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LINK(2) System Calls Manual LINK(2)

link — make hard link to a file

#include <unistd.h>

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

#include <fcntl.h>
#include <unistd.h>

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

The () function 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. As mandated by POSIX.1 name1 may not be a directory.

The () function is equivalent to link() except that where name1 or name2 specifies a relative path, the directory entries linked are resolved relative to the directories associated with file descriptors fd1 or fd2 (respectively) instead of the current working directory.

If () is passed the special value AT_FDCWD (defined in <fcntl.h>) in the fd1 or fd2 parameter, the current working directory is used for resolving the respective name1 or name2 argument.

The flag argument is the bitwise OR of zero or more of the following values:

If name1 names a symbolic link, a new link for the target of the symbolic link is created.

If the AT_SYMLINK_FOLLOW flag is clear and name1 names a symbolic link, a new link is created for the symbolic link name1 and not its target.

Upon successful completion, a value of 0 is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error.

link() and linkat() 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 a pathname exceeded {NAME_MAX} characters, or an entire path name exceeded {PATH_MAX} 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 LINK_MAX.
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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 file named by name1 is a directory and the effective user ID is not superuser, or the file system containing the file does not permit the use of link() on a directory.
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The file named by name1 is flagged immutable or append-only.
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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.

Additionally, linkat() will fail if:

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The value of the flag argument was neither zero nor AT_SYMLINK_FOLLOW.
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The name1 or name2 argument specifies a relative path and the fd1 or fd2 argument, respectively, is neither AT_FDCWD nor a valid file descriptor.
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The name1 or name2 argument specifies a relative path and the fd1 or fd2 argument, respectively, is a valid file descriptor but it does not reference a directory.
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The name1 or name2 argument specifies a relative path but search permission is denied for the directory which the fd1 or fd2 file descriptor, respectively, references.

ln(1), readlink(2), symlink(2), unlink(2)

The link() and linkat() functions are expected to conform to IEEE Std 1003.1-2008 (“POSIX.1”).

The link() system call first appeared in Version 1 AT&T UNIX. The linkat() function appeared in OpenBSD 5.0.

February 13, 2014 OpenBSD-5.5