UNLINK(3P) POSIX Programmer's Manual UNLINK(3P)
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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 relative to directory file
descriptor
SYNOPSIS
#include <unistd.h>
int unlink(const char *path);
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 sta-
tus 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 file descriptor was opened without
O_SEARCH, the function shall check whether directory searches are per-
mitted 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 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 direc-
tory, 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 imple-
mentation 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 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 process does not sat-
isfy the criteria specified in the Base Definitions volume of
POSIX.1-2008, Section 4.2, Directory Protection.
EROFS The directory entry to be unlinked is part of a read-only file
system.
The unlinkat() function shall fail if:
EACCES fd was not opened with 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 resolu-
tion 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 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 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-con-
forming 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 expo-
sure 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.1-2008, Section 4.2, Directory
Protection, <fcntl.h>, <unistd.h>
COPYRIGHT
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 Electri-
cal 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.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 UNLINK(3P)
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