man-pages/man3p/unlink.3p

.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "UNLINK" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" unlink 
.SH NAME
unlink \- remove a directory entry
.SH SYNOPSIS
.LP
\fB#include <unistd.h>
.br
.sp
int unlink(const char *\fP\fIpath\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
The \fIunlink\fP() function shall remove a link to a file. If \fIpath\fP
names a symbolic link, \fIunlink\fP() shall remove
the symbolic link named by \fIpath\fP and shall not affect any file
or directory named by the contents of the symbolic link.
Otherwise, \fIunlink\fP() shall remove the link named by the pathname
pointed to by \fIpath\fP and shall decrement the link count
of the file referenced by the link.
.LP
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\fP() returns, but the removal of the file
contents shall be postponed until all references to the file
are closed.
.LP
The \fIpath\fP argument shall not name a directory unless the process
has appropriate privileges and the implementation
supports using \fIunlink\fP() on directories.
.LP
Upon successful completion, \fIunlink\fP() shall mark for update the
\fIst_ctime\fP and \fIst_mtime\fP fields of the parent
directory. Also, if the file's link count is not 0, the \fIst_ctime\fP
field of the file shall be marked for update.
.SH RETURN VALUE
.LP
Upon successful completion, 0 shall be returned. Otherwise, -1 shall
be returned and \fIerrno\fP set to indicate the error. If
-1 is returned, the named file shall not be changed.
.SH ERRORS
.LP
The \fIunlink\fP() function shall fail and shall not unlink the file
if:
.TP 7
.B 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 7
.B EBUSY
The file named by the \fIpath\fP argument cannot be unlinked because
it is being used by the system or another process and the
implementation considers this an error.
.TP 7
.B ELOOP
A loop exists in symbolic links encountered during resolution of the
\fIpath\fP argument.
.TP 7
.B ENAMETOOLONG
The length of the \fIpath\fP argument exceeds {PATH_MAX} or a pathname
component is longer than {NAME_MAX}.
.TP 7
.B ENOENT
A component of \fIpath\fP does not name an existing file or \fIpath\fP
is an empty string.
.TP 7
.B ENOTDIR
A component of the path prefix is not a directory.
.TP 7
.B EPERM
The file named by \fIpath\fP is a directory, and either the calling
process does not have appropriate privileges, or the
implementation prohibits using \fIunlink\fP() on directories.
.TP 7
.B EPERM \fRor\fP EACCES
.sp
The S_ISVTX flag is set on the directory containing the file referred
to by the \fIpath\fP argument and the caller is not the file
owner, nor is the caller the directory owner, nor does the caller
have appropriate privileges. 
.TP 7
.B EROFS
The directory entry to be unlinked is part of a read-only file system.
.sp
.LP
The \fIunlink\fP() function may fail and not unlink the file if:
.TP 7
.B EBUSY
The file named by \fIpath\fP is a named STREAM. 
.TP 7
.B ELOOP
More than {SYMLOOP_MAX} symbolic links were encountered during resolution
of the \fIpath\fP argument.
.TP 7
.B ENAMETOOLONG
As a result of encountering a symbolic link in resolution of the \fIpath\fP
argument, the length of the substituted pathname
string exceeded {PATH_MAX}.
.TP 7
.B ETXTBSY
The entry to be unlinked is the last directory entry to a pure procedure
(shared text) file that is being executed.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.SS Removing a Link to a File
.LP
The following example shows how to remove a link to a file named \fB/home/cnd/mod1\fP
by removing the entry named
\fB/modules/pass1\fP.
.sp
.RS
.nf

\fB#include <unistd.h>
.sp

char *path = "/modules/pass1";
int   status;
\&...
status = unlink(path);
\fP
.fi
.RE
.SS Checking for an Error
.LP
The following example fragment creates a temporary password lock file
named \fBLOCKFILE\fP, which is defined as
\fB/etc/ptmp\fP, and gets a file descriptor for it. If the file cannot
be opened for writing, \fIunlink\fP() is used to remove
the link between the file descriptor and \fBLOCKFILE\fP.
.sp
.RS
.nf

\fB#include <sys/types.h>
#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <sys/stat.h>
.sp

#define LOCKFILE "/etc/ptmp"
.sp

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);
}
.sp

/* 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);
}
\fP
.fi
.RE
.SS Replacing Files
.LP
The following example fragment uses \fIunlink\fP() to discard links
to files, so that they can be replaced with new versions of
the files. The first call removes the link to \fBLOCKFILE\fP if an
error occurs. Successive calls remove the links to
\fBSAVEFILE\fP and \fBPASSWDFILE\fP so that new links can be created,
then removes the link to \fBLOCKFILE\fP when it is no
longer needed.
.sp
.RS
.nf

\fB#include <sys/types.h>
#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <sys/stat.h>
.sp

#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);
}
.sp

/* Change permissions on new password file. */
chmod(LOCKFILE, S_IRUSR | S_IRGRP | S_IROTH);
.sp

/* Remove saved password file. */
unlink(SAVEFILE);
.sp

/* Save current password file. */
link(PASSWDFILE, SAVEFILE);
.sp

/* Remove current password file. */
unlink(PASSWDFILE);
.sp

/* Save new password file as current password file. */
link(LOCKFILE,PASSWDFILE);
.sp

/* Remove lock file. */
unlink(LOCKFILE);
.sp

exit(0);
\fP
.fi
.RE
.SH APPLICATION USAGE
.LP
Applications should use \fIrmdir\fP() to remove a directory.
.SH RATIONALE
.LP
Unlinking a directory is restricted to the superuser in many historical
implementations for reasons given in \fIlink\fP() (see also \fIrename\fP()).
.LP
The meaning of [EBUSY] in historical implementations is "mount point
busy". Since this volume of
IEEE\ Std\ 1003.1-2001 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.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIclose\fP() , \fIlink\fP() , \fIremove\fP() , \fIrmdir\fP() , the
Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI<unistd.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .