Provided by: manpages-posix-dev_2013a-1_all 
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 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 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 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 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.1‐2008, 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 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.1‐2008 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 [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.
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 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 .