NAME

link, linkat — hard link one file to another file

SYNOPSIS

#include <unistd.h>

int link(const char *path1, const char *path2);

[OH] #include <fcntl.h>

int linkat(int fd1, const char *path1, int fd2,
       const char *path2, int flag);


DESCRIPTION

The link() function shall create a new hard link (directory entry) for the existing file, path1.

The path1 argument points to a pathname naming an existing file. The path2 argument points to a pathname naming the new directory entry to be created. The link() function shall atomically create a new hard link for the existing file and the link count of the file shall be incremented by one.

If path1 names a directory, link() shall fail unless the process has appropriate privileges and the implementation supports using link() on directories.

If path1 names a symbolic link, it is implementation-defined whether link() follows the symbolic link, or creates a new hard link to the symbolic link itself.

Upon successful completion, link() shall mark for update the last file status change timestamp of the file. Also, the last data modification and last file status change timestamps of the directory that contains the new entry shall be marked for update.

If link() fails, no link shall be created and the link count of the file shall remain unchanged.

The implementation may require that the calling process has permission to access the existing file.

The linkat() function shall be equivalent to the link() function except that symbolic links shall be handled as specified by the value of flag (see below) and except in the case where either path1 or path2 or both are relative paths. In this case a relative path path1 is interpreted relative to the directory associated with the file descriptor fd1 instead of the current working directory and similarly for path2 and the file descriptor fd2. 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_SYMLINK_FOLLOW

If path1 names a symbolic link, a new hard link for the target of the symbolic link is created.

If linkat() is passed the special value AT_FDCWD in the fd1 or fd2 parameter, the current working directory shall be used for the respective path argument. If both fd1 and fd2 have value AT_FDCWD, the behavior shall be identical to a call to link(), except that symbolic links shall be handled as specified by the value of flag.

If the AT_SYMLINK_FOLLOW flag is clear in the flag argument and the path1 argument names a symbolic link, a new hard link is created for the symbolic link path1 and not its target.

RETURN VALUE

Upon successful completion, these functions shall return 0. Otherwise, these functions shall return -1 and set errno to indicate the error.

ERRORS

These functions shall fail if:

[EACCES]

A component of either path prefix denies search permission, or the requested link requires writing in a directory that denies write permission, or the calling process does not have permission to access the existing file and this is required by the implementation.

[EEXIST]

The path2 argument resolves to an existing directory entry or refers to a symbolic link.

[EILSEQ]

The last pathname component of path2 is not a portable filename, and cannot be created in the target directory.

[ELOOP]

A loop exists in symbolic links encountered during resolution of the path1 or path2 argument.

[EMLINK]

The number of hard links to the file named by path1 would exceed {LINK_MAX}.

[ENAMETOOLONG]

The length of a component of a pathname is longer than {NAME_MAX}.

[ENOENT]

A component of either path prefix does not exist; the file named by path1 does not exist; or path1 or path2 points to an empty string.

[ENOENT] or [ENOTDIR]

The path1 argument names an existing non-directory file, and the path2 argument contains at least one non-<slash> character and ends with one or more trailing <slash> characters. If path2 without the trailing <slash> characters would name an existing file, an [ENOENT] error shall not occur.

[ENOSPC]

The directory to contain the link cannot be extended.

[ENOTDIR]

A component of either path prefix names an existing file that is neither a directory nor a symbolic link to a directory, or the path1 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, or the path1 argument names an existing non-directory file and the path2 argument names a nonexistent file, contains at least one non-<slash> character, and ends with one or more trailing <slash> characters.

[EPERM]

The file named by path1 is a directory and either the calling process does not have appropriate privileges or the implementation prohibits using link() on directories.

[EROFS]

The requested link requires writing in a directory on a read-only file system.

[EXDEV]

The file named by path1 and the directory in which the directory entry named by path2 is to be created are on different file systems and the implementation does not support hard links between file systems.

The linkat() function shall fail if:

[EACCES]

The access mode of the open file description associated with fd1 or fd2 is not O_SEARCH and the permissions of the directory underlying fd1 or fd2, respectively, do not permit directory searches.

[EBADF]

The path1 or path2 argument does not specify an absolute path and the fd1 or fd2 argument, respectively, is neither AT_FDCWD nor a valid file descriptor open for reading or searching.

[ENOTDIR]

The path1 or path2 argument is not an absolute path and fd1 or fd2, respectively, is a file descriptor associated with a non-directory file.

These functions may fail if:

[ELOOP]

More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path1 or path2 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}.

The linkat() function may fail if:

[EINVAL]

The value of the flag argument is not valid.

EXAMPLES

Creating a Hard Link to a File

The following example shows how to create an additional hard link to a file named /home/cnd/mod1 by creating a new directory entry named /modules/pass1.

#include <unistd.h>


char *path1 = "/home/cnd/mod1";
char *path2 = "/modules/pass1";
int   status;
...
status = link (path1, path2);

Creating a Hard Link to a File Within a Program

In the following program example, the link() function hard links the /etc/passwd file (defined as PASSWDFILE) to a file named /etc/opasswd (defined as SAVEFILE), which is used to save the current password file. Then, after removing the current password file (defined as PASSWDFILE), the new password file is saved as the current password file using the link() function again.

#include <unistd.h>


#define LOCKFILE "/etc/ptmp"
#define PASSWDFILE "/etc/passwd"
#define SAVEFILE "/etc/opasswd"
...
/* Save current password file */
link (PASSWDFILE, SAVEFILE);


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


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

APPLICATION USAGE

Some implementations do allow hard links between file systems.

If path1 refers to a symbolic link, application developers should use linkat() with appropriate flags to select whether or not the symbolic link should be resolved.

RATIONALE

Creating additional hard links to a directory is restricted to the superuser in most historical implementations because this capability may produce loops in the file hierarchy or otherwise corrupt the file system. This volume of POSIX.1-2024 continues that philosophy by prohibiting link() and unlink() from doing this. Other functions could do it if the implementor designed such an extension.

Some historical implementations allow hard linking of files on different file systems. Wording was added to explicitly allow this optional behavior.

The exception for cross-file system hard links is intended to apply only to links that are programmatically indistinguishable from traditional hard links.

The purpose of the linkat() function is to link files 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 link(), resulting in unspecified behavior. By opening a file descriptor for the directory of both the existing file and the target location and using the linkat() function it can be guaranteed that the both filenames are in the desired directories.

Earlier versions of this standard specified only the link() function, and required it to behave like linkat() with the AT_SYMLINK_FOLLOW flag. However, historical practice from SVR4 and Linux kernels had link() behaving like linkat() with no flags, and many systems that attempted to provide a conforming link() function did so in a way that was rarely used, and when it was used did not conform to the standard (e.g., by not being atomic, or by dereferencing the symbolic link incorrectly). Since applications could not rely on link() following symbolic links in practice, the linkat() function was added taking a flag to specify the desired behavior for the application.

Implementations are encouraged to have link() and linkat() report an [EILSEQ] error if the file named by path2 did not previously exist, and the last component of that pathname contains any bytes that have the encoded value of a <newline> character.

FUTURE DIRECTIONS

None.

SEE ALSO

rename, symlink, unlink

XBD <fcntl.h>, <unistd.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 6

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

• The [ELOOP] mandatory error condition is added.

• A second [ENAMETOOLONG] is added as an optional error condition.

The following changes were made to align with the IEEE P1003.1a draft standard:

• An explanation is added of the action when path2 refers to a symbolic link.

• The [ELOOP] optional error condition is added.

Issue 7

Austin Group Interpretation 1003.1-2001 #143 is applied.

SD5-XSH-ERN-93 is applied, adding RATIONALE.

The linkat() function is added from The Open Group Technical Standard, 2006, Extended API Set Part 2.

Functionality relating to XSI STREAMS is marked obsolescent.

Changes are made related to support for finegrained timestamps.

The [EOPNOTSUPP] error is removed.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0354 [326], XSH/TC1-2008/0355 [461], XSH/TC1-2008/0356 [326], XSH/TC1-2008/0357 [324], XSH/TC1-2008/0358 [147,429], XSH/TC1-2008/0359 [277], XSH/TC1-2008/0360 [278], and XSH/TC1-2008/0361 [278] are applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0195 [873], XSH/TC2-2008/0196 [591], XSH/TC2-2008/0197 [817], XSH/TC2-2008/0198 [822], and XSH/TC2-2008/0199 [817] are applied.

Issue 8

Austin Group Defect 251 is applied, encouraging implementations to disallow the creation of filenames containing any bytes that have the encoded value of a <newline> character.

Austin Group Defect 293 is applied, adding the [EILSEQ] error.

Austin Group Defect 1330 is applied, removing obsolescent interfaces.

Austin Group Defect 1380 is applied, changing text using the term "link" in line with its updated definition and removing a paragraph from the RATIONALE section.