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Each input and output stream is represented by a unique Prolog term, a stream object. In general, this term is of the form
user
Stands for the standard input or output stream, depending on context.
'$stream'(X)
A stream connected to some file. X is an integer.
A stream alias. Aliases can be associated with
streams using the alias(Atom)
option of
open/4
. There are also three predefined aliases:
user_input
An alias initially referring to the UNIX stdin
stream. The alias can be changed with prolog_flag/3
and accessed by the C variable SP_stdin
.
user_output
An alias initially referring to the UNIX stdout
stream. The alias can be changed with prolog_flag/3
and accessed by the C variable SP_stdout
.
user_error
An alias initially referring to the UNIX stderr
stream. The alias can be changed with prolog_flag/3
and accessed by the C variable SP_stderr
.
This stream is used by the Prolog top level and debugger, and
for all unsolicited messages by built-in predicates.
Stream objects are created by the predicate open/[3,4]
ref-iou-sfh-opn and passed as arguments to
those predicates that need them.
Representation for stream objects to be used in C code is different.
Use stream_code/2
to convert from one to the other when
appropriate.
See mpg-ref-stream_code.