LINK(2) System Calls Manual LINK(2)

link, linkat — make a hard file link

#include <unistd.h>

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

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

The () function call atomically creates the specified directory entry (hard link) path2 with the attributes of the underlying object pointed at by path1. If the link is successful, the link count of the underlying object is incremented; path1 and path2 share equal access and rights to the underlying object.

If path1 is removed, the file path2 is not deleted and the link count of the underlying object is decremented.

In order for the system call to succeed, path1 must exist and both path1 and path2 must be in the same file system. As mandated by POSIX.1, path1 may not be a directory.

() will resolve and follow symbolic links contained within both path1 and path2. If the last component of path1 is a symbolic link, link() will point the hard link, path2, to the underlying object pointed to by path1, not to the symbolic link itself.

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.

Values for flag are constructed by a bitwise-inclusive OR of flags from the following list, defined in <fcntl.h>:

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

If () is passed the special value AT_FDCWD in the fd1 or fd2 parameter, the current working directory is used for the respective name argument. If both fd1 and fd2 have value AT_FDCWD, the behavior is identical to a call to link(). Unless flag contains the AT_SYMLINK_FOLLOW flag, if name1 names a symbolic link, a new link is created for the symbolic link name1 and not its target. On OS X, not assigning AT_SYMLINK_FOLLOW to flag may result in some file systems returning an error.

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

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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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The current process cannot access the existing file.
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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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The link named by path2 already exists.
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One of the pathnames specified is outside the process's allocated address space.
[]
An I/O error occurs while reading from or writing to the file system to make the directory entry.
[]
Too many symbolic links are encountered in translating one of the pathnames. This is taken to be indicative of a looping symbolic link.
The file already has {LINK_MAX} links.
[]
A component of a pathname exceeds {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, or is a dangling symbolic link.
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The file named by path1 does not exist, or is a dangling symbolic link.
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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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A component of either path prefix is not a directory.
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The file named by path1 is a directory.
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The requested link requires writing in a directory on a read-only file system.
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The link named by path2 and the file named by path1 are on different file systems.
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The file named by path1 is a “dataless” file that must be materialized before being linked and the I/O policy of the current thread or process disallows file materialization (see getiopolicy_np(3)).

In addition to the errors returned by the link(), 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.
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The value of the flag argument is not valid.
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flag was not set to AT_SYMLINK_FOLLOW (some file systems only).
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The underlying file system does not support this call.
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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.

symlink(2), unlink(2)

The link() function is expected to conform to IEEE Std 1003.1-1988 (“POSIX.1”). The linkat() system call is expected to conform to POSIX.1-2008 .

June 3, 2021 BSD 4