreign code that wishes long term access to a stream to maintain a handle to the stream as a (registered) atom_t object rather than a IOSTREAM*.

IOSTREAM * PL_acquire_stream(IOSTREAM *s)

Obtain ownership of s and return s. The application must call PL_release_stream() when done. Only one thread can own a stream and this call blocks if some other thread owns the stream. This function may be called multiple times by the same thread (recursive lock). Note that PL_get_stream() also acquires ownership.

int PL_release_stream(IOSTREAM *s)

Give up ownership acquired using PL_acquire_stream() or PL_get_stream(). If the stream is an an error state, return FALSE with an exception. Otherwise return TRUE.

In general, stream functions do not set any Prolog error state; that is done by PL_release_stream(). Once a stream is in an error state, all subsequent functions act as no-ops (returning -1) unless Sclearerr() is called. Sferror() may be used to check whether a stream is in an error condition. This error may be turned into a Prolog exception by calling PL_acquire_stream() followed by PL_release_stream(). In this case, PL_release_stream() will set the Prolog exception and return FALSE.

Below is an example that writes “Hello World” to a stream provided by Prolog. Note that PL_release_stream() raises an exception if the Sfprintf() failed and (thus) left the stream in an error state.

static foreign_t
hello_world(term_t to)
{ IOSTREAM *s;

  if ( PL_get_stream(to, &s, SIO_OUTPUT) )
  { Sfprintf(s, "Hello World!\n");
    return PL_release_stream(s);
  }

  return FALSE;
}

  ... // fragment from install function
  PL_register_foreign("hello world", 1, hello_world, 0);