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unlink: odebrat položku adresáře

Originální popis anglicky: unlink - remove a directory entry

Návod, kniha: POSIX Programmer's Manual


#include <unistd.h>
int unlink(const char * path);


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 st_ctime and st_mtime fields of the parent directory. Also, if the file's link count is not 0, the st_ctime field of the file shall be marked for update.


Upon successful completion, 0 shall be returned. Otherwise, -1 shall be returned and errno set to indicate the error. If -1 is returned, the named file shall not be changed.


The unlink() function shall fail and shall not unlink the file if:
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.
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.
A loop exists in symbolic links encountered during resolution of the path argument.
The length of the path argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}.
A component of path does not name an existing file or path is an empty string.
A component of the path prefix is not a directory.
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.
The S_ISVTX flag is set on the directory containing the file referred to by the path argument and the caller is not the file owner, nor is the caller the directory owner, nor does the caller have appropriate privileges.
The directory entry to be unlinked is part of a read-only file system.
The unlink() function may fail and not unlink the file if:
The file named by path is a named STREAM.
More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.
As a result of encountering a symbolic link in resolution of the path argument, the length of the substituted pathname string exceeded {PATH_MAX}.
The entry to be unlinked is the last directory entry to a pure procedure (shared text) file that is being executed.
The following sections are informative.


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);


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 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.




close() , link() , remove() , rmdir() , the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h> 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 .
2003 IEEE/The Open Group
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