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. Supported Options are:
[’’]. 
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.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.