Streams

### 8.10 Streams

- Introduction
- `current_input/1`
- `current_output/1`
- `set_input/1`
- `set_output/1`
- `open/4`, `open/3`
- `close/2`, `close/1`
- `flush_output/1`, `flush_output/0`
- `current_stream/1`
- `stream_property/2`
- `at_end_of_stream/1`, `at_end_of_stream/0`
- `stream_position/2`
- `set_stream_position/2`
- `seek/4`
- `character_count/2`
- `line_count/2`
- `line_position/2`
- `stream_line_column/3`
- `set_stream_line_column/3`
- `add_stream_alias/2`
- `current_alias/2`
- `add_stream_mirror/2`
- `remove_stream_mirror/2`
- `current_mirror/2`
- `set_stream_type/2`
- `set_stream_eof_action/2`
- `set_stream_buffering/2`

#### 8.10.1 Introduction

A stream provides a logical view of a source/sink.

Sources and sinks: a program can output results to a sink or input data from a source. A source/sink may be a file (regular file, terminal, device,…), a constant term, a pipe, a socket,…

Associating a stream to a source/sink: to manipulate a source/sink it must be associated with a stream. This provides a logical and uniform view of the source/sink whatever its type. Once this association has been established, i.e. a stream has been created, all subsequent references to the source/sink are made by referring the stream. A stream is unidirectional: it is either an input stream or an output stream. For a classical file, the association is done by opening the file (whose name is specified as an atom) with the `open/4` (section 8.10.6). GNU Prolog makes it possible to treat a Prolog constant term as a source/sink and provides built-in predicates to associate a stream to such a term (section 8.11). GNU Prolog provides operating system interface predicates defining pipes between GNU Prolog and child processes with streams associated with these pipes, e.g. `popen/3` (section 8.27.21). Similarly, socket interface predicates associate streams to a socket to allow the communication, e.g. `socket_connect/4` (section 8.28.5).

Stream-term: a stream-term identifies a stream during a call of an input/output built-in predicate. It is created as a result of associating a stream to a source/sink (section above). A stream-term is a compound term of the form `’$stream’(I)` where `I` is an integer.

Stream aliases: any stream may be associated with a stream alias which is an atom which may be used to refer to that stream. The association can be done at open time or using `add_stream_alias/2` (section 8.10.20). Such an association automatically ends when the stream is closed. A particular alias only refers to at most one stream at any one time. However, more than one alias can be associated with a stream. Most built-in predicates which have a stream-term as an input argument also accept a stream alias as that argument. However, built-in predicates which return a stream-term do not accept a stream alias.

Standard streams: three streams are predefined and open during the execution of every goal: the standard input stream which has the alias `user_input`, the standard output stream which has the alias `user_output` and the standard error stream which has the alias `user_error`. A goal which attempts to close either standard stream succeeds, but does not close the stream.

Current streams: during execution there is a current input stream and a current output stream. By default, the current input and output streams are the standard input and output streams, but the built-in predicates `set_input/1` (section 8.10.4) and `set_output/1` (section 8.10.5) can be used to change them. When the current input stream is closed, the standard input stream becomes the current input stream. When the current output stream is closed, the standard output stream becomes the current output stream.

Text streams and binary streams: a text stream is a sequence of characters. A text stream is also regarded as a sequence of lines where each line is a possibly empty sequence of characters followed by a new line character. GNU Prolog may add or remove space characters at the ends of lines in order to conform to the conventions for representing text streams in the operating system. A binary stream is a sequence of bytes. Only a few built-in predicates can deal with binary streams, e.g. `get_byte/2` (section 8.13).

Stream positions: the stream position of a stream identifies an absolute position of the source/sink to which the stream is connected and defines where in the source/sink the next input or output will take place. A stream position is a ground term of the form `’$stream_position’(I1, I2, I3, I4)` where `I1`, `I2`, `I3` and `I4` are integers. Stream positions are used to reposition a stream (when possible) using for instance `set_stream_position/2` (section 8.10.13).

The position end of stream: when all data of a stream `S` has been input `S` has a stream position end-of-stream. At this stream position a goal to input more data will return a specific value to indicate that end of stream has been reached (e.g. `-1` for `get_code/2` or `end_of_file` for `get_char/2`,…). When this terminating value has been input, the stream has a stream position past-end-of-stream.

Buffering mode: input/output on a stream can be buffered (line-buffered or block-buffered) or not buffered at all. The buffering mode can be specified at open time or using `set_stream_buffering/2` (section 8.10.27). Line buffering is used on output streams, output data are only written to the sink when a new-line character is output (or at the close time). Block buffering is used on input or output. On input streams, when an input is requested on the source, if the buffer is empty, all available characters are read (within the limits of the size of the buffer), subsequent reads will first use the characters in the buffer. On output streams, output data are stored in the buffer and only when the buffer is full is it physically written on the sink. Thus, an output to a buffered stream may not be sent immediately to the sink connected to that stream. When it is necessary to be certain that output has been delivered, the built-in predicate `flush_output/1` (section 8.10.8) should be used. Finally, it is also possible to use non-buffered streams, in that case input/output are directly done on the connected source/sink. This can be useful for communication purposes (e.g. sockets) or when a precise control is needed, e.g. `select/5` (section 8.27.25).

Stream mirrors: any stream may be associated with mirror streams specified at open time or using `add_stream_mirror/2` (section 8.10.22). Then, all characters/bytes read from/written to the stream are also written on each mirror stream. The association automatically ends when either the stream or the mirror stream is closed. It is also possible to explicitly remove a mirror stream using `remove_stream_mirror/2` (section 8.10.23).

#### 8.10.2 `current_input/1`

Templates

  `current_input(?stream)`

Description

`current_input(Stream)` unifies `Stream` with the stream-term identifying the current input stream.

Errors

`Stream` is neither a variable nor a stream |  | `domain_error(stream, Stream)`

Portability

ISO predicate.

#### 8.10.3 `current_output/1`

Templates

  `current_output(?stream)`

Description

`current_output(Stream)` unifies `Stream` with the stream-term identifying the current output stream.

Errors

`Stream` is neither a variable nor a stream |  | `domain_error(stream, Stream)`

Portability

ISO predicate.

#### 8.10.4 `set_input/1`

Templates

  `set_input(+stream_or_alias)`

Description

`set_input(SorA)` sets the current input stream to be the stream associated with the stream-term or alias `SorA`.

Errors

`SorA` is a variable |  | `instantiation_error`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`
`SorA` is an output stream |  | `permission_error(input, stream, SorA)`

Portability

ISO predicate.

#### 8.10.5 `set_output/1`

Templates

  `set_output(+stream_or_alias)`

Description

`set_output(SorA)` sets the current output stream to be the stream associated with the stream-term or alias `SorA`.

Errors

`SorA` is a variable |  | `instantiation_error`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`
`SorA` is an input stream |  | `permission_error(output, stream, SorA)`

Portability

ISO predicate.

#### 8.10.6 `open/4`, `open/3`

Templates

  `open(+source_sink, +io_mode, -stream, +stream_option_list)
  open(+source_sink, +io_mode, -stream)`

Description

`open(SourceSink, Mode, Stream, Options)` opens the source/sink `SourceSink` for input or output as indicated by `Mode` and the list of stream-options `Options` and unifies `Stream` with the stream-term which is associated with this stream. See `absolute_file_name/2` for information about the syntax of `SourceSink` (section 8.26.1).

Input/output modes: `Mode` is an atom which defines the input/output operations that may be performed the stream. Possible modes are:

- `read`: the source/sink is a source and must already exist. Input starts at the beginning of the source.
- `write`: the source/sink is a sink. If the sink already exists then it is emptied else an empty sink is created. Output starts at the beginning of that sink.
- `append`: the source/sink is a sink. If the sink does not exist it is created. Output starts at the end of that sink.

Stream options: `Options` is a list of stream options. If this list contains contradictory options, the rightmost option is the one which applies. Possible options are:

- `type(text`/`binary)`: specifies whether the stream is a text stream or a binary stream. The default value is `text`.
- `reposition(true`/`false)`: specifies whether it is possible to reposition the stream. The default value is `true` except if the stream cannot be repositioned (e.g. a terminal).
- `eof_action(error`/`eof_code`/`reset)`: specifies the effect of attempting to input from a stream whose stream position is past-end-of-stream:
  - `error`: a `permission_error` is raised signifying that no more input exists in this stream.
  - `eof_code`: the result of input is as if the stream position is end-of-stream.
  - `reset`: the stream position is reset so that it is not past-end-of-stream, and another attempt is made to input from it (this is useful when inputting from a terminal).

  The default value is `eof_code`.

- `alias(Alias)`: specifies that the atom `Alias` is to be an alias for the stream. By default no alias is attached to the stream. Several aliases can be defined for a same stream.
- `mirror(Mirror)`: specifies the stream associated with the stream-term or alias `Mirror` is a mirror for the stream. By default no mirror is attached to the stream. Several mirrors can be defined for a same stream.
- `buffering(none`/`line`/`block)`: specifies which type of buffering is used by input/output operations on this stream:
  - `none`: no buffering.
  - `line`: output operations buffer data emitted until a new-line occurs
  - `block`: input/output operations buffer data until a given number (implementation dependant) of characters/bytes have been treated.

  The default value is `line` for a terminal (TTY), `block` otherwise.

`open(SourceSink, Mode, Stream)` is equivalent to `open(SourceSink, Mode, Stream, [])`.

Errors

`SourceSink` is a variable |  | `instantiation_error`
`Mode` is a variable |  | `instantiation_error`
`Options` is a partial list or a list with an element `E` which is a variable |  | `instantiation_error`
`Mode` is neither a variable nor an atom |  | `type_error(atom, Mode)`
`Options` is neither a partial list nor a list |  | `type_error(list, Options)`
`Stream` is not a variable |  | `uninstantiation_error(Stream)`
`SourceSink` is neither a variable nor a source/sink |  | `domain_error(source_sink, SourceSink)`
`Mode` is an atom but not an input/output mode |  | `domain_error(io_mode, Mode)`
an element `E` of the `Options` list is neither a variable nor a stream-option |  | `domain_error(stream_option, E)`
the source/sink specified by `SourceSink` does not exist |  | `existence_error(source_sink, SourceSink)`
the source/sink specified by `SourceSink` cannot be opened |  | `permission_error(open, source_sink, SourceSink)`
an element `E` of the `Options` list is `alias(A)` and `A` is already associated with an open stream |  | `permission_error(open, source_sink, alias(A))`
an element `E` of the `Options` list is `mirror(M)` and `M` is not associated with an open stream |  | `existence_error(stream, M)`
an element `E` of the `Options` list is `mirror(M)` and `M` is an input stream |  | `permission_error(output, stream, M)`
an element `E` of the `Options` list is `reposition(true)` and it is not possible to reposition this stream |  | `permission_error(open, source_sink, reposition(true))`

Portability

ISO predicates. The `mirror` and `buffering` stream options are GNU Prolog extensions.

#### 8.10.7 `close/2`, `close/1`

Templates

  `close(+stream_or_alias, +close_option_list)
  close(+stream_or_alias)`

Description

`close(SorA, Options)` closes the stream associated with the stream-term or alias `SorA`. If `SorA` is the standard input stream or the standard output stream `close/2` simply succeeds else the associated source/sink is physically closed. If `SorA` is the current input stream the current input stream becomes the standard input stream `user_input`. If `SorA` is the current output stream the current output stream becomes the standard output stream `user_output`.

Close options: `Options` is a list of close options. For the moment only one option is available:

- `force(true`/`false)`: with `false`, if an error occurs when trying to close the source/sink, the stream is not closed and an error (`system_error` or `resource_error`) is raised (but `close/2` succeeds). With `true`, if an error occurs it is ignored and the stream is closed. The purpose of `force/1` option is to allow an error handling routine to do its best to reclaim resources. The default value is `false`.

`close(SorA)` is equivalent to `close(SorA, [])`.

Errors

`SorA` is a variable |  | `instantiation_error`
`Options` is a partial list or a list with an element `E` which is a variable |  | `instantiation_error`
`Options` is neither a partial list nor a list |  | `type_error(list, Options)`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
an element `E` of the `Options` list is neither a variable nor a close-option |  | `domain_error(close_option, E)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`
`SorA` needs a special close (section 8.11) |  | `system_error(needs_special_close)`

Portability

ISO predicates. The `system_error(needs_special_close)` is a GNU Prolog extension.

#### 8.10.8 `flush_output/1`, `flush_output/0`

Templates

  `flush_output(+stream_or_alias)
  flush_output`

Description

`flush_output(SorA)` sends any buffered output characters/bytes to the stream.

`flush_output/0` applies to the current output stream.

Errors

`SorA` is a variable |  | `instantiation_error`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`
`SorA` is an input stream |  | `permission_error(output, stream, SorA)`

Portability

ISO predicates.

#### 8.10.9 `current_stream/1`

Templates

  `current_stream(?stream)`

Description

`current_stream(Stream)` succeeds if there exists a stream-term that unifies with `Stream`. This predicate is re-executable on backtracking.

Errors

`Stream` is neither a variable nor a stream-term |  | `domain_error(stream, Stream)`

Portability

GNU Prolog predicate.

#### 8.10.10 `stream_property/2`

Templates

  `stream_property(?stream, ?stream_property)`

Description

`stream_property(Stream, Property)` succeeds if `current_stream(Stream)` succeeds (section 8.10.9) and if `Property` unifies with one of the properties of the stream. This predicate is re-executable on backtracking.

Stream properties:

- `file_name(F)`: the name of the connected source/sink.
- `mode(M)`: `M` is the open mode (`read`, `write`, `append`).
- `input`: if it is an input stream.
- `output`: if it is an output stream.
- `alias(A)`: `A` is an alias of the stream.
- `mirror(M)`: `M` is a mirror stream of the stream.
- `type(T)`: `T` is the type of the stream (`text`, `binary`).
- `reposition(R)`: `R` is the reposition boolean (`true`, `false`).
- `eof_action(A)`: `A` is the end-of-file action (`error`, `eof_code`, `reset`).
- `buffering(B)`: `B` is the buffering mode (`none`, `line`, `block`).
- `end_of_stream(E)`: `E` is the current end-of-stream status (`not`, `at`, `past`). If the stream position is end-of-stream then `E` is unified with `at` else if the stream position is past-end-of-stream then `E` is unified with `past` else `E` is unified with `not`.
- `position(P)`: `P` is the stream-position term associated with the current position.

Errors

`Stream` is a variable |  | `instantiation_error`
`Stream` is neither a variable nor a stream-term |  | `domain_error(stream, Stream)`
`Property` is neither a variable nor a stream property |  | `domain_error(stream_property, Property)`
`Property` = `file_name(E)`,`mode(E)`, `alias(E)`, `end_of_stream(E)`, `eof_action(E)`, `reposition(E)`, `type(E)` or `buffering(E)` and `E` is neither a variable nor an atom |  | `type_error(atom, E)`

Portability

ISO predicate. The `buffering/1` property is a GNU Prolog extension.

#### 8.10.11 `at_end_of_stream/1`, `at_end_of_stream/0`

Templates

  `at_end_of_stream(+stream_or_alias)
  at_end_of_stream`

Description

`at_end_of_stream(SorA)` succeeds if the stream associated with stream-term or alias `SorA` has a stream position end-of-stream or past-end-of-stream. This predicate can be defined using `stream_property/2` (section 8.10.10).

`at_end_of_stream/0` applies to the current input stream.

Errors

`SorA` is a variable |  | `instantiation_error`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`
`SorA` is an output stream |  | `permission_error(input, stream, SorA)`

Portability

ISO predicates. The `permission_error(input, stream, SorA)` is a GNU Prolog extension.

#### 8.10.12 `stream_position/2`

Templates

  `stream_position(+stream_or_alias, ?stream_position)`

Description

`stream_position(SorA, Position)` succeeds unifying `Position` with the stream-position term associated with the current position of the stream-term or alias `SorA`. This predicate can be defined using `stream_property/2` (section 8.10.10).

Errors

`SorA` is a variable |  | `instantiation_error`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`Position` is neither a variable nor a stream-position term |  | `domain_error(stream_position, Position)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`

Portability

GNU Prolog predicate.

#### 8.10.13 `set_stream_position/2`

Templates

  `set_stream_position(+stream_or_alias, +stream_position)`

Description

`set_stream_position(SorA, Position)` sets the position of the stream associated with the stream-term or alias `SorA` to `Position`. `Position` should have previously been returned by `stream_property/2` (section 8.10.10) or by `stream_position/2` (section 8.10.12).

Errors

`SorA` is a variable |  | `instantiation_error`
`Position` is a variable |  | `instantiation_error`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`Position` is neither a variable nor a stream-position term |  | `domain_error(stream_position, Position)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`
`SorA` has stream property `reposition(false)` |  | `permission_error(reposition, stream, SorA)`

Portability

ISO predicate.

#### 8.10.14 `seek/4`

Templates

  `seek(+stream_or_alias, +stream_seek_method, +integer, ?integer)`

Description

`seek(SorA, Whence, Offset, NewOffset)` sets the position of the stream associated with the stream-term or alias `SorA` to `Offset` according to `Whence` and unifies `NewOffset` with the new offset from the beginning of the file. `seek/4` can only be used on binary streams. `Whence` is an atom from:

- `bof`: the position is set relatively to the begin of the file (`Offset` should be ≥ 0).
- `current`: the position is set relatively to the current position (`Offset` can be ≥ 0 or ≤ 0).
- `eof`: the position is set relatively to the end of the file (`Offset` should be ≤ 0).

This predicate is an interface to the C Unix function `lseek(2)`.

Errors

`SorA` is a variable |  | `instantiation_error`
`Whence` is a variable |  | `instantiation_error`
`Offset` is a variable |  | `instantiation_error`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`Whence` is neither a variable nor an atom |  | `type_error(atom, Whence)`
`Whence` is an atom but not a valid stream seek method |  | `domain_error(stream_seek_method, Whence)`
`Offset` is neither a variable nor an integer |  | `type_error(integer, Offset)`
`NewOffset` is neither a variable nor an integer |  | `type_error(integer, NewOffset)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`
`SorA` has stream property `reposition(false)` |  | `permission_error(reposition, stream, SorA)`
`SorA` is associated with a text stream |  | `permission_error(reposition, text_stream, SorA)`

Portability

GNU Prolog predicate.

#### 8.10.15 `character_count/2`

Templates

  `character_count(+stream_or_alias, ?integer)`

Description

`character_count(SorA, Count)` unifies `Count` with the number of characters/bytes read/written on the stream associated with stream-term or alias `SorA`.

Errors

`SorA` is a variable |  | `instantiation_error`
`Count` is neither a variable nor an integer |  | `type_error(integer, Count)`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`

Portability

GNU Prolog predicate.

#### 8.10.16 `line_count/2`

Templates

  `line_count(+stream_or_alias, ?integer)`

Description

`line_count(SorA, Count)` unifies `Count` with the number of lines read/written on the stream associated with the stream-term or alias `SorA`. This predicate can only be used on text streams.

Errors

`SorA` is a variable |  | `instantiation_error`
`Count` is neither a variable nor an integer |  | `type_error(integer, Count)`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`
`SorA` is associated with a binary stream |  | `permission_error(access, binary_stream, SorA)`

Portability

GNU Prolog predicate.

#### 8.10.17 `line_position/2`

Templates

  `line_position(+stream_or_alias, ?integer)`

Description

`line_position(SorA, Count)` unifies `Count` with the number of characters read/written on the current line of the stream associated with the stream-term or alias `SorA`. This predicate can only be used on text streams.

Errors

`SorA` is a variable |  | `instantiation_error`
`Count` is neither a variable nor an integer |  | `type_error(integer, Count)`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`
`SorA` is associated with a binary stream |  | `permission_error(access, binary_stream, SorA)`

Portability

GNU Prolog predicate.

#### 8.10.18 `stream_line_column/3`

Templates

  `stream_line_column(+stream_or_alias, ?integer, ?integer)`

Description

`stream_line_column(SorA, Line, Column)` unifies `Line` (resp. `Column`) with the current line number (resp. column number) of the stream associated with the stream-term or alias `SorA`. This predicate can only be used on text streams. Note that `Line` corresponds to the value returned by `line_count/2` + 1 (section 8.10.16) and `Column` to the value returned by `line_position/2` + 1 (section 8.10.17).

Errors

`SorA` is a variable |  | `instantiation_error`
`Line` is neither a variable nor an integer |  | `type_error(integer, Line)`
`Column` is neither a variable nor an integer |  | `type_error(integer, Column)`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`
`SorA` is associated with a binary stream |  | `permission_error(access, binary_stream, SorA)`

Portability

GNU Prolog predicate.

#### 8.10.19 `set_stream_line_column/3`

Templates

  `set_stream_line_column(+stream_or_alias, +integer, +integer)`

Description

`set_stream_line_column(SorA, Line, Column)` sets the stream position of the stream associated with the stream-term or alias `SorA` according to the line number `Line` and the column number `Column`. This predicate can only be used on text streams. It first repositions the stream to the beginning of the file and then reads character by character until the required position is reached.

Errors

`SorA` is a variable |  | `instantiation_error`
`Line` is a variable |  | `instantiation_error`
`Column` is a variable |  | `instantiation_error`
`Line` is neither a variable nor an integer |  | `type_error(integer, Line)`
`Column` is neither a variable nor an integer |  | `type_error(integer, Column)`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`
`SorA` is associated with a binary stream |  | `permission_error(reposition, binary_stream, SorA)`
`SorA` has stream property `reposition(false)` |  | `permission_error(reposition, stream, SorA)`

Portability

GNU Prolog predicate.

#### 8.10.20 `add_stream_alias/2`

Templates

  `add_stream_alias(+stream_or_alias, +atom)`

Description

`add_stream_alias(SorA, Alias)` adds `Alias` as a new alias to the stream associated with the stream-term or alias `SorA`.

Errors

`SorA` is a variable |  | `instantiation_error`
`Alias` is a variable |  | `instantiation_error`
`Alias` is neither a variable nor an atom |  | `type_error(atom, Alias)`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`
`Alias` is already associated with an open stream |  | `permission_error(add_alias, source_sink, alias(Alias))`

Portability

GNU Prolog predicate.

#### 8.10.21 `current_alias/2`

Templates

  `current_alias(?stream, ?atom)`

Description

`current_alias(Stream, Alias)` succeeds if `current_stream(Stream)` succeeds (section 8.10.9) and if `Alias` unifies with one of the aliases of the stream. It can be defined using `stream_property/2` (section 8.10.10). This predicate is re-executable on backtracking.

Errors

`Stream` is neither a variable nor a stream-term |  | `domain_error(stream, Stream)`
`Alias` is neither a variable nor an atom |  | `type_error(atom, Alias)`

Portability

GNU Prolog predicate.

#### 8.10.22 `add_stream_mirror/2`

Templates

  `add_stream_mirror(+stream_or_alias, +stream_or_alias)`

Description

`add_stream_mirror(SorA, Mirror)` adds the stream associated with the stream-term or alias `Mirror` as a new mirror to the stream associated with the stream-term or alias `SorA`. After this, all characters (or bytes) read from (or written to) `SorA` are also written to `Mirror`. This mirroring occurs until `Mirror` is explicitly removed using `remove_stream_mirror/2` (section 8.10.23) or implicitly when `Mirror` is closed. Several mirror streams can be associated with a same stream. If `Mirror` represents the same stream as `SorA` or if `Mirror` is already a mirror for `SorA`, no mirror is added.

Errors

`SorA` is a variable |  | `instantiation_error`
`Mirror` is a variable |  | `instantiation_error`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`Mirror` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, Mirror)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`
`Mirror` is not associated with an open stream |  | `existence_error(stream, Mirror)`
`Mirror` is an input stream |  | `permission_error(output, stream, Mirror)`

Portability

GNU Prolog predicate.

#### 8.10.23 `remove_stream_mirror/2`

Templates

  `remove_stream_mirror(+stream_or_alias, +stream_or_alias)`

Description

`remove_stream_mirror(SorA, Mirror)` removes the stream associated with the stream-term or alias `Mirror` from the list of mirrors of the stream associated with the stream-term or alias `SorA`. This predicate fails if `Mirror` is not a mirror stream for `SorA`.

Errors

`SorA` is a variable |  | `instantiation_error`
`Mirror` is a variable |  | `instantiation_error`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`Mirror` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, Mirror)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`
`Mirror` is not associated with an open stream |  | `existence_error(stream, Mirror)`

Portability

GNU Prolog predicate.

#### 8.10.24 `current_mirror/2`

Templates

  `current_mirror(?stream, ?stream)`

Description

`current_mirror(Stream, M)` succeeds if `current_stream(Stream)` succeeds (section 8.10.9) and if `M` unifies with one of the mirrors of the stream. It can be defined using `stream_property/2` (section 8.10.10). This predicate is re-executable on backtracking.

Errors

`Stream` is neither a variable nor a stream-term |  | `domain_error(stream, Stream)`
`M` is neither a variable nor a stream-term |  | `domain_error(stream, M)`

Portability

GNU Prolog predicate.

#### 8.10.25 `set_stream_type/2`

Templates

  `set_stream_type(+stream_or_alias, +atom)`

Description

`set_stream_type(SorA, Type)` updates the type associated with stream-term or alias `SorA`. The value of `Type` is an atom in `text` or `binary` as for `open/4` (section 8.10.6). The type of a stream can only be changed before any input/output operation is executed.

Errors

`SorA` is a variable |  | `instantiation_error`
`Type` is a variable |  | `instantiation_error`
`Type` is neither a variable nor a valid type |  | `domain_error(stream_type, Type)`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`
An I/O operation has already been executed on `SorA` |  | `permission_error(modify, stream, SorA)`

Portability

GNU Prolog predicate.

#### 8.10.26 `set_stream_eof_action/2`

Templates

  `set_stream_eof_action(+stream_or_alias, +atom)`

Description

`set_stream_eof_action(SorA, Action)` updates the `eof_action` option associated with the stream-term or alias `SorA`. The value of `Action` is one of the atoms `error`, `eof_code`, `reset` as for `open/4` (section 8.10.6).

Errors

`SorA` is a variable |  | `instantiation_error`
`Action` is a variable |  | `instantiation_error`
`Action` is neither a variable nor a valid eof action |  | `domain_error(eof_action, Action)`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`
`SorA` is an output stream |  | `permission_error(modify, stream, SorA)`

Portability

GNU Prolog predicate.

#### 8.10.27 `set_stream_buffering/2`

Templates

  `set_stream_buffering(+stream_or_alias, +atom)`

Description

`set_stream_buffering(SorA, Buffering)` updates the buffering mode associated with the stream-term or alias `SorA`. The value of `Buffering` is one of the atoms `none`, `line` or `block` as for `open/4` (section 8.10.6). This predicate may only be used after opening a stream and before any other operations have been performed on it.

Errors

`SorA` is a variable |  | `instantiation_error`
`Buffering` is a variable |  | `instantiation_error`
`Buffering` is neither a variable nor a valid buffering mode |  | `domain_error(buffering_mode, Buffering)`
`SorA` is neither a variable nor a stream-term or alias |  | `domain_error(stream_or_alias, SorA)`
`SorA` is not associated with an open stream |  | `existence_error(stream, SorA)`

Portability

GNU Prolog predicate.

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