link(2) | System Calls Manual | link(2) |
link, linkat - make a new name for a file
Standard C library (libc, -lc)
#include <unistd.h>
int link(const char *oldpath, const char *newpath);
#include <fcntl.h> /* Definition of AT_* constants */ #include <unistd.h>
int linkat(int olddirfd, const char *oldpath, int newdirfd, const char *newpath, int flags);
linkat():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_ATFILE_SOURCE
link() creates a new link (also known as a hard link) to an existing file.
If newpath exists, it will not be overwritten.
This new name may be used exactly as the old one for any operation; both names refer to the same file (and so have the same permissions and ownership) and it is impossible to tell which name was the "original".
The linkat() system call operates in exactly the same way as link(), except for the differences described here.
If the pathname given in oldpath is relative, then it is interpreted relative to the directory referred to by the file descriptor olddirfd (rather than relative to the current working directory of the calling process, as is done by link() for a relative pathname).
If oldpath is relative and olddirfd is the special value AT_FDCWD, then oldpath is interpreted relative to the current working directory of the calling process (like link()).
If oldpath is absolute, then olddirfd is ignored.
The interpretation of newpath is as for oldpath, except that a relative pathname is interpreted relative to the directory referred to by the file descriptor newdirfd.
The following values can be bitwise ORed in flags:
linkat(AT_FDCWD, "/proc/self/fd/<fd>", newdirfd,
newname, AT_SYMLINK_FOLLOW);
Before Linux 2.6.18, the flags argument was unused, and had to be specified as 0.
See openat(2) for an explanation of the need for linkat().
On success, zero is returned. On error, -1 is returned, and errno is set to indicate the error.
The following additional errors can occur for linkat():
open(path, O_TMPFILE | O_EXCL, mode);
linkat() was added in Linux 2.6.16; library support was added in glibc 2.4.
link(): SVr4, 4.3BSD, POSIX.1-2001 (but see NOTES), POSIX.1-2008.
linkat(): POSIX.1-2008.
Hard links, as created by link(), cannot span filesystems. Use symlink(2) if this is required.
POSIX.1-2001 says that link() should dereference oldpath if it is a symbolic link. However, since Linux 2.0, Linux does not do so: if oldpath is a symbolic link, then newpath is created as a (hard) link to the same symbolic link file (i.e., newpath becomes a symbolic link to the same file that oldpath refers to). Some other implementations behave in the same manner as Linux. POSIX.1-2008 changes the specification of link(), making it implementation-dependent whether or not oldpath is dereferenced if it is a symbolic link. For precise control over the treatment of symbolic links when creating a link, use linkat().
On older kernels where linkat() is unavailable, the glibc wrapper function falls back to the use of link(), unless the AT_SYMLINK_FOLLOW is specified. When oldpath and newpath are relative pathnames, glibc constructs pathnames based on the symbolic links in /proc/self/fd that correspond to the olddirfd and newdirfd arguments.
On NFS filesystems, the return code may be wrong in case the NFS server performs the link creation and dies before it can say so. Use stat(2) to find out if the link got created.
ln(1), open(2), rename(2), stat(2), symlink(2), unlink(2), path_resolution(7), symlink(7)
2023-02-05 | Linux man-pages 6.03 |