UNLINK(3POSIX)

📦 version 2017a, in debian bookworm, debian trixie, ubuntu jammy, ubuntu noble, ubuntu resolute, ubuntu stonking

✨ PROLOG ✨

okay so bestie, welcome to the UNLINK(3POSIX) page 💖 This manual page is part of the POSIX Programmer's Manual, periodt. But heads up: 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. So like, don't come for me if Linux does its own thing 💅.

👑 NAME 👑

unlink, unlinkat — remove a directory entry 💅

🌸 SYNOPSIS 🌸

#include <unistd.h>

int unlink(const char *path);

#include <fcntl.h>

int unlinkat(int fd, const char *path, int flag);

💅 DESCRIPTION 💅

okay so unlink() is the main character here: it removes 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.

🎀 RETURN VALUE 🎀

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. 😭

🔥 ERRORS 🔥

  • 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.
  • 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.

🌈 EXAMPLES 🌈

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);

💜 APPLICATION USAGE 💜

Applications should use rmdir() to remove a directory. 🫶

💖 RATIONALE 💖

okay so bestie, why is this whole thing like this? history, mostly. let's get into it 🫶

Unlinking a directory is restricted to the superuser in lots of historical implementations, and the reasons for that are given over in link() (also peep rename() while you're there, iykyk).

The meaning of [EBUSY] in those historical implementations was literally mount point busy''</hotpink>. But since this volume of POSIX.1‐2017 does **not** cover the system administration concepts of mounting and unmounting, the description of the error got changed to <hotpink>resource busy'' instead. (and lowkey, this meaning is what some device drivers use when a second process tries to open an exclusive use device 👀). The wording is also intended to let implementations refuse to remove a directory if it's the root or the current working directory of any process. so like, don't come for them, it's on purpose ✨

The standard developers reviewed TR 24715‐2006 and noted that LSB-conforming implementations may return [EISDIR] instead of [EPERM] when unlinking a directory. They considered changing the requirement for [EPERM] to [EPERM] or [EISDIR] to permit that behavior... and then decided against it, because it would break existing strictly conforming and conforming applications 😭. So the tea is: applications written for portability to both POSIX.1‐2008 and the LSB should be prepared to handle either error code. fr fr, handle both. 👑

Now, the purpose of the unlinkat() function? main character energy, honestly. It exists so you can remove directory entries in directories other than the current working directory without exposure to race conditions. Because any part of the path of a file could be changed in parallel to a call to unlink(), which results in unspecified behavior 💀. By opening a file descriptor for the target directory and using the unlinkat() function instead, it can be guaranteed that the removed directory entry is located relative to the desired directory. chef's kiss, no race, we love to see it 🎀

🔮 FUTURE DIRECTIONS 🔮

None.

that's it. that's the section. nothing coming. the future is empty, bestie 🌈

👀 SEE ALSO 👀

  • close()
  • link()
  • remove()
  • rename()
  • rmdir()
  • symlink()

and also, from The Base Definitions volume of POSIX.1‐2017:

  • Section 4.3, Directory Protection
  • <fcntl.h>
  • <unistd.h>

go read those, they're giving very much "required reading" 📚✨

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 .

and that's the whole page, bestie!! 💜 this masterpiece was brought to you by IEEE/The Open Group, version 2017, dated UNLINK(3POSIX) — and you know it was written with love (and race conditions in mind). now go unlink something responsibly, mwah 🫶✨💅

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