unlink(3p) - phpMan

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

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 - remove a directory entry
SYNOPSIS
       #include <unistd.h>
       int unlink(const char *path);

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 st_ctime
       and  st_mtime  fields of the parent directory. Also, if the file's link
       count is not 0, the st_ctime field of the  file  shall  be  marked  for
       update.
RETURN VALUE
       Upon successful completion, 0 shall be returned. Otherwise, -1 shall be
       returned and errno set to indicate the error. If -1  is  returned,  the
       named file shall not be changed.
ERRORS
       The unlink() function 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 imple-
              mentation considers this an error.
       ELOOP  A loop exists in symbolic links encountered during resolution of
              the path argument.
       ENAMETOOLONG
              The length of the path argument exceeds {PATH_MAX} or a pathname
              component 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 is not a directory.
       EPERM  The file named by path is a directory, and  either  the  calling
              process does not have appropriate privileges, or the implementa-
              tion 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 caller is not the file
              owner, nor is the caller the directory owner, nor does the call-
              er have appropriate privileges.
       EROFS  The  directory  entry to be unlinked is part of a read-only file
              system.

       The unlink() function 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
              As a result of encountering a symbolic link in resolution of the
              path argument, the length of  the  substituted  pathname  string
              exceeded {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 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 descrip-
       tor  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 IEEE Std 1003.1-2001  does  not  cover  the
       system administration concepts of mounting and unmounting, the descrip-
       tion 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.
FUTURE DIRECTIONS
       None.
SEE ALSO
       close(), link(), remove(), rmdir(),  the  Base  Definitions  volume  of
       IEEE Std 1003.1-2001, <unistd.h>
COPYRIGHT
       Portions  of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       --  Portable  Operating  System  Interface (POSIX), The Open Group Base
       Specifications Issue 6, Copyright (C) 2001-2003  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 .

IEEE/The Open Group                  2003                           UNLINK(3P)