SWI-Prolog provides two different packages for input and output. The native I/O system is based on the ISO standard predicates open/3, close/1 and friends.37Actually based on Quintus Prolog, providing this interface before the ISO standard existed. Being more widely portable and equipped with a clearer and more robust specification, new code is encouraged to use these predicates for manipulation of I/O streams.
Section 4.16.2 describes tell/1, see/1 and friends, providing I/O in the spirit of the outdated Edinburgh standard. These predicates are layered on top of the ISO predicates. Both packages are fully integrated; the user may switch freely between them.
The predicates described in this section provide ISO compliant I/O, where 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.
This schema is not vulnerable to filename and stream ambiguities as well as changes to the working directory. New code is advised to use these predicates to manage input and output streams.
pipe(Command)', like see/1 and tell/1. Mode is one of
appendopens the file for writing, positioning the file-pointer at the end. Mode
updateopens the file for writing, positioning the file-pointer at the beginning of the file without truncating the file. 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.38New code should use the
alias(Alias)option for compatibility to the ISO standard The Options list can contain the following options:
text(default), Prolog will write a text-file in an operating-system compatible way. Using type
binarythe bytes will be read or written without any translation. See also the option
?- open(data, read, Fd, [alias(input)]). ..., read(input, Term), ...
textis derived from the Prolog flag encoding. For
binarystreams the default encoding is
octet. For details on encoding issues, see section 2.17.1.
write. See also stream_property/2 and especially section 22.214.171.124 for a discussion on this feature.
eof_codemakes 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
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,
linebuffering by line, and
falseimplies 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
sharedmeans other processes may read the file, but not write it. The value
exclusivemeans no other process may read or write the file.
Locks are acquired through the POSIX function fcntl() using the
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, especially
related to NFS (network file system), please study the fcntl() manual
page before trusting your locks!
lock option is a SWI-Prolog extension.
reposition is not supported in SWI-Prolog.
All streams connected to a file may be repositioned.
/dev/null) or exploit the counting properties. The initial encoding of Stream is
utf8, enabling arbitrary Unicode output. The encoding can be changed to determine byte-counts of the output in a particular encoding or validate output is possible in a particular encoding. For example, the code below determines the number of characters emitted when writing Term.
write_length(Term, Len) :- open_null_stream(Out), write(Out, Term), character_count(Out, Len0), close(Out), Len = Len0.
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.
false. See also open/4.
true, a BOM (Byte Order Mark) was detected while opening the file for reading or a BOM was written while opening the stream. See section 126.96.36.199 for details.
past. See also at_end_of_stream/[0,1].
error. See open/4 for details.
appendand the SWI-Prolog extension
dos, text-streams will emit
\rfrom input streams. Default depends on the operating system.
error(throw and I/O error exception),
\...\escape code or
&#...;XML character entity). The initial mode is
prologfor the user streams and
errorfor all other streams. See also section 2.17.1 and set_stream/2.
trueif the stream is associated with a terminal. See also set_stream/2.
if the stream refers to some other object. Mode is one of
position(Pos)property. See also seek/4.
position(Position)property. Field is one of
byte_count. See also line_count/2, line_position/2, character_count/2 and byte_count/2.39Introduced in version 5.6.4 after extending the position term with a byte-count. Compatible with SICStus Prolog.
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.
Positions are counted in `units'. A unit is 1 byte, except for text-files using 2-byte Unicode encoding (2 bytes) or wchar encoding (sizeof(wchar_t)). The latter guarantees comfortable interaction with wide-character text-objects. Otherwise, the use of seek/4 on non-binary files (see open/4) is of limited use, especially when using multi-byte text-encodings (e.g. UTF-8) or multi-byte newline files (e.g. DOS/Windows). On text-files, SWI-Prolog offers reliable backup to an old position using stream_property/2 and set_stream_position/2. Skipping N character codes is achieved calling get_code/2 N times or using copy_stream_data/3, directing the output to a null-stream (see open_null_stream/1). 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 repositioned, a
is raised. If applying the offset would result in a file-position less
then zero, a
domain_error is raised. Behaviour when seeking
to positions beyond the size of the underlying object depend on the
object and possibly the operating system. The predicate seek/4
is compatible to Quintus Prolog, though the error conditions and
signalling is ISO compliant. See also stream_property/2
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.
detect. It will be set to
\rcharacter was removed.
timeout_error(read, Stream), _)
current_inputof the calling thread. Out becomes
current_output. If Error equals Out an unbuffered stream is associated to the same destination and linked to
user_error. Otherwise Error is used for
user_error. Output buffering for Out is set to
lineand buffering on Error is disabled. See also prolog/0 and set_stream/2. The clib package provides the library
library(prolog_server)creating a TCP/IP server for creating an interactive session to Prolog.
The package for implicit input and output destination is (almost) compatible to Edinburgh 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.
Source and destination are either a file,
user, or a
term `pipe(Command)'. The reserved stream name
refers to the terminal.40The ISO
I/O layer uses
In the predicate descriptions below we will call the source/destination
argument `SrcDest'. Below are some examples of
|% Start reading from file `data'.|
|% Start writing to the terminal.|
|% Start writing to the printer.|
Another example of using the pipe/1 construct is shown below.41As of version 5.3.15, the pipe construct is supported in the MS-Windows version, both for plcon.exe and plwin.exe. The implementation uses code from the LUA programming language (http://www.lua.org). 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().
Unlike Edinburgh Prolog systems, telling/1 and seeing/1 do not return the filename of the current input/output, but the stream-identifier, to ensure the design pattern below works under all circumstances.42Filenames can be ambiguous and SWI-Prolog streams can refer to much more than just files.
..., telling(Old), tell(x), ..., told, tell(Old), ...,
The predicates tell/1
first check for
pipe(command) and a stream-handle. Otherwise, if the
argument is an atom it is first compared to open streams associated to a
file with exactly the same name. If such a stream, created
exists, output (input) is switch to the open stream. Otherwise a file
with the specified name is opened.
The behaviour is compatible to Edinburgh Prolog. This is not without problems. Changing directory, non-file streams, multiple names referring to the same file easily lead to unexpected behaviour. New code, especially when managing multiple I/O channels should consider using the ISO I/O predicates defined in section 4.16.1.
useris returned if the current input is the stream
user_inputto improve compatibility with traditional Edinburgh I/O. See the introduction of section 4.16.2 for details.
useris returned if the current output is the stream
user_outputto improve compatibility with traditional Edinburgh I/O. See the introduction of section 4.16.2 for details.
The predicates below can be used for switching between the implicit- and the explicit stream based I/O predicates.
Applications should generally avoid creating atoms by breaking and concatenating other atoms as the creation of large numbers of intermediate atoms generally leads to poor performance, even more so in multi-threaded applications. This predicate supports creating difference-lists from character data efficiently. The example below defines the DCG rule term/3 to insert a term in the output:
term(Term, In, Tail) :- with_output_to(codes(In, Tail), write(Term)). ?- phrase(term(hello), X). X = [104, 101, 108, 108, 111]