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7.2.3 File System

These procedures allow querying and setting file system attributes (such as owner, permissions, sizes and types of files); deleting, copying, renaming and linking files; creating and removing directories and querying their contents; syncing the file system and creating special files.

Scheme Procedure: access? path how
C Function: scm_access (path, how)

Test accessibility of a file under the real UID and GID of the calling process. The return is #t if path exists and the permissions requested by how are all allowed, or #f if not.

how is an integer which is one of the following values, or a bitwise-OR (logior) of multiple values.

Variable: R_OK

Test for read permission.

Variable: W_OK

Test for write permission.

Variable: X_OK

Test for execute permission.

Variable: F_OK

Test for existence of the file. This is implied by each of the other tests, so there’s no need to combine it with them.

It’s important to note that access? does not simply indicate what will happen on attempting to read or write a file. In normal circumstances it does, but in a set-UID or set-GID program it doesn’t because access? tests the real ID, whereas an open or execute attempt uses the effective ID.

A program which will never run set-UID/GID can ignore the difference between real and effective IDs, but for maximum generality, especially in library functions, it’s best not to use access? to predict the result of an open or execute, instead simply attempt that and catch any exception.

The main use for access? is to let a set-UID/GID program determine what the invoking user would have been allowed to do, without the greater (or perhaps lesser) privileges afforded by the effective ID. For more on this, see Testing File Access in The GNU C Library Reference Manual.

Scheme Procedure: stat object [exception-on-error?]
C Function: scm_stat (object, exception_on_error)

Return an object containing various information about the file determined by object. object can be a string containing a file name or a port or integer file descriptor which is open on a file (in which case fstat is used as the underlying system call).

If the optional exception_on_error argument is true, which is the default, an exception will be raised if the underlying system call returns an error, for example if the file is not found or is not readable. Otherwise, an error will cause stat to return #f.

The object returned by stat can be passed as a single parameter to the following procedures, all of which return integers:

Scheme Procedure: stat:dev st

The device number containing the file.

Scheme Procedure: stat:ino st

The file serial number, which distinguishes this file from all other files on the same device.

Scheme Procedure: stat:mode st

The mode of the file. This is an integer which incorporates file type information and file permission bits. See also stat:type and stat:perms below.

The number of hard links to the file.

Scheme Procedure: stat:uid st

The user ID of the file’s owner.

Scheme Procedure: stat:gid st

The group ID of the file.

Scheme Procedure: stat:rdev st

Device ID; this entry is defined only for character or block special files. On some systems this field is not available at all, in which case stat:rdev returns #f.

Scheme Procedure: stat:size st

The size of a regular file in bytes.

Scheme Procedure: stat:atime st

The last access time for the file, in seconds.

Scheme Procedure: stat:mtime st

The last modification time for the file, in seconds.

Scheme Procedure: stat:ctime st

The last modification time for the attributes of the file, in seconds.

Scheme Procedure: stat:atimensec st
Scheme Procedure: stat:mtimensec st
Scheme Procedure: stat:ctimensec st

The fractional part of a file’s access, modification, or attribute modification time, in nanoseconds. Nanosecond timestamps are only available on some operating systems and file systems. If Guile cannot retrieve nanosecond-level timestamps for a file, these fields will be set to 0.

Scheme Procedure: stat:blksize st

The optimal block size for reading or writing the file, in bytes. On some systems this field is not available, in which case stat:blksize returns a sensible suggested block size.

Scheme Procedure: stat:blocks st

The amount of disk space that the file occupies measured in units of 512 byte blocks. On some systems this field is not available, in which case stat:blocks returns #f.

In addition, the following procedures return the information from stat:mode in a more convenient form:

Scheme Procedure: stat:type st

A symbol representing the type of file. Possible values are ‘regular’, ‘directory’, ‘symlink’, ‘block-special’, ‘char-special’, ‘fifo’, ‘socket’, and ‘unknown’.

Scheme Procedure: stat:perms st

An integer representing the access permission bits.

Scheme Procedure: lstat path
C Function: scm_lstat (path)

Similar to stat, but does not follow symbolic links, i.e., it will return information about a symbolic link itself, not the file it points to. path must be a string.

Return the value of the symbolic link named by path (a string), i.e., the file that the link points to.

Scheme Procedure: chown object owner group
C Function: scm_chown (object, owner, group)

Change the ownership and group of the file referred to by object to the integer values owner and group. object can be a string containing a file name or, if the platform supports fchown (see File Owner in The GNU C Library Reference Manual), a port or integer file descriptor which is open on the file. The return value is unspecified.

If object is a symbolic link, either the ownership of the link or the ownership of the referenced file will be changed depending on the operating system (lchown is unsupported at present). If owner or group is specified as -1, then that ID is not changed.

Scheme Procedure: chmod object mode
C Function: scm_chmod (object, mode)

Changes the permissions of the file referred to by object. object can be a string containing a file name or a port or integer file descriptor which is open on a file (in which case fchmod is used as the underlying system call). mode specifies the new permissions as a decimal number, e.g., (chmod "foo" #o755). The return value is unspecified.

Scheme Procedure: utime pathname [actime [modtime [actimens [modtimens [flags]]]]]
C Function: scm_utime (pathname, actime, modtime, actimens, modtimens, flags)

utime sets the access and modification times for the file named by pathname. If actime or modtime is not supplied, then the current time is used. actime and modtime must be integer time values as returned by the current-time procedure.

The optional actimens and modtimens are nanoseconds to add actime and modtime. Nanosecond precision is only supported on some combinations of file systems and operating systems.

(utime "foo" (- (current-time) 3600))

will set the access time to one hour in the past and the modification time to the current time.

Last, flags may be either 0 or the AT_SYMLINK_NOFOLLOW constant, to set the time of pathname even if it is a symbolic link.

Scheme Procedure: delete-file str
C Function: scm_delete_file (str)

Deletes (or “unlinks”) the file whose path is specified by str.

Scheme Procedure: copy-file oldfile newfile
C Function: scm_copy_file (oldfile, newfile)

Copy the file specified by oldfile to newfile. The return value is unspecified.

Scheme Procedure: sendfile out in count [offset]
C Function: scm_sendfile (out, in, count, offset)

Send count bytes from in to out, both of which must be either open file ports or file descriptors. When offset is omitted, start reading from in’s current position; otherwise, start reading at offset. Return the number of bytes actually sent.

When in is a port, it is often preferable to specify offset, because in’s offset as a port may be different from the offset of its underlying file descriptor.

On systems that support it, such as GNU/Linux, this procedure uses the sendfile libc function, which usually corresponds to a system call. This is faster than doing a series of read and write system calls. A typical application is to send a file over a socket.

In some cases, the sendfile libc function may return EINVAL or ENOSYS. In that case, Guile’s sendfile procedure automatically falls back to doing a series of read and write calls.

In other cases, the libc function may send fewer bytes than count—for instance because out is a slow or limited device, such as a pipe. When that happens, Guile’s sendfile automatically retries until exactly count bytes were sent or an error occurs.

Scheme Procedure: rename-file oldname newname
C Function: scm_rename (oldname, newname)

Renames the file specified by oldname to newname. The return value is unspecified.

Creates a new name newpath in the file system for the file named by oldpath. If oldpath is a symbolic link, the link may or may not be followed depending on the system.

Create a symbolic link named newpath with the value (i.e., pointing to) oldpath. The return value is unspecified.

Scheme Procedure: mkdir path [mode]
C Function: scm_mkdir (path, mode)

Create a new directory named by path. If mode is omitted then the permissions of the directory are set to #o777 masked with the current umask (see umask). Otherwise they are set to the value specified with mode. The return value is unspecified.

Scheme Procedure: rmdir path
C Function: scm_rmdir (path)

Remove the existing directory named by path. The directory must be empty for this to succeed. The return value is unspecified.

Scheme Procedure: opendir dirname
C Function: scm_opendir (dirname)

Open the directory specified by dirname and return a directory stream.

Before using this and the procedures below, make sure to see the higher-level procedures for directory traversal that are available (see File Tree Walk).

Scheme Procedure: directory-stream? object
C Function: scm_directory_stream_p (object)

Return a boolean indicating whether object is a directory stream as returned by opendir.

Scheme Procedure: readdir stream
C Function: scm_readdir (stream)

Return (as a string) the next directory entry from the directory stream stream. If there is no remaining entry to be read then the end of file object is returned.

Scheme Procedure: rewinddir stream
C Function: scm_rewinddir (stream)

Reset the directory port stream so that the next call to readdir will return the first directory entry.

Scheme Procedure: closedir stream
C Function: scm_closedir (stream)

Close the directory stream stream. The return value is unspecified.

Here is an example showing how to display all the entries in a directory:

(define dir (opendir "/usr/lib"))
(do ((entry (readdir dir) (readdir dir)))
    ((eof-object? entry))
  (display entry)(newline))
(closedir dir)
Scheme Procedure: sync
C Function: scm_sync ()

Flush the operating system disk buffers. The return value is unspecified.

Scheme Procedure: mknod path type perms dev
C Function: scm_mknod (path, type, perms, dev)

Creates a new special file, such as a file corresponding to a device. path specifies the name of the file. type should be one of the following symbols: ‘regular’, ‘directory’, ‘symlink’, ‘block-special’, ‘char-special’, ‘fifo’, or ‘socket’. perms (an integer) specifies the file permissions. dev (an integer) specifies which device the special file refers to. Its exact interpretation depends on the kind of special file being created.


(mknod "/dev/fd0" 'block-special #o660 (+ (* 2 256) 2))

The return value is unspecified.

Scheme Procedure: tmpnam
C Function: scm_tmpnam ()

Return an auto-generated name of a temporary file, a file which doesn’t already exist. The name includes a path, it’s usually in /tmp but that’s system dependent.

Care must be taken when using tmpnam. In between choosing the name and creating the file another program might use that name, or an attacker might even make it a symlink pointing at something important and causing you to overwrite that.

The safe way is to create the file using open with O_EXCL to avoid any overwriting. A loop can try again with another name if the file exists (error EEXIST). mkstemp below does that.

Scheme Procedure: mkstemp tmpl [mode]

Create a new unique file in the file system and return a new buffered port open for reading and writing to the file.

tmpl is a string specifying where the file should be created: it must end with ‘XXXXXX’. The name of the newly created file will be the same as tmpl, but with those ‘X’s changed, and can be determined by calling port-filename on the returned port.

Note that the newly created file is not deleted automatically by Guile; probably the caller should arrange to call delete-file when the file is no longer needed.

POSIX doesn’t specify the permissions mode of the file. On GNU and most systems it’s #o600; an application can use chmod to relax that if desired. For example #o666 less umask, which is usual for ordinary file creation,

(let ((port (mkstemp "/tmp/myfile-XXXXXX")))
  (chmod port (logand #o666 (lognot (umask))))

The optional mode argument specifies a mode with which to open the new file, as a string in the same format that open-file takes. It defaults to "w+".

Scheme Procedure: tmpfile
C Function: scm_tmpfile ()

Return an input/output port to a unique temporary file named using the path prefix P_tmpdir defined in stdio.h. The file is automatically deleted when the port is closed or the program terminates.

Scheme Procedure: mkdtemp tmpl
C Function: scm_mkdtemp (tmpl)

Create a new directory named in accordance with the template string tmpl.

tmpl is a string specifying the directory’s name. The last six characters of tmpl must be ‘XXXXXX’. Upon successful execution, the name of the new directory is returned which has the same form as tmpl but with the ‘XXXXXX’ characters modified to ensure the directory name is unique.

The permissions of the directory created are OS dependent, but, are usually #o700.

An error may be thrown if the template has the wrong format or if the directory cannot be created.

Scheme Procedure: dirname filename
C Function: scm_dirname (filename)

Return the directory name component of the file name filename. If filename does not contain a directory component, . is returned.

Scheme Procedure: basename filename [suffix]
C Function: scm_basename (filename, suffix)

Return the base name of the file name filename. The base name is the file name without any directory components. If suffix is provided, and is equal to the end of basename, it is removed also.

(basename "/tmp/test.xml" ".xml")
⇒ "test"
Scheme Procedure: canonicalize-path path
C Function: scm_canonicalize_path (path)

Return the canonical (absolute) path of path. A canonical path has no . or .. components, nor any repeated path separators (/) nor symlinks.

Raises an error if any component of path does not exist.

(canonicalize-path "test.xml")
⇒ "/tmp/test.xml"
Scheme Procedure: file-exists? filename

Return #t if the file named filename exists, #f if not.

Many operating systems, such as GNU, use / (forward slash) to separate the components of a file name; any file name starting with / is considered an absolute file name. These conventions are specified by the POSIX Base Definitions, which refer to conforming file names as “pathnames”. Some operating systems use a different convention; in particular, Windows uses \ (backslash) as the file name separator, and also has the notion of volume names like C:\ for absolute file names. The following procedures and variables provide support for portable file name manipulations.

Scheme Procedure: system-file-name-convention

Return either posix or windows, depending on what kind of system this Guile is running on.

Scheme Procedure: file-name-separator? c

Return true if character c is a file name separator on the host platform.

Scheme Procedure: absolute-file-name? file-name

Return true if file-name denotes an absolute file name on the host platform.

Scheme Variable: file-name-separator-string

The preferred file name separator.

Note that on MinGW builds for Windows, both / and \ are valid separators. Thus, programs should not assume that file-name-separator-string is the only file name separator—e.g., when extracting the components of a file name.

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