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



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

NAME
       unlink, unlinkat -- remove a directory entry

SYNOPSIS
       #include <unistd.h>

       int unlink(const char *path);

       #include <fcntl.h>

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

DESCRIPTION
       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 access mode of the open file
       description associated with the file descriptor is not O_SEARCH, the
       function shall check whether directory searches are permitted using the
       current permissions of the directory underlying the file descriptor. If
       the access mode is 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>:

       AT_REMOVEDIR
             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
       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
       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.

       ENAMETOOLONG
              The length of a component of a pathname is longer than
              {NAME_MAX}.

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

       ENOTDIR
              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.1-2017, Section 4.3, Directory Protection.

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

       The unlinkat() function shall fail if:

       EACCES The access mode of the open file description associated with fd
              is not 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.

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

       EEXIST or ENOTEMPTY
              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.

       ENOTDIR
              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.

       ENAMETOOLONG
              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}.

       ETXTBSY
              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
   Removing aa Link to aa 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");
               exit(1);
           }

           /* Lock file created; proceed with fdopen of lock file so that
              putpwent() can be used.
            */
           if ((fpfd = fdopen(pfd, "w")) == NULL) {
               close(pfd);
               unlink(LOCKFILE);
               exit(1);
           }

   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);
               unlink(LOCKFILE);
               exit(1);
           }

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

           /* Remove saved password file. */
           unlink(SAVEFILE);

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

           /* Remove current password file. */
           unlink(PASSWDFILE);

           /* Save new password file as current password file. */
           link(LOCKFILE,PASSWDFILE);

           /* Remove lock file. */
           unlink(LOCKFILE);

           exit(0);

APPLICATION USAGE
       Applications should use rmdir() to remove a directory.

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

       The meaning of [EBUSY] in historical implementations is ``mount point
       busy''. Since this volume of POSIX.1-2017 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.

FUTURE DIRECTIONS
       None.

SEE ALSO
       close(), link(), remove(), rename(), rmdir(), symlink()

       The Base Definitions volume of POSIX.1-2017, Section 4.3, Directory
       Protection, <fcntl.h>, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  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.opengroup.org/unix/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                  2017                            UNLINK(3)
