The predicates in this section provide interaction with the file system and (syntactic) operations on file names. SWI-Prolog file system interaction is based on the POSIX standard. On Windows we use the Unicode (wide character) versions of the C runtime library functions.
The file operations define a large
set of error conditions. Errors are mapped to Prolog exceptions using a
generic function that receives the
action (e.g., make_directory), type
(e.g.,
directory), the term that describes the object (name) of
the file system and the errno value. Unfortunately, the
resulting exceptions are often misleading. For example, calling make_directory/1
such that it must create mutiple directories (e.g., d1/d2/d3)
returns an existence error on the directory d1/d2/d3 rather
than the missing component. On Windows the situation is even worse
because many of its runtime functions distinguish only a few error
codes. For example, _wmkdir() only produces EEXIST
or ENOENT and all failures except for an already existing
target result in an existence_error exception. We can only improve on
this situation by implementing the translation of errno to
a Prolog exception specifically for each file operation and perform
additional tests to distinguish the different error conditions that are
represented by the same errno value. This is hard, in
particular because this translation needs to depend on the OS and
specific file system limitations.
SWI-Prolog uses the Windows
Unicode functions to access the file system. Internally all Prolog's
file handling is based on C char* strings. On POSIX systems
these strings use multibyte encodings according to the current locale
(nowadays often UTF-8). On Windows the encoding is fixed to UTF-8 and a
wrapper around the low-level file functions translates this to UTF-16152Before
8.5.16 to UCS-2, allowing only for Unicode code points up to 0xffff.
before calling the Win32 *W() or C runtime _w*() functions.
Windows absolute file paths are traditionally limited to 260
characters (PATH_MAX). More recent versions of Windows
support long files. Many of the Unicode file functions support longer
paths. For others, support for long paths can be forced by prefixing the
path with “\\?\” (``\\?UNC\” for
UNC paths (//server/path)). This syntax does not allow for
relative paths. Thus, SWI-Prolog file functions use GetFullPathName()
to arrive at absolute canonical paths and apply the appropriate prefix
before calling Windows low-level functions.
Unfortunately the support is not very robust. Some functions still apply length limits while others do not work with the above mentioned prefix. The result depends on the Windows version, several Windows registry entries and the file system. Exceeding the length limit is often reported as non-existence of the target file or directory.
read, write, append, execute,
search, exist, or none. Fails
silently otherwise.
File may also be the name of a directory.
access_file(File, none) simply succeeds without testing
anything.
If Mode is write or append, this
predicate also succeeds if the file does not exist and the user has
write access to the directory of the specified location.
The mode execute is only intended for use with regular
files and the mode search only with directories. However,
the two modes are currently equivalent and both can be used with either
files or directories. This may change in the future, so the results of
checking execute access on directories or search
access on regular files should not be relied on.
The behaviour is backed up by the POSIX access() API. The Windows replacement (_waccess()) returns incorrect results because it does not consider ACLs (Access Control Lists). The Prolog flag win_file_access_check may be used to control the level of checking performed by Prolog. Please note that checking access never provides a guarantee that a subsequent open succeeds without errors due to inherent concurrency in file operations. It is generally more robust to try and open the file and handle possible exceptions. See open/4 and catch/3.
/*[^/]*/*$. If
the result is empty it binds Directory to //.//a/b/ into /a/b. Volker
Wysk pointed at this incorrect behaviour.
See also directory_file_path/3
from library(filesex). The system ensures that for every
valid Path using the Prolog (POSIX) directory separators,
following is true on systems with a sound implementation of
same_file/2.154On
some systems, Path and Path2 refer to the same
entry in the file system, but same_file/2
may fail. See also prolog_to_os_filename/2.
...,
file_directory_name(FilePath, Dir),
file_base_name(FilePath, File),
directory_file_path(Dir, File, Path2),
same_file(FilePath, Path2).
/*([^/]*)/*$, now capturing the non-///'') otherwise. The behaviour is consistent with the
POSIX basename program.155Before
SWI-Prolog 7.7.13, if argPath ended with a /st_dev and st_inode, same_file/2
is implemented by comparing the device and inode identifiers. On
Windows,
same_file/2
uses GetFileInformationByHandle() and compares the volume serial
number and file index.156As of
version 8.5.16. Earlier versions only compare the canonical name
obtained using GetFullPathName().The rename() system call has a large number of error conditions. Errors are mapped to Prolog exceptions using a generic conversion based on the File1 argument. As a result, the errors may be confusing. Future versions may improve on this.
., .. and
repeated directory separators (//This predicate has a different history than absolute_file_name/3 and should primarily be used to get an absolute canonical name from a relative name. If File is a term Alias(Relative) is behaviour is defined as below, i.e., if an accessible file can be found using the provided search path this is returned. Otherwise it returns the the expansion of the alias path.158The SICStus implementation behaves as absolute_file_name/3 with an empty option list. Users are advised to use absolute_file_name/3 with appropriate options for resolving an Alias(Relative) term.
absolute_file_name(Spec, AbsFile) :-
absolute_file_name(Spec, File, [access(read), file_errors(fail)]),
!,
AbsFile = File.
absolute_file_name(Spec, AbsFile) :-
absolute_file_name(Spec, AbsFile, []).
See also absolute_file_name/3, file_search_path/2, and expand_file_name/2.
(library(lists)), a
relative filename or an absolute filename. The primary intention of this
predicate is to resolve files specified as Alias(Relative), which use
file_search_path/2
to look up the possibilities for Alias. This predicate only returns
non-directories, unless the option
file_type(directory) is specified or the requested access
is
none. The result always uses the directory separator /[’’].
For each extension, absolute_file_name/3
will first add the extension and then verify the conditions imposed by
the other options. If the condition fails, the next extension on the
list is tried. Extensions may be specified both as .ext or
plain ext.Up to version 9.3.9, the system tried both the directory holding the current source and the current working directory.
read, write, append, execute,
search, exist or none. See also access_file/2.
The default is none which, if file_type is not
specified as directory or regular, returns
absolute file names that result from expanding aliases without
inspecting the actual file system.txt implies [’’],
prolog implies [’.pl’,’’], executable
implies
[’.so’,’’] and qlf
implies [’.qlf’,’’]. The
Type directory implies [’’]
and causes this predicate to generate (only) directories. The Type regular
is the opposite of directory and is the default if no file
type is specified and the effective access mode is none.
The file type source is an alias for prolog
for compatibility with SICStus Prolog. See also prolog_file_type/2.
error (default), throw an existence_error
exception if the file cannot be found. If fail, stay
silent.159Silent operation was the
default up to version 3.2.6.first (default), the predicate leaves no choice point.
Otherwise a choice point will be left and backtracking may yield more
solutions.true (default is false) and Spec
is atomic, call expand_file_name/2
followed by member/2
on Spec before proceeding. This is a SWI-Prolog extension
intended to minimise porting effort after SWI-Prolog stopped expanding
environment variables and the ~*
The Prolog flag verbose_file_search
can be set to true to help debugging Prolog's search for
files. See also file_search_path/2.
This predicate is derived from Quintus Prolog. In Quintus Prolog, the
argument order was absolute_file_name(+Spec, +Options, -Path).
The argument order has been changed for compatibility with ISO and
SICStus. The Quintus argument order is still accepted.
<letter>:
or //<host>/. This predicate is intended
to provide platform-independent checking for absolute paths. See also absolute_file_name/2
and
prolog_to_os_filename/2...... See also expand_file_name/2.160This
predicate should be considered a misnomer because it returns entries
rather than files. We stick to this name for compatibility with, e.g.,
SICStus, Ciao and YAP.?*[
...’and‘{...}’are recognised. The
interpretation of‘{...}’is slightly different
from the C shell (csh(1)). The comma-separated argument can be arbitrary
patterns, including‘{...}’patterns. The empty
pattern is legal as well:‘{.pl,}’matches either‘.pl’or
the empty string.
If the pattern contains wildcard characters, only existing files and directories are returned. Expanding a‘pattern’without wildcard characters returns the argument, regardless of whether or not it exists.
Before expanding wildcards, the construct $\arg{var} is
expanded to the value of the environment variable var, and a
possible leading ~ character is expanded to the user's home
directory.161On Windows, the home
directory is determined as follows: if the environment variable HOME
exists, this is used. If the variables HOMEDRIVE and HOMEPATH
exist (Windows-NT), these are used. At initialisation, the system will
set the environment variable HOME to point to the
SWI-Prolog home directory if neither HOME nor HOMEPATH
and HOMEDRIVE are defined.
\/Because it is possible to guess the generated filename, attackers may create the filesystem entry as a link and possibly create a security issue. New code should use tmp_file_stream/3.
O_EXCL, which
guarantees that the file did not exist before this call. The directory
for temporary files is defined by the Prolog flag tmp_dir.
The following options are processed:
binary
opens the file in binary mode.This predicate is a safe replacement of tmp_file/2. Note that in those cases where the temporary file is needed to store output from an external command, the file must be closed first. E.g., the following downloads a file from a URL to a temporary file and opens the file for reading (on Unix systems you can delete the file for cleanup after opening it for reading):
open_url(URL, In) :-
tmp_file_stream(text, File, Stream),
close(Stream),
process_create(curl, ['-o', File, URL], []),
open(File, read, In),
delete_file(File). % Unix-only
Temporary files created using this call are removed if the Prolog process terminates gracefully. Calling delete_file/1 using FileName removes the file and removes the entry from the administration of files-to-be-deleted.
working_directory(CWD, CWD) to get the current directory.
See also absolute_file_name/2
and chdir/1.bugSome
of the file I/O predicates use local filenames. Changing directory while
file-bound streams are open causes wrong results on telling/1, seeing/1
and current_stream/3.
Note that the working directory is shared between all threads.
Applications are strongly encouraged not to change the working directory
or change the working directory once during the initialization.