unlink(3p) — Linux manual page


UNLINK(3P)                POSIX Programmer's Manual               UNLINK(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior), or
       the interface may not be implemented on Linux.

NAME         top

       unlink, unlinkat — remove a directory entry relative to directory
       file descriptor

SYNOPSIS         top

       #include <unistd.h>

       int unlink(const char *path);
       int unlinkat(int fd, const char *path, int flag);

DESCRIPTION         top

       The unlink() function shall remove a link to a file. If path names a
       symbolic link, unlink() shall remove the symbolic link named by path
       and shall not affect any file or directory named by the contents of
       the symbolic link. Otherwise, unlink() shall remove the link named by
       the pathname pointed to by path and shall decrement the link count of
       the file referenced by the link.

       When the file's link count becomes 0 and no process has the file
       open, the space occupied by the file shall be freed and the file
       shall no longer be accessible. If one or more processes have the file
       open when the last link is removed, the link shall be removed before
       unlink() returns, but the removal of the file contents shall be
       postponed until all references to the file are closed.

       The path argument shall not name a directory unless the process has
       appropriate privileges and the implementation supports using unlink()
       on directories.

       Upon successful completion, unlink() shall mark for update the last
       data modification and last file status change timestamps of the
       parent directory. Also, if the file's link count is not 0, the last
       file status change timestamp of the file shall be marked for update.

       The unlinkat() function shall be equivalent to the unlink() or
       rmdir() function except in the case where path specifies a relative
       path. In this case the directory entry to be removed is determined
       relative to the directory associated with the file descriptor fd
       instead of the current working directory. If the file descriptor was
       opened without O_SEARCH, the function shall check whether directory
       searches are permitted using the current permissions of the directory
       underlying the file descriptor. If the file descriptor was opened
       with O_SEARCH, the function shall not perform the check.

       Values for flag are constructed by a bitwise-inclusive OR of flags
       from the following list, defined in <fcntl.h>:

             Remove the directory entry specified by fd and path as a
             directory, not a normal file.

       If unlinkat() is passed the special value AT_FDCWD in the fd
       parameter, the current working directory shall be used and the
       behavior shall be identical to a call to unlink() or rmdir()
       respectively, depending on whether or not the AT_REMOVEDIR bit is set
       in flag.

RETURN VALUE         top

       Upon successful completion, these functions shall return 0.
       Otherwise, these functions shall return −1 and set errno to indicate
       the error. If −1 is returned, the named file shall not be changed.

ERRORS         top

       These functions shall fail and shall not unlink the file if:

       EACCES Search permission is denied for a component of the path
              prefix, or write permission is denied on the directory
              containing the directory entry to be removed.

       EBUSY  The file named by the path argument cannot be unlinked because
              it is being used by the system or another process and the
              implementation considers this an error.

       ELOOP  A loop exists in symbolic links encountered during resolution
              of the path argument.

              The length of a component of a pathname is longer than

       ENOENT A component of path does not name an existing file or path is
              an empty string.

              A component of the path prefix names an existing file that is
              neither a directory nor a symbolic link to a directory, or the
              path argument contains at least one non-<slash> character and
              ends with one or more trailing <slash> characters and the last
              pathname component names an existing file that is neither a
              directory nor a symbolic link to a directory.

       EPERM  The file named by path is a directory, and either the calling
              process does not have appropriate privileges, or the
              implementation prohibits using unlink() on directories.

       EPERM or EACCES
              The S_ISVTX flag is set on the directory containing the file
              referred to by the path argument and the process does not
              satisfy the criteria specified in the Base Definitions volume
              of POSIX.1‐2008, Section 4.2, Directory Protection.

       EROFS  The directory entry to be unlinked is part of a read-only file

       The unlinkat() function shall fail if:

       EACCES fd was not opened with O_SEARCH and the permissions of the
              directory underlying fd do not permit directory searches.

       EBADF  The path argument does not specify an absolute path and the fd
              argument is neither AT_FDCWD nor a valid file descriptor open
              for reading or searching.

              The path argument is not an absolute path and fd is a file
              descriptor associated with a non-directory file.

              The flag parameter has the AT_REMOVEDIR bit set and the path
              argument names a directory that is not an empty directory, or
              there are hard links to the directory other than dot or a
              single entry in dot-dot.

              The flag parameter has the AT_REMOVEDIR bit set and path does
              not name a directory.

       These functions may fail and not unlink the file if:

       EBUSY  The file named by path is a named STREAM.

       ELOOP  More than {SYMLOOP_MAX} symbolic links were encountered during
              resolution of the path argument.

              The length of a pathname exceeds {PATH_MAX}, or pathname
              resolution of a symbolic link produced an intermediate result
              with a length that exceeds {PATH_MAX}.

              The entry to be unlinked is the last directory entry to a pure
              procedure (shared text) file that is being executed.

       The unlinkat() function may fail if:

       EINVAL The value of the flag argument is not valid.

       The following sections are informative.

EXAMPLES         top

   Removing a Link to a File
       The following example shows how to remove a link to a file named
       /home/cnd/mod1 by removing the entry named /modules/pass1.

           #include <unistd.h>

           char *path = "/modules/pass1";
           int   status;
           status = unlink(path);

   Checking for an Error
       The following example fragment creates a temporary password lock file
       named LOCKFILE, which is defined as /etc/ptmp, and gets a file
       descriptor for it. If the file cannot be opened for writing, unlink()
       is used to remove the link between the file descriptor and LOCKFILE.

           #include <sys/types.h>
           #include <stdio.h>
           #include <fcntl.h>
           #include <errno.h>
           #include <unistd.h>
           #include <sys/stat.h>

           #define LOCKFILE "/etc/ptmp"

           int pfd;  /* Integer for file descriptor returned by open call. */
           FILE *fpfd;  /* File pointer for use in putpwent(). */
           /* Open password Lock file. If it exists, this is an error. */
           if ((pfd = open(LOCKFILE, O_WRONLY| O_CREAT | O_EXCL, S_IRUSR
               | S_IWUSR | S_IRGRP | S_IROTH)) == -1)  {
               fprintf(stderr, "Cannot open /etc/ptmp. Try again later.\n");

           /* Lock file created; proceed with fdopen of lock file so that
              putpwent() can be used.
           if ((fpfd = fdopen(pfd, "w")) == NULL) {

   Replacing Files
       The following example fragment uses unlink() to discard links to
       files, so that they can be replaced with new versions of the files.
       The first call removes the link to LOCKFILE if an error occurs.
       Successive calls remove the links to SAVEFILE and PASSWDFILE so that
       new links can be created, then removes the link to LOCKFILE when it
       is no longer needed.

           #include <sys/types.h>
           #include <stdio.h>
           #include <fcntl.h>
           #include <errno.h>
           #include <unistd.h>
           #include <sys/stat.h>

           #define LOCKFILE "/etc/ptmp"
           #define PASSWDFILE "/etc/passwd"
           #define SAVEFILE "/etc/opasswd"
           /* If no change was made, assume error and leave passwd unchanged. */
           if (!valid_change) {
               fprintf(stderr, "Could not change password for user %s\n", user);

           /* Change permissions on new password file. */
           chmod(LOCKFILE, S_IRUSR | S_IRGRP | S_IROTH);

           /* Remove saved password file. */

           /* Save current password file. */
           link(PASSWDFILE, SAVEFILE);

           /* Remove current password file. */

           /* Save new password file as current password file. */

           /* Remove lock file. */



       Applications should use rmdir() to remove a directory.

RATIONALE         top

       Unlinking a directory is restricted to the superuser in many
       historical implementations for reasons given in link() (see also

       The meaning of [EBUSY] in historical implementations is ``mount point
       busy''. Since this volume of POSIX.1‐2008 does not cover the system
       administration concepts of mounting and unmounting, the description
       of the error was changed to ``resource busy''. (This meaning is used
       by some device drivers when a second process tries to open an
       exclusive use device.) The wording is also intended to allow
       implementations to refuse to remove a directory if it is the root or
       current working directory of any process.

       The standard developers reviewed TR 24715‐2006 and noted that LSB-
       conforming implementations may return [EISDIR] instead of [EPERM]
       when unlinking a directory. A change to permit this behavior by
       changing the requirement for [EPERM] to [EPERM] or [EISDIR] was
       considered, but decided against since it would break existing
       strictly conforming and conforming applications. Applications written
       for portability to both POSIX.1‐2008 and the LSB should be prepared
       to handle either error code.

       The purpose of the unlinkat() function is to remove directory entries
       in directories other than the current working directory without
       exposure to race conditions. Any part of the path of a file could be
       changed in parallel to a call to unlink(), resulting in unspecified
       behavior. By opening a file descriptor for the target directory and
       using the unlinkat() function it can be guaranteed that the removed
       directory entry is located relative to the desired directory.



SEE ALSO         top

       close(3p), link(3p), remove(3p), rename(3p), rmdir(3p), symlink(3p)

       The Base Definitions volume of POSIX.1‐2008, Section 4.2, Directory
       Protection, fcntl.h(0p), unistd.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) In the event of any discrepancy between this version and
       the original IEEE and The Open Group Standard, the original IEEE and
       The Open Group Standard is the referee document. The original
       Standard can be obtained online at http://www.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the
       source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                          UNLINK(3P)

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