SWI-Prolog also allows SrcDest to be a term pipe(Command). In this form, Command is started as a child process and if Mode is write, output written to Stream is sent to the standard input of Command. Viso versa, if Mode is read, data written by Command to the standard output may be read from Stream. On Unix systems, Command is handed to popen() which hands it to the Unix shell. On Windows, Command is executed directly. See also process_create/3 from library(process).

The following Options are recognised by open/4:

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),
        ...

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 2.19.1.1 for a discussion of this feature.

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).

create(+List)

Specifies how a new file is created when opening in write, append or update mode. Currently, List is a list of atoms that describe the permissions of the created file.^{83Added after feedback from Joachim Shimpf and Per Mildner.} Defined values are below. Not recognised values are silently ignored, allowing for adding platform specific extensions to this set.

read

Allow read access to the file.

write

Allow write access to the file.

execute

Allow execution access to the file.

default

Allow read and write access to the file.

all

Allow any access provided by the OS.

Note that if List is empty, the created file has no associated access permissions. The create options map to the POSIX mode option of open(), where read map to 0444, write to 0222 and execute to 0111. On POSIX systems, the final permission is defined as (mode & ~umask).

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 2.19.1.

eof_action(Action)

Defines what happens if the end of the input stream is reached. The default value for Action is eof_code, which 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.

locale(+Locale)

Set the locale that is used by notably format/2 for output on this stream. See section 4.23.

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.

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.

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.