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



PPRROOLLOOGG
       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.


NNAAMMEE
       unlink, unlinkat -- remove a directory entry relative to directory file
       descriptor

SSYYNNOOPPSSIISS
       #include <unistd.h>

       int unlink(const char *_p_a_t_h);
       int unlinkat(int _f_d, const char *_p_a_t_h, int _f_l_a_g);

DDEESSCCRRIIPPTTIIOONN
       The _u_n_l_i_n_k() function shall remove a link to a file. If _p_a_t_h names a
       symbolic link, _u_n_l_i_n_k() shall remove the symbolic link named by _p_a_t_h
       and shall not affect any file or directory named by the contents of the
       symbolic link. Otherwise, _u_n_l_i_n_k() shall remove the link named by the
       pathname pointed to by _p_a_t_h 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 _u_n_l_i_n_k()
       returns, but the removal of the file contents shall be postponed until
       all references to the file are closed.

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

       Upon successful completion, _u_n_l_i_n_k() 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 _u_n_l_i_n_k_a_t() function shall be equivalent to the _u_n_l_i_n_k() or _r_m_d_i_r()
       function except in the case where _p_a_t_h specifies a relative path. In
       this case the directory entry to be removed is determined relative to
       the directory associated with the file descriptor _f_d 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 _f_l_a_g are constructed by a bitwise-inclusive OR of flags from
       the following list, defined in _<_f_c_n_t_l_._h_>:

       AT_REMOVEDIR
             Remove the directory entry specified by _f_d and _p_a_t_h as a
             directory, not a normal file.

       If _u_n_l_i_n_k_a_t() is passed the special value AT_FDCWD in the _f_d parameter,
       the current working directory shall be used and the behavior shall be
       identical to a call to _u_n_l_i_n_k() or _r_m_d_i_r() respectively, depending on
       whether or not the AT_REMOVEDIR bit is set in _f_l_a_g.

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

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

       EEAACCCCEESS 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.

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

       EELLOOOOPP  A loop exists in symbolic links encountered during resolution of
              the _p_a_t_h argument.

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

       EENNOOEENNTT A component of _p_a_t_h does not name an existing file or _p_a_t_h is an
              empty string.

       EENNOOTTDDIIRR
              A component of the path prefix names an existing file that is
              neither a directory nor a symbolic link to a directory, or the
              _p_a_t_h 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.

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

       EEPPEERRMM or EEAACCCCEESS
              The S_ISVTX flag is set on the directory containing the file
              referred to by the _p_a_t_h argument and the process does not
              satisfy the criteria specified in the Base Definitions volume of
              POSIX.1-2008, _S_e_c_t_i_o_n _4_._2, _D_i_r_e_c_t_o_r_y _P_r_o_t_e_c_t_i_o_n.

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

       The _u_n_l_i_n_k_a_t() function shall fail if:

       EEAACCCCEESS _f_d was not opened with O_SEARCH and the permissions of the
              directory underlying _f_d do not permit directory searches.

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

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

       EEEEXXIISSTT or EENNOOTTEEMMPPTTYY
              The _f_l_a_g parameter has the AT_REMOVEDIR bit set and the _p_a_t_h
              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.

       EENNOOTTDDIIRR
              The _f_l_a_g parameter has the AT_REMOVEDIR bit set and _p_a_t_h does
              not name a directory.

       These functions may fail and not unlink the file if:

       EEBBUUSSYY  The file named by _p_a_t_h is a named STREAM.

       EELLOOOOPP  More than {SYMLOOP_MAX} symbolic links were encountered during
              resolution of the _p_a_t_h argument.

       EENNAAMMEETTOOOOLLOONNGG
              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}.

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

       The _u_n_l_i_n_k_a_t() function may fail if:

       EEIINNVVAALL The value of the _f_l_a_g argument is not valid.

       _T_h_e _f_o_l_l_o_w_i_n_g _s_e_c_t_i_o_n_s _a_r_e _i_n_f_o_r_m_a_t_i_v_e_.

EEXXAAMMPPLLEESS
   RReemmoovviinngg aa LLiinnkk ttoo aa FFiillee
       The following example shows how to remove a link to a file named
       //hhoommee//ccnndd//mmoodd11 by removing the entry named //mmoodduulleess//ppaassss11.

           ##iinncclluuddee <<uunniissttdd..hh>>

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

   CChheecckkiinngg ffoorr aann EErrrroorr
       The following example fragment creates a temporary password lock file
       named LLOOCCKKFFIILLEE, which is defined as //eettcc//ppttmmpp, and gets a file
       descriptor for it. If the file cannot be opened for writing, _u_n_l_i_n_k()
       is used to remove the link between the file descriptor and LLOOCCKKFFIILLEE.

           ##iinncclluuddee <<ssyyss//ttyyppeess..hh>>
           ##iinncclluuddee <<ssttddiioo..hh>>
           ##iinncclluuddee <<ffccnnttll..hh>>
           ##iinncclluuddee <<eerrrrnnoo..hh>>
           ##iinncclluuddee <<uunniissttdd..hh>>
           ##iinncclluuddee <<ssyyss//ssttaatt..hh>>

           #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);
           }

   RReeppllaacciinngg FFiilleess
       The following example fragment uses _u_n_l_i_n_k() to discard links to files,
       so that they can be replaced with new versions of the files. The first
       call removes the link to LLOOCCKKFFIILLEE if an error occurs. Successive calls
       remove the links to SSAAVVEEFFIILLEE and PPAASSSSWWDDFFIILLEE so that new links can be
       created, then removes the link to LLOOCCKKFFIILLEE when it is no longer needed.

           ##iinncclluuddee <<ssyyss//ttyyppeess..hh>>
           ##iinncclluuddee <<ssttddiioo..hh>>
           ##iinncclluuddee <<ffccnnttll..hh>>
           ##iinncclluuddee <<eerrrrnnoo..hh>>
           ##iinncclluuddee <<uunniissttdd..hh>>
           ##iinncclluuddee <<ssyyss//ssttaatt..hh>>

           #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);

AAPPPPLLIICCAATTIIOONN UUSSAAGGEE
       Applications should use _r_m_d_i_r() to remove a directory.

RRAATTIIOONNAALLEE
       Unlinking a directory is restricted to the superuser in many historical
       implementations for reasons given in _l_i_n_k() (see also _r_e_n_a_m_e()).

       The meaning of [[EEBBUUSSYY]] 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 [[EEIISSDDIIRR]] instead of [[EEPPEERRMM]] when
       unlinking a directory. A change to permit this behavior by changing the
       requirement for [[EEPPEERRMM]] to [[EEPPEERRMM]] or [[EEIISSDDIIRR]] 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 _u_n_l_i_n_k_a_t() 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 _u_n_l_i_n_k(), resulting in unspecified
       behavior. By opening a file descriptor for the target directory and
       using the _u_n_l_i_n_k_a_t() function it can be guaranteed that the removed
       directory entry is located relative to the desired directory.

FFUUTTUURREE DDIIRREECCTTIIOONNSS
       None.

SSEEEE AALLSSOO
       _c_l_o_s_e(), _l_i_n_k(), _r_e_m_o_v_e(), _r_e_n_a_m_e(), _r_m_d_i_r(), _s_y_m_l_i_n_k()

       The Base Definitions volume of POSIX.1-2008, _S_e_c_t_i_o_n _4_._2, _D_i_r_e_c_t_o_r_y
       _P_r_o_t_e_c_t_i_o_n, <<ffccnnttll..hh>>, <<uunniissttdd..hh>>

CCOOPPYYRRIIGGHHTT
       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(3)
