UNLINK(P) POSIX Programmer's Manual UNLINK(P)NAMEunlink - 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 ALSOclose() , 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(P)
