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File System Interaction

4.36 File System Interaction

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-16154Before 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.

access_file(+File, +Mode)
True if File exists and can be accessed by this Prolog process under mode Mode. Mode is one of the atoms 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.

exists_file(+File)
True if File exists and is a regular file. This does not imply the user has read or write access to the file. See also exists_directory/1 and access_file/2. The current implementation fails silently, also on error error conditions such as File being too long.
file_directory_name(+File, -Directory)
Extracts the directory part of File. This predicate removes the longest match for the regular expression /*[^/]*/*$. If the result is empty it binds Directory to / if the first character of File is / and . otherwise. The behaviour is consistent with the POSIX dirname program.155Before SWI-Prolog 7.7.13 trailing / where not removed, translation /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/2.156On 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).
file_base_name(+File, -Name)
Extracts the file name part from name that may include directories. Similar to file_directory_name/2 the extraction is based on the regex /*([^/]*)/*$, now capturing the non-/ segment. If the segment is empty it unifies -Name with / if File starts with / and the empty atom ('') otherwise. The behaviour is consistent with the POSIX basename program.157Before SWI-Prolog 7.7.13, if argPath ended with a / -Name was unified with the empty atom.
same_file(+File1, +File2)
True if both filenames refer to the same physical file. That is, if File1 and File2 are the same string or both names exist and point to the same file (due to hard or symbolic links and/or relative vs. absolute paths). On systems that provide stat() with meaningful values for 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.158As of version 8.5.16. Earlier versions only compare the canonical name obtained using GetFullPathName().
exists_directory(+Directory)
True if Directory exists and is a directory. This does not imply the user has read, search or write permission for the directory. The current implementation fails silently, also on error error conditions such as Directory being too long.
delete_file(+File)
Remove File from the file system. Note that on POSIX systems the remove() call works on read-only files as long as the containing directory has write access. The Windows remove() call raises a permission error if the file is read-only. SWI-Prolog removes the read-only attribute if remove() fails and tries again. If the file still cannot be removed it restores the read-only attribute and delete_file/1 raises a permission error. As a consequence a read-only file that cannot be removed is briefly read-write. Also note that while an open file can be removed on POSIX systems (where it is actually deleted when closed), deleting an open file on Windows is not possible.
rename_file(+File1, +File2)
Rename File1 as File2. The semantics is compatible to the semantics of the POSIX rename() system call as far as the operating system allows. Notably, if File2 exists, the operation succeeds (except for possible permission errors) and is atomic (meaning there is no window where File2 does not exist). Note that File2 cannot be an existing directory.159The POSIX semantics describe one exception: a directory can be moved to an existing empty directory. To move a file to another directory one must create File2 from the target directory and the base name of File1. See file_base_name/2.

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.

size_file(+File, -Size)
Unify Size with the size of File in bytes.
time_file(+File, -Time)
Unify the last modification time of File with Time. Time is a floating point number expressing the seconds elapsed since Jan 1, 1970. See also convert_time/[2,8] and get_time/1.
absolute_file_name(+File, -Absolute)
Expand a local filename into an absolute path. The absolute path is canonicalised: references to ., .. and repeated directory separators (/) are deleted. This predicate ensures that expanding a filename returns the same absolute path regardless of how the file is addressed. Notably, if a file appears in multiple directories due to symbolic or hard links absolute_file_name/2 returns the same absolute filename. SWI-Prolog uses absolute filenames to register source files independent of the current working directory. The directory separators are always /; prolog_to_os_filename/2 can be used to obtain the the operating system's preferred form.

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.160The 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.

absolute_file_name(+Spec, -Absolute, +Options)
Convert the given file specification into an absolute path. Spec is a term Alias(Relative) (e.g., (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 /; if the operating system uses something different, SWI-Prolog converts the file name before it makes an OS call. If you need the filename in the OS's preferred form, use prolog_to_os_filename/2. Supported Options are:
extensions(ListOfExtensions)
List of file extensions to try. Default is [’’]. 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.
relative_to(+FileOrDir)
Resolve the path relative to the given directory or the directory holding the given file. Without this option, paths are resolved relative to the working directory (see working_directory/2) or, if Spec is atomic and absolute_file_name/[2,3] is executed in a directive, it uses the current source file as reference. Note that a directive using initialization/1 is executed after loading the file. This implies that such paths are resolved relative to the working directory for the toplevel file and relative to the file loading the file holding the initialization/1 directive otherwise.

Up to version 9.3.9, the system tried both the directory holding the current source and the current working directory.

access(Mode)
Imposes the condition access_file(File, Mode). Mode is one of 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.
file_type(Type)
Defines extensions. Current mapping: 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.

file_errors(fail/error)
If error (default), throw an existence_error exception if the file cannot be found. If fail, stay silent.161Silent operation was the default up to version 3.2.6.
solutions(first/all)
If first (default), the predicate leaves no choice point. Otherwise a choice point will be left and backtracking may yield more solutions.
expand(Boolean)
If 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 ~ by default. This option should be considered deprecated. In particular the use of wildcard patterns such as * should be avoided.

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.

is_absolute_file_name(+File)
True if File specifies an absolute path name. On POSIX systems, this implies the path starts with a‘/’. For Microsoft-based systems this implies the path starts with <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.
file_name_extension(?Base, ?Extension, ?Name)
This predicate is used to add, remove or test filename extensions. The main reason for its introduction is to deal with different filename properties in a portable manner. If the file system is case-insensitive, testing for an extension will also be done case-insensitive. Extension may be specified with or without a leading dot (.). If an Extension is generated, it will not have a leading dot.
directory_files(+Directory, -Entries)
Unify Entries with a list of entries in Directory. Each member of Entries is an atom denoting an entry relative to Directory. Entries contains all entries, including hidden files and, if supplied by the OS, the special entries . and ... See also expand_file_name/2.162This 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.
expand_file_name(+WildCard, -List)
Unify List with a sorted list of files or directories matching WildCard. The normal Unix wildcard constructs‘?’,‘*’,‘[ ...’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.163On 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.

prolog_to_os_filename(?PrologPath, ?OsPath)
Convert between the internal Prolog path name conventions and the operating system path name conventions. The internal conventions follow the POSIX standard, which implies that this predicate is equivalent to =/2 (unify) on POSIX (e.g., Unix) systems. On Windows systems it changes the directory separator from \ into /.
read_link(+File, -Link, -Target)
If File points to a symbolic link, unify Link with the value of the link and Target to the file the link is pointing to. Target points to a file, directory or non-existing entry in the file system, but never to a link. Fails if File is not a link. Fails always on systems that do not support symbolic links.
[deprecated]tmp_file(+Base, -TmpName)
Create a name for a temporary file. Base is an identifier for the category of file. The TmpName is guaranteed to be unique. If the system halts, it will automatically remove all created temporary files. Base is used as part of the final filename. Portable applications should limit themselves to alphanumeric characters. The directory for temporary files is defined by the Prolog flag tmp_dir.

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.

tmp_file_stream(+Encoding, -FileName, -Stream)
tmp_file_stream(-FileName, -Stream, +Options)
Create a temporary filename FileName, open it for writing and unify Stream with the output stream. If the OS supports it, the created file is only accessible to the current user and the file is created using the open()-flag 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:
encoding(+Encoding)
Encoding of Stream. Default is the value of the Prolog flag encoding. The value binary opens the file in binary mode.
extension(+Ext)
Ensure the created file has the given extension. Default is no extension. Using an extension may be necessary to run external programs on the file.

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.

make_directory(+Directory)
Create a new directory (folder) on the filesystem. Raises an exception on failure. On Unix systems, the directory is created with default permissions (defined by the process umask setting).
delete_directory(+Directory)
Delete directory (folder) from the filesystem. Raises an exception on failure. Please note that in general it will not be possible to delete a non-empty directory.
working_directory(-Old, +New)
Unify Old with an absolute path to the current working directory and change working directory to New. Use the pattern 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.
chdir(+Path)
Compatibility predicate. New code should use working_directory/2.

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