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unlink.3p (12175B)

'\" et
.TH UNLINK "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH 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.
.\"
.SH NAME
unlink, unlinkat
\(em remove a directory entry
.SH SYNOPSIS
.LP
.nf
#include <unistd.h>
.P
int unlink(const char *\fIpath\fP);
.P
#include <fcntl.h>
.P
int unlinkat(int \fIfd\fP, const char *\fIpath\fP, int \fIflag\fP);
.fi
.SH DESCRIPTION
The
\fIunlink\fR()
function shall remove a link to a file. If
.IR path
names a symbolic link,
\fIunlink\fR()
shall remove the symbolic link named by
.IR path
and shall not affect any file or directory named by the contents of the
symbolic link. Otherwise,
\fIunlink\fR()
shall remove the link named by the pathname pointed to by
.IR path
and shall decrement the link count of the file referenced by the link.
.P
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
\fIunlink\fR()
returns, but the removal of the file contents shall be postponed until
all references to the file are closed.
.P
The
.IR path
argument shall not name a directory unless the process has appropriate
privileges and the implementation supports using
\fIunlink\fR()
on directories.
.P
Upon successful completion,
\fIunlink\fR()
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.
.P
The
\fIunlinkat\fR()
function shall be equivalent to the
\fIunlink\fR()
or
\fIrmdir\fR()
function except in the case where
.IR 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
.IR 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.
.P
Values for
.IR flag
are constructed by a bitwise-inclusive OR of flags from the following
list, defined in
.IR <fcntl.h> :
.IP AT_REMOVEDIR 6
.br
Remove the directory entry specified by
.IR fd
and
.IR path
as a directory, not a normal file.
.P
If
\fIunlinkat\fR()
is passed the special value AT_FDCWD in the
.IR fd
parameter, the current working directory shall be used and the behavior
shall be identical to a call to
\fIunlink\fR()
or
\fIrmdir\fR()
respectively, depending on whether or not the AT_REMOVEDIR bit is set in
.IR flag .
.SH "RETURN VALUE"
Upon successful completion, these functions shall return 0. Otherwise,
these functions shall return \-1 and set
.IR errno
to indicate the error. If \-1 is returned, the named file shall not
be changed.
.SH ERRORS
These functions shall fail and shall not unlink the file if:
.TP
.BR 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.
.TP
.BR EBUSY
The file named by the
.IR path
argument cannot be unlinked because it is being used by the system or
another process and the implementation considers this an error.
.TP
.BR ELOOP
A loop exists in symbolic links encountered during resolution of the
.IR path
argument.
.TP
.BR ENAMETOOLONG
.br
The length of a component of a pathname is longer than
{NAME_MAX}.
.TP
.BR ENOENT
A component of
.IR path
does not name an existing file or
.IR path
is an empty string.
.TP
.BR 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
.IR path
argument contains at least one non-\c
<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.
.TP
.BR EPERM
The file named by
.IR path
is a directory, and either the calling process does not have
appropriate privileges, or the implementation prohibits using
\fIunlink\fR()
on directories.
.TP
.BR EPERM " or " EACCES
.br
The S_ISVTX flag is set on the directory containing the file referred
to by the
.IR path
argument and the process does not satisfy the criteria specified in the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 4.3" ", " "Directory Protection".
.TP
.BR EROFS
The directory entry to be unlinked is part of a read-only file system.
.P
The
\fIunlinkat\fR()
function shall fail if:
.TP
.BR EACCES
The access mode of the open file description associated with
.IR fd
is not O_SEARCH and the permissions of the directory underlying
.IR fd
do not permit directory searches.
.TP
.BR EBADF
The
.IR path
argument does not specify an absolute path and the
.IR fd
argument is neither AT_FDCWD nor a valid file descriptor open for reading
or searching.
.TP
.BR ENOTDIR
The
.IR path
argument is not an absolute path and
.IR fd
is a file descriptor associated with a non-directory file.
.TP
.BR EEXIST " or " ENOTEMPTY
.br
The
.IR flag
parameter has the AT_REMOVEDIR bit set and the
.IR 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.
.TP
.BR ENOTDIR
The
.IR flag
parameter has the AT_REMOVEDIR bit set and
.IR path
does not name a directory.
.P
These functions may fail and not unlink the file if:
.TP
.BR EBUSY
The file named by
.IR path
is a named STREAM.
.TP
.BR ELOOP
More than
{SYMLOOP_MAX}
symbolic links were encountered during resolution of the
.IR path
argument.
.TP
.BR ENAMETOOLONG
.br
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}.
.TP
.BR ETXTBSY
The entry to be unlinked is the last directory entry to a pure
procedure (shared text) file that is being executed.
.br
.P
The
\fIunlinkat\fR()
function may fail if:
.TP
.BR EINVAL
The value of the
.IR flag
argument is not valid.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Removing a Link to a File"
.P
The following example shows how to remove a link to a file named
.BR /home/cnd/mod1
by removing the entry named
.BR /modules/pass1 .
.sp
.RS 4
.nf
#include <unistd.h>
.P
char *path = "/modules/pass1";
int   status;
\&...
status = unlink(path);
.fi
.P
.RE
.SS "Checking for an Error"
.P
The following example fragment creates a temporary password lock file
named
.BR LOCKFILE ,
which is defined as
.BR /etc/ptmp ,
and gets a file descriptor for it. If the file cannot be opened for
writing,
\fIunlink\fR()
is used to remove the link between the file descriptor and
.BR LOCKFILE .
.sp
.RS 4
.nf
#include <sys/types.h>
#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <sys/stat.h>
.P
#define LOCKFILE "/etc/ptmp"
.P
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.\en");
    exit(1);
}
.P
/* 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);
}
.fi
.P
.RE
.SS "Replacing Files"
.P
The following example fragment uses
\fIunlink\fR()
to discard links to files, so that they can be replaced with new
versions of the files. The first call removes the link to
.BR LOCKFILE
if an error occurs. Successive calls remove the links to
.BR SAVEFILE
and
.BR PASSWDFILE
so that new links can be created, then removes the link to
.BR LOCKFILE
when it is no longer needed.
.sp
.RS 4
.nf
#include <sys/types.h>
#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <sys/stat.h>
.P
#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\en", user);
    unlink(LOCKFILE);
    exit(1);
}
.P
/* Change permissions on new password file. */
chmod(LOCKFILE, S_IRUSR | S_IRGRP | S_IROTH);
.P
/* Remove saved password file. */
unlink(SAVEFILE);
.P
/* Save current password file. */
link(PASSWDFILE, SAVEFILE);
.P
/* Remove current password file. */
unlink(PASSWDFILE);
.P
/* Save new password file as current password file. */
link(LOCKFILE,PASSWDFILE);
.P
/* Remove lock file. */
unlink(LOCKFILE);
.P
exit(0);
.fi
.P
.RE
.SH "APPLICATION USAGE"
Applications should use
\fIrmdir\fR()
to remove a directory.
.SH RATIONALE
Unlinking a directory is restricted to the superuser
in many historical implementations for reasons given in
\fIlink\fR()
(see also
\fIrename\fR()).
.P
The meaning of
.BR [EBUSY] 
in historical implementations is ``mount point busy''. Since this volume of POSIX.1\(hy2017 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.
.P
The standard developers reviewed TR 24715\(hy2006 and noted that
LSB-conforming implementations may return
.BR [EISDIR] 
instead of
.BR [EPERM] 
when unlinking a directory. A change to permit this behavior by
changing the requirement for
.BR [EPERM] 
to
.BR [EPERM] 
or
.BR [EISDIR] 
was considered, but decided against since it would break existing
strictly conforming and conforming applications. Applications written
for portability to both POSIX.1\(hy2008 and the LSB should be prepared to
handle either error code.
.P
The purpose of the
\fIunlinkat\fR()
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
\fIunlink\fR(),
resulting in unspecified behavior. By opening a file descriptor for
the target directory and using the
\fIunlinkat\fR()
function it can be guaranteed that the removed directory entry is
located relative to the desired directory.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIclose\fR\^(\|)",
.IR "\fIlink\fR\^(\|)",
.IR "\fIremove\fR\^(\|)",
.IR "\fIrename\fR\^(\|)",
.IR "\fIrmdir\fR\^(\|)",
.IR "\fIsymlink\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 4.3" ", " "Directory Protection",
.IR "\fB<fcntl.h>\fP",
.IR "\fB<unistd.h>\fP"
.\"
.SH COPYRIGHT
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 .
.PP
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 .