True when SrcDest can be opened in Mode and Stream
is an I/O stream to/from the object. SrcDest is normally the
name of a file, represented as an atom or string. Mode is one
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.97New
code should use the
alias(Alias) option for compatibility
with the ISO standard.
SWI-Prolog also allows SrcDest to be a term
In this form, Command is started as a child process and if
write, output written to Stream
is sent to the standard input of Command. Vice versa, if Mode
read, data written by Command to the standard
output can 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 and therefore shell syntax such as redirecting
> file) does not work. Use of
pipe(Command) feature is deprecated. The predicate
library(process) provides a richer and more portable
alternative for interacting with processes including handling all three
If SrcDest is an IRI, i.e., starts with
://, where <scheme>
is a non-empty sequence of lowercase ASCII letters open/3,4
calls hooks registered by register_iri_scheme/3.
Currently the only predefined IRI scheme is
access to the resource database. See
The following Options are recognised by open/4:
- 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)]).
- Check for a BOM (Byte Order Marker) or write one. If omitted,
the default is
true for mode
false for mode
write. See also stream_property/2
and especially section
126.96.36.199 for a discussion of this feature.
- Defines output buffering. The atom
full (default) defines
line buffering by line, and
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
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
- Specifies how a new file is created when opening in
update mode. Currently, List
is a list of atoms that describe the permissions of the created file.98Added
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.
- Allow read access to the file.
- Allow write access to the file.
- Allow execution access to the file.
- Allow read and write access to the file.
- 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,
to 0222 and
execute to 0111. On POSIX systems, the final
permission is defined as (mode &
- 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.
binary streams the default encoding is
For details on encoding issues, see section
- 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
error is like
repetitive reading will raise an error. With action
Prolog will examine the file again and return more data if the file has
- Set the locale that is used by notably format/2
for output on this stream. See section
- Try to obtain a lock on the open file. Default is
which does not lock the file. The value
means other processes may read the file, but not write it. The value
exclusive means 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.
- Set end-of-line processing for the stream. Mode is one of
detect. This option
is ignored for binary streams. Using
detect on an output
stream raises an exception. See also set_stream/2.
- Using type
text (default), Prolog will write a text file in
an operating system compatible way. Using type
bytes will be read or written without any translation. See also the
- This option can be combined with the
lock option. If
true), the open call returns
immediately with an exception if the file is locked. The exception has
permission_error(lock, source_sink, SrcDest).
reposition is not supported in SWI-Prolog.
All streams connected to a file may be repositioned.