## 4.16 Input and output

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.58Actually 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 4.16.3 describes tell/1, see/1 and friends, providing I/O in the spirit of the traditional Edinburgh standard. These predicates are layered on top of the ISO predicates. Both packages are fully integrated; the user may switch freely between them.

### 4.16.1 Predefined stream aliases

Each thread has five stream aliases: `user_input`, `user_output`, `user_error`, `current_input`, and `current_output`. Newly created threads inherit these stream aliases from theyr parent. The `user_input`, `user_output` and `user_error` aliases of the `main` thread are initially bound to the standard operating system I/O streams (stdin, stdout and stderr, normally bound to the POSIX file handles 0, 1 and 2). These aliases may be re-bound, for example if standard I/O refers to a window such as in the swipl-win.exe GUI executable for Windows. They can be re-bound by the user using set_prolog_IO/3 and set_stream/2 by setting the alias of a stream (e.g, `set_stream(S, alias(user_output))`). An example of rebinding can be found in library `library(prolog_server)`, providing a telnet service. The aliases `current_input` and `current_output` define the source and destination for predicates that do not take a stream argument (e.g., read/1, write/1, get_code/1, ... ). Initially, these are bound to the same stream as `user_input` and `user_error`. They are re-bound by see/1, tell/1, set_input/1 and set_output/1. The `current_output` stream is also temporary re-bound by with_output_to/2 or format/3 using e.g., `format(atom(A), ...`. Note that code which explicitly writes to the streams `user_output` and `user_error` will not be redirected by with_output_to/2.

Compatibility

Note that the ISO standard only defines the `user_*` streams. The `current' streams can be accessed using current_input/1 and current_output/1. For example, an ISO compatible implementation of write/1 is

    write(Term) :- current_output(Out), write_term(Out, Term).

while SWI-Prolog additionally allows for

    write(Term) :- write(current_output, Term).

### 4.16.2 ISO Input and Output Streams

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. On the other hand, using the notion of current-I/O simplifies reusability of code without the need to pass arguments around. E.g., see with_output_to/2.

SWI-Prolog streams are, compatible with the ISO standard, either input or output streams. To accommodate portability to other systems, a pair of streams can be packed into a stream-pair. See stream_pair/3 for details.

SWI-Prolog stream handles are unique symbols that have no syntactical representation. They are written as `\bnfmeta{stream}(hex-number)`, which is not valid input for read/1. They are realised using a blob of type `stream` (see blob/2 and section 9.4.7).

[ISO]open(+SrcDest, +Mode, -Stream, +Options)
  ISO compliant predicate to open a stream. SrcDest is either an atom specifying a file, or a term ``pipe(Command)`', 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. 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.59New code should use the `alias(Alias)` option for compatibility with the ISO standard. The Options list can contain the following options:
  type(Type)
    Using type `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. See also the option `encoding`.
  alias(Atom)
    Gives the stream a name. Below is an example. Be careful with this option as stream names are global. See also set_stream/2.

        ?- open(data, read, Fd, [alias(input)]).

                ...,
                read(input, Term),
                ...

  encoding(Encoding)
    Define the encoding used for reading and writing text to this stream. The default encoding for type `text` is derived from the Prolog flag encoding. For `binary` streams the default encoding is `octet`. For details on encoding issues, see section 2.18.1.
  bom(Bool)
    Check for a BOM (Byte Order Marker) or write one. If omitted, the default is `true` for mode `read` and `false` for mode `write`. See also stream_property/2 and especially section 2.18.1.1 for a discussion of this feature.
  eof_action(Action)
    Defines what happens if the end of the input stream is reached. Action `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.
  buffer(Buffering)
    Defines output buffering. The atom `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.
  close_on_abort(Bool)
    If `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, however, it will be flushed. Useful for logfiles and if the stream is associated to a process (using the `pipe/1` construct).
  locale(+Locale)
    Set the locale that is used by notably format/2 for output on this stream. See section 4.22.
  lock(LockingMode)
    Try to obtain a lock on the open file. Default is `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, especially related to NFS (network file system), please study the fcntl() manual page before trusting your locks!

    The `lock` option is a SWI-Prolog extension.

  wait(Bool)
    This option can be combined with the `lock` option. If `false` (default `true`), the open call returns immediately with an exception if the file is locked. The exception has the format `permission_error(lock, source_sink, SrcDest)`.

  The option `reposition` is not supported in SWI-Prolog. All streams connected to a file may be repositioned.

[ISO]open(+SrcDest, +Mode, ?Stream)
  Equivalent to open/4 with an empty option list.
open_null_stream(?Stream)
  Open an output stream that produces no output. All counting functions are enabled on such a stream. It can be used to discard output (like Unix `/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 if 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.

[ISO]close(+Stream)
  Close the specified stream. If Stream is not open, an existence error is raised. However, closing a stream multiple times may crash Prolog. This is particularly true for multithreaded applications.

  If the closed stream is the current input or output stream, the terminal is made the current input or output.

[ISO]close(+Stream, +Options)
  Provides `close(Stream, [force(true)])` as the only option. Called this way, any resource errors (such as write errors while flushing the output buffer) are ignored.
[ISO]stream_property(?Stream, ?StreamProperty)
  ISO compatible predicate for querying the status of open I/O streams. StreamProperty is one of:
  alias(Atom)
    If Atom is bound, test if the stream has the specified alias. Otherwise unify Atom with the first alias of the stream.bugBacktracking does not give other aliases.
  buffer(Buffering)
    SWI-Prolog extension to query the buffering mode of this stream. Buffering is one of `full`, `line` or `false`. See also open/4.
  buffer_size(Integer)
    SWI-Prolog extension to query the size of the I/O buffer associated to a stream in bytes. Fails if the stream is not buffered.
  bom(Bool)
    If present and `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 2.18.1.1 for details.
  close_on_abort(Bool)
    Determine whether or not abort/0 closes the stream. By default streams are closed.
  close_on_exec(Bool)
    Determine whether or not the stream is closed when executing a new process (exec() in Unix, CreateProcess() in Windows). Default is to close streams. This maps to fcntl() `F_SETFD` using the flag `FD_CLOEXEC` on Unix and (negated) `HANDLE_FLAG_INHERIT` on Windows.
  encoding(Encoding)
    Query the encoding used for text. See section 2.18.1 for an overview of wide character and encoding issues in SWI-Prolog.
  end_of_stream(E)
    If Stream is an input stream, unify E with one of the atoms `not`, `at` or `past`. See also at_end_of_stream/[0,1].
  eof_action(A)
    Unify A with one of `eof_code`, `reset` or `error`. See open/4 for details.
  file_name(Atom)
    If Stream is associated to a file, unify Atom to the name of this file.
  file_no(Integer)
    If the stream is associated with a POSIX file descriptor, unify Integer with the descriptor number. SWI-Prolog extension used primarily for integration with foreign code. See also Sfileno() from `SWI-Stream.h`.
  input
    True if Stream has mode `read`.
  locale(Locale)
    True when Locale is the current locale associated with the stream. See section 4.22.
  mode(IOMode)
    Unify IOMode to the mode given to open/4 for opening the stream. Values are: `read`, `write`, `append` and the SWI-Prolog extension `update`.
  newline(NewlineMode)
    One of `posix` or `dos`. If `dos`, text streams will emit `\r\n` for `\n` and discard `\r` from input streams. Default depends on the operating system.
  nlink(-Count)
    Number of hard links to the file. This expresses the number of `names' the file has. Not supported on all operating systems and the value might be bogus. See the documentation of fstat() for your OS and the value `st_nlink`.
  output
    True if Stream has mode `write`, `append` or `update`.
  position(Pos)
    Unify Pos with the current stream position. A stream position is an opaque term whose fields can be extracted using stream_position_data/3. See also set_stream_position/2.
  reposition(Bool)
    Unify Bool with true if the position of the stream can be set (see seek/4). It is assumed the position can be set if the stream has a seek-function and is not based on a POSIX file descriptor that is not associated to a regular file.
  representation_errors(Mode)
    Determines behaviour of character output if the stream cannot represent a character. For example, an ISO Latin-1 stream cannot represent Cyrillic characters. The behaviour is one of `error` (throw an I/O error exception), `prolog` (write `\...\` escape code) or `xml` (write `&#...;` XML character entity). The initial mode is `prolog` for the user streams and `error` for all other streams. See also section 2.18.1 and set_stream/2.
  timeout(-Time)
    Time is the timeout currently associated with the stream. See set_stream/2 with the same option. If no timeout is specified, Time is unified to the atom `infinite`.
  type(Type)
    Unify Type with `text` or `binary`.
  tty(Bool)
    This property is reported with Bool equal to `true` if the stream is associated with a terminal. See also set_stream/2.
current_stream(?Object, ?Mode, ?Stream)
  The predicate current_stream/3 is used to access the status of a stream as well as to generate all open streams. Object is the name of the file opened if the stream refers to an open file, an integer file descriptor if the stream encapsulates an operating system stream, or the atom `[]` if the stream refers to some other object. Mode is one of `read` or `write`.
is_stream(+Term)
  True if Term is a stream name or valid stream handle. This predicate realises a safe test for the existence of a stream alias or handle.
stream_pair(?StreamPair, ?Read, ?Write)
  This predicate can be used in mode (-,+,+) to create a stream-pair from an input stream and an output stream. Stream-pairs can be used by all I/O operations on streams, where the operation selects the appropriate member of the pair. The predicate close/1 closes both streams of the pair. Mode (+,-,-) can be used to get access to the underlying streams.
[ISO]set_stream_position(+Stream, +Pos)
  Set the current position of Stream to Pos. Pos is a term as returned by stream_property/2 using the `position(Pos)` property. See also seek/4.
stream_position_data(?Field, +Pos, -Data)
  Extracts information from the opaque stream position term as returned by stream_property/2 requesting the `position(Pos)` property. Field is one of `line_count`, `line_position`, `char_count` or `byte_count`. See also line_count/2, line_position/2, character_count/2 and byte_count/2.60Introduced in version 5.6.4 after extending the position term with a byte count. Compatible with SICStus Prolog.
seek(+Stream, +Offset, +Method, -NewLocation)
  Reposition the current point of the given Stream. Method is one of `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.

  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 `permission_error` is raised. If applying the offset would result in a file position less than 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 with Quintus Prolog, though the error conditions and signalling is ISO compliant. See also stream_property/2 and set_stream_position/2.

set_stream(+Stream, +Attribute)
  Modify an attribute of an existing stream. Attribute specifies the stream property to set. See also stream_property/2 and open/4.
  alias(AliasName)
    Set the alias of an already created stream. If AliasName is the name of one of the standard streams, this stream is rebound. Thus, `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.
  buffer(Buffering)
    Set the buffering mode of an already created stream. Buffering is one of `full`, `line` or `false`.
  buffer_size(+Size)
    Set the size of the I/O buffer of the underlying stream to Size bytes.
  close_on_abort(Bool)
    Determine whether or not the stream is closed by abort/0. By default, streams are closed.
  close_on_exec(Bool)
    Set the `close_on_exec` property. See stream_property/2.
  encoding(Atom)
    Defines the mapping between bytes and character codes used for the stream. See section 2.18.1 for supported encodings.
  eof_action(Action)
    Set end-of-file handling to one of `eof_code`, `reset` or `error`.
  file_name(FileName)
    Set the filename associated to this stream. This call can be used to set the file for error locations if Stream corresponds to FileName and is not obtained by opening the file directly but, for example, through a network service.
  line_position(LinePos)
    Set the line position attribute of the stream. This feature is intended to correct position management of the stream after sending a terminal escape sequence (e.g., setting ANSI character attributes). Setting this attribute raises a permission error if the stream does not record positions. See line_position/2 and stream_property/2 (property `position`).
  locale(+Locale)
    Change the locale of the stream. See section 4.22.
  newline(NewlineMode)
    Set input or output translation for newlines. See corresponding stream_property/2 for details. In addition to the detected modes, an input stream can be set in mode `detect`. It will be set to `dos` if a `\r` character was removed.
  timeout(Seconds)
    This option can be used to make streams generate an exception if it takes longer than Seconds before any new data arrives at the stream. The value infinite (default) makes the stream block indefinitely. Like wait_for_input/3, this call only applies to streams that support the select() system call. For further information about timeout handling, see wait_for_input/3. The exception is of the form

    > `error(`timeout_error(read, Stream)`, _)`

  type(Type)
    Set the type of the stream to one of `text` or `binary`. See also open/4 and the `encoding` property of streams. Switching to `binary` sets the encoding to `octet`. Switching to `text` sets the encoding to the default text encoding.
  record_position(Bool)
    Do/do not record the line count and line position (see line_count/2 and line_position/2).
  representation_errors(Mode)
    Change the behaviour when writing characters to the stream that cannot be represented by the encoding. See also stream_property/2 and section 2.18.1.
  tty(Bool)
    Modify whether Prolog thinks there is a terminal (i.e. human interaction) connected to this stream. On Unix systems the initial value comes from isatty(). On Windows, the initial user streams are supposed to be associated to a terminal. See also stream_property/2.
set_prolog_IO(+In, +Out, +Error)
  Prepare the given streams for interactive behaviour normally associated to the terminal. In becomes the `user_input` and `current_input` of the calling thread. Out becomes `user_output` and `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 `line` and 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.

### 4.16.3 Edinburgh-style I/O

The package for implicit input and output destinations is (almost) compatible with Edinburgh DEC-10 and C-Prolog. The reading and writing predicates refer to, resp., the current input and output streams. 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 stream's current value can be obtained using telling/1 for output and seeing/1 for input.

Source and destination are either a file, `user`, or a term `pipe(Command)'. The reserved stream name `user` refers to the terminal.61The ISO I/O layer uses `user_input`, `user_output` and `user_error`. 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).` | % Start writing to the terminal.
`?- tell(pipe(lpr)).` | % Start writing to the printer.

Another example of using the `pipe/1` construct is shown below.62As of version 5.3.15, the pipe construct is supported in the MS-Windows version, both for swipl.exe and swipl-win.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([]).

The effect of tell/1 is not undone on backtracking, and since the stream handle is not specified explicitly in further I/O operations when using Edinburgh-style I/O, you may write to unintended streams more easily than when using ISO compliant I/O. For example, the following query writes both "a" and "b" into the file `out' :

    ?- (tell(out), write(a), false ; write(b)), told.

#### Compatibility notes

Unlike Edinburgh Prolog systems, telling/1 and seeing/1 do not return the filename of the current input/output but rather the stream identifier, to ensure the design pattern below works under all circumstances:63Filenames 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 and see/1 first check for `user`, the `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 exists, created using tell/1 or see/1, output (input) is switched to the open stream. Otherwise a file with the specified name is opened.

The behaviour is compatible with Edinburgh Prolog. This is not without problems. Changing directory, non-file streams, and 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 4.16.2.

see(+SrcDest)
  Open SrcDest for reading and make it the current input (see set_input/1). If SrcDest is a stream handle, just make this stream the current input. See the introduction of section 4.16.3 for details.
tell(+SrcDest)
  Open SrcDest for writing and make it the current output (see set_output/1). If SrcDest is a stream handle, just make this stream the current output. See the introduction of section 4.16.3 for details.
append(+File)
  Similar to tell/1, but positions the file pointer at the end of File rather than truncating an existing file. The pipe construct is not accepted by this predicate.
seeing(?SrcDest)
  Same as current_input/1, except that `user` is returned if the current input is the stream `user_input` to improve compatibility with traditional Edinburgh I/O. See the introduction of section 4.16.3 for details.
telling(?SrcDest)
  Same as current_output/1, except that `user` is returned if the current output is the stream `user_output` to improve compatibility with traditional Edinburgh I/O. See the introduction of section 4.16.3 for details.
seen
  Close the current input stream. The new input stream becomes `user_input`.
told
  Close the current output stream. The new output stream becomes `user_output`.

### 4.16.4 Switching between Edinburgh and ISO I/O

The predicates below can be used for switching between the implicit and the explicit stream-based I/O predicates.

[ISO]set_input(+Stream)
  Set the current input stream to become Stream. Thus, `open(file, read, Stream), set_input(Stream)` is equivalent to `see(file)`.
[ISO]set_output(+Stream)
  Set the current output stream to become Stream. See also with_output_to/2.
[ISO]current_input(-Stream)
  Get the current input stream. Useful for getting access to the status predicates associated with streams.
[ISO]current_output(-Stream)
  Get the current output stream.

### 4.16.5 Write onto atoms, code-lists, etc.

with_output_to(+Output, :Goal)
  Run Goal as once/1, while characters written to the current output are sent to Output. The predicate is SWI-Prolog-specific, inspired by various posts to the mailinglist. It provides a flexible replacement for predicates such as sformat/3 , swritef/3, term_to_atom/2, atom_number/2 converting numbers to atoms, etc. The predicate format/3 accepts the same terms as output argument.

  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 multithreaded 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]

  A Stream handle or alias
    Temporarily switch current output to the given stream. Redirection using with_output_to/2 guarantees the original output is restored, also if Goal fails or raises an exception. See also call_cleanup/2.
  atom(-Atom)
    Create an atom from the emitted characters. Please note the remark above.
  string(-String)
    Create a string object as defined in section 4.24.
  codes(-Codes)
    Create a list of character codes from the emitted characters, similar to atom_codes/2.
  codes(-Codes, -Tail)
    Create a list of character codes as a difference list.
  chars(-Chars)
    Create a list of one-character atoms from the emitted characters, similar to atom_chars/2.
  chars(-Chars, -Tail)
    Create a list of one-character atoms as a difference list.