stream_position(+Stream, -Current)

True when Current represents the current position of Stream.

stream_position(+Stream, -Current, +New)

Unifies the current position of the read/write pointer for Stream with Current, then sets the position to New.


Stream stream_object
an open stream
Current term
stream position object representing the current position of Stream.
New term
stream position object


A stream position object is represented by a special Prolog term. The only safe way of obtaining such an object is via stream_position/3 or stream_position/2. You should not try to construct, change, or rely on the form of this object. It may vary under different operating systems and/or change in subsequent versions of Quintus Prolog. On some systems, a stream position object currently has the form:

     '$stream_position'(CharCount, LineCount, LinePos, Magic1, Magic2)


Character count, line count and line position determine the position of the pointer in the stream. Such information is found by using character_count/2, line_count/2 and line_position/2.

stream_position/2 may be used on any stream at all: streams that are connected to disk files, streams that are connected to sockets, streams that are connected to the terminal (including the standard streams user_input, user_output, user_error), and even streams defined using QP_make_stream().

stream_position/3 may only be used on streams that are connected to disk files.

Standard term comparison of two stream position objects for the same stream will work as one expects. When SP1 and SP2 refer to positions in the same stream, SP1 @< SP2 if and only if SP1 is before SP2 in the stream.


Stream errors (see ref-iou-sfh-est), plus:

Current, New are not valid stream position objects.


To find the current position of a stream without changing it, one can ask

     | ?- stream_position(Stream, Current).

See Also

seek/4, open/[3,4], character_count/2, line_count/2, line_position/2. ref-iou