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LINK(2) System Calls Manual LINK(2)

link, linkat — make a hard file link

library “libc”

#include <unistd.h>

int
link(const char *name1, const char *name2);

int
linkat(int fd1, const char *name1, int fd2, const char *name2, int flags);

The () function call atomically creates the specified directory entry (hard link) name2 with the attributes of the underlying object pointed at by name1. If the link is successful: the link count of the underlying object is incremented; name1 and name2 share equal access and rights to the underlying object.

If name1 is removed, the file name2 is not deleted and the link count of the underlying object is decremented.

Name1 must exist for the hard link to succeed and both name1 and name2 must be in the same file system. name1 may not be a directory.

When operating on a symlink, () resolves the symlink and creates a hard link on the target. linkat() will do the same if AT_SYMLINK_FOLLOW is set in flags, but it will link on the symlink itself if the flag is clear.

The () system call is equivalent to link() except in the case where either name1 or name2 or both are relative paths. In this case a relative path name1 is interpreted relative to the directory associated with the file descriptor fd1 instead of the current working directory and similarly for name2 and the file descriptor fd2. If fd1 or fd2 has the special value AT_FDCWD then name1 or name2 respectively is interpreted relative to the current working directory.

The link() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

Link() will fail and no link will be created if:

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A component of either path prefix is not a directory.
[]
A component of either pathname exceeded 255 characters, or entire length of either path name exceeded 1023 characters.
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A component of either path prefix does not exist.
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The file system containing the file named by name1 does not support links.
The link count of the file named by name1 would exceed 32767.
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A component of either path prefix denies search permission.
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The requested link requires writing in a directory with a mode that denies write permission.
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Too many symbolic links were encountered in translating one of the pathnames.
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The file named by name1 does not exist.
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The link named by name2 does exist.
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The named file has its immutable or append-only flag set (see chflags(2)).
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The file named by name1 is a directory.
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The link named by name2 and the file named by name1 are on different file systems.
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The directory in which the entry for the new link is being placed cannot be extended because there is no space left on the file system containing the directory.
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The directory in which the entry for the new link is being placed cannot be extended because the user's quota of disk blocks on the file system containing the directory has been exhausted.
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An I/O error occurred while reading from or writing to the file system to make the directory entry.
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The requested link requires writing in a directory on a read-only file system.
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One of the pathnames specified is outside the process's allocated address space.

In addition to the errors returned by the link() syscall, the linkat() system call may fail if:

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The name1 or name2 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 searching.
[]
The name1 or name2 argument is not an absolute path and fd1 or fd2, respectively, is neither AT_FDCWD nor a file descriptor associated with a directory.

readlink(2), symlink(2), unlink(2)

The link() function call is expected to conform to IEEE Std 1003.1-1990 (“POSIX.1”).

A link() function call appeared in Version 7 AT&T UNIX. The linkat() system call appeared in DragonFly 3.1.

The link() system call traditionally allows the super-user to link directories which corrupts the filesystem coherency. This implementation no longer permits it.

July 31, 2012 DragonFly-5.6.1