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UNLINK(3P) POSIX Programmer's Manual UNLINK(3P)
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.
unlink, unlinkat — remove a directory entry relative to directory
file descriptor
#include <unistd.h>
int unlink(const char *path);
int unlinkat(int fd, const char *path, int flag);
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.
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.
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.
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);
Applications should use rmdir() to remove a directory.
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.
None.
close(3p), link(3p), remove(3p), rename(3p), rmdir(3p), symlink(3p)
The Base Definitions volume of POSIX.1‐2008, Section 4.2, Directory
Protection, fcntl.h(0p), unistd.h(0p)
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 .
IEEE/The Open Group 2013 UNLINK(3P)
Pages that refer to this page: unistd.h(0p), cp(1p), ln(1p), rm(1p), rmdir(1p), unlink(1p), close(3p), fstatvfs(3p), link(3p), posix_fallocate(3p), remove(3p), rename(3p), rmdir(3p), symlink(3p), tempnam(3p), tmpfile(3p), tmpnam(3p)
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