SWI-Prolog provides two different packages for input and output. One confirms to the Edinburgh standard. This package has a notion of `current-input' and `current-output'. The reading and writing predicates implicitly refer to these streams. In the second package, streams are opened explicitly and the resulting handle is used as an argument to the reading and writing predicate to specify the source or destination. Both packages are fully integrated; the user may switch freely between them.
The package for implicit input and output destination is upwards
compatible to DEC-10 and C-Prolog. The reading and writing predicates
refer to resp. the current input- and output stream. 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 streams current value can be obtained using telling/1
for output- and seeing/1
for input streams. The table below shows the valid stream
specifications. The reserved names user_input,
user_output and user_error are for neat
integration with the explicit streams.
user | This reserved name refers to the terminal |
user_input | Input from the terminal |
user_output | Output to the terminal |
user_error | Unix error stream (output only) |
| <Atom> | Name of a Unix file |
pipe(<Atom>) | Name of a Unix command |
Source and destination are either a file, one of the reserved words above, or a term `pipe(Command)'. In the predicate descriptions below we will call the source/destination argument `SrcDest'. Below are some examples of source/destination specifications.
?- see(data). | % Start reading from file `data'. |
?- tell(user_error). | % Start writing on the error stream. |
?- tell(pipe(lpr)). | % Start writing to the printer. |
Another example of using the pipe/1 construct is shown below. 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 predicates below are part of the Quintus compatible stream-based I/O package. In this package 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.
pipe(Command)',
just like see/1
and tell/1. Mode
is one of read,
write, append or update. Mode append
opens the file for writing, positioning the file-pointer at the end.
Mode update opens the file for writing, positioning the
file-pointer at the beginning of the file without truncating the file.
See also stream_position/3. 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. The Options list can contain the following
options:
text (default), Prolog will write a text-file in
an operating-system compatible way. Using type binary the
bytes will be read or written without any translation. Note there is no
difference between the two on Unix systems.
?- open(data, read, Fd, [alias(input)]).
...,
read(input, Term),
...
|
eof_code 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 error is like 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.
full (default) defines
full buffering, line buffering by line, and false
implies 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, it will be flushed however. Useful for logfiles and if the
stream is associated to a process (using the pipe/1 construct).
none,
which does not lock the file. The value read or shared
means other processes may read the file, but not write it. The value
write or exclusive means no other process may
read or write the file.
Locks are acquired through the POSIX function fcntl() using the
command
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, expecially
related to NFS (network file system), please study the fcntl() manual
page before trusting your locks!
The lock option is a SWI-Prolog extension.
The option reposition is not supported in SWI-Prolog.
All streams connected to a file may be repositioned.
/dev/null.
Stream can be an atom, giving the null-stream an alias name.
close(Stream, [force(true)]) as the only option.
Called this way, any resource error (such as write-errors while flushing
the output buffer) are ignored.
read, write, append
and the SWI-Prolog extension update.
read.
write, append
or
update.
$stream_position(CharIndex,
LineNo, LinePos). See also term_position/3.
not, at or past. See
also
at_end_of_stream/[0,1].
eof_code, reset
or
error. See open/4
for details.
text or binary.
SWI-Stream.h.
full, line or false.
See also open/4.
read or write.
position(Pos) property. See also seek/4.
bof, current or 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.
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 repostioned, a reposition error
is raised. The predicate seek/4
is compatible to Quintus Prolog, though the error conditions and
signalling is ISO compliant. See also
stream_position/3.
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.
full, line
or false.
eof_code, reset
or
error.
The predicates below can be used for switching between the implicit- and the explicit stream based I/O predicates.