|
HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
|
|
|
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
#include <unistd.h>
int unlink(const char *path);
#include <fcntl.h>
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
access mode of the open file description associated with the file
descriptor is not O_SEARCH, the function shall check whether
directory searches are permitted using the current permissions of
the directory underlying the file descriptor. If the access mode
is 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.1‐2017, Section 4.3, Directory Protection.
EROFS The directory entry to be unlinked is part of a read-only
file system.
The unlinkat() function shall fail if:
EACCES The access mode of the open file description associated
with fd is not 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.1‐2017 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.1‐2017, Section 4.3,
Directory Protection, fcntl.h(0p), unistd.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
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 2017 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)
|
HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|