# unlink

## PROLOG



           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



           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 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");
                   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.
    


* * *