Manpage of LINK
LINKSection: Linux Programmer's Manual (2)
NAMElink, linkat - make a new name for a file
#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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
- Since glibc 2.10:
- _POSIX_C_SOURCE >= 200809L
- Before glibc 2.10:
DESCRIPTIONlink() creates a new link (also known as a hard link) to an existing file.
If newpathexists, it will notbe 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".
linkat()The linkat() system call operates in exactly the same way as link(), except for the differences described here.
If the pathname given in oldpathis 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 oldpathis relative and olddirfdis the special value AT_FDCWD, then oldpathis interpreted relative to the current working directory of the calling process (like link()).
If oldpathis absolute, then olddirfdis ignored.
The interpretation of newpathis 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:
- AT_EMPTY_PATH (since Linux 2.6.39)
- If oldpathis an empty string, create a link to the file referenced by olddirfd(which may have been obtained using the open(2) O_PATHflag). In this case, olddirfdcan refer to any type of file except a directory. This will generally not work if the file has a link count of zero (files created with O_TMPFILEand without O_EXCLare an exception). The caller must have the CAP_DAC_READ_SEARCHcapability in order to use this flag. This flag is Linux-specific; define _GNU_SOURCEto obtain its definition.
- AT_SYMLINK_FOLLOW (since Linux 2.6.18)
- By default, linkat(), does not dereference oldpathif it is a symbolic link (like link()). The flag AT_SYMLINK_FOLLOWcan be specified in flagsto cause oldpathto be dereferenced if it is a symbolic link. If procfs is mounted, this can be used as an alternative to AT_EMPTY_PATH, like this:
- linkat(AT_FDCWD, "/proc/self/fd/<fd>", newdirfd,
Before kernel 2.6.18, the flagsargument was unused, and had to be specified as 0.
See openat(2) for an explanation of the need for linkat().
RETURN VALUEOn success, zero is returned. On error, -1 is returned, and errnois set appropriately.
- Write access to the directory containing newpathis denied, or search permission is denied for one of the directories in the path prefix of oldpathor newpath. (See also path_resolution(7).)
- The user's quota of disk blocks on the filesystem has been exhausted.
- newpathalready exists.
- oldpath or newpath points outside your accessible address space.
- An I/O error occurred.
- Too many symbolic links were encountered in resolving oldpath or newpath.
- The file referred to by oldpathalready has the maximum number of links to it. For example, on an ext4(5) filesystem that does not employ the dir_indexfeature, the limit on the number of hard links to a file is 65,000; on btrfs(5), the limit is 65,535 links.
- oldpath or newpath was too long.
- A directory component in oldpath or newpathdoes not exist or is a dangling symbolic link.
- Insufficient kernel memory was available.
- The device containing the file has no room for the new directory entry.
- A component used as a directory in oldpath or newpathis not, in fact, a directory.
- oldpathis a directory.
- The filesystem containing oldpath and newpathdoes not support the creation of hard links.
- EPERM (since Linux 3.6)
- The caller does not have permission to create a hard link to this file (see the description of /proc/sys/fs/protected_hardlinksin proc(5)).
- oldpathis marked immutable or append-only. (See ioctl_iflags(2).)
- The file is on a read-only filesystem.
- oldpath and newpathare not on the same mounted filesystem. (Linux permits a filesystem to be mounted at multiple points, but link() does not work across different mount points, even if the same filesystem is mounted on both.)
The following additional errors can occur for linkat():
- olddirfdor newdirfdis not a valid file descriptor.
- An invalid flag value was specified in flags.
- AT_EMPTY_PATHwas specified in flags, but the caller did not have the CAP_DAC_READ_SEARCHcapability.
- An attempt was made to link to the /proc/self/fd/NNfile corresponding to a file descriptor created with
open(path, O_TMPFILE | O_EXCL, mode);
- See open(2).
- oldpathis a relative pathname and olddirfdrefers to a directory that has been deleted, or newpathis a relative pathname and newdirfdrefers to a directory that has been deleted.
- oldpathis relative and olddirfdis a file descriptor referring to a file other than a directory; or similar for newpathand newdirfd
- AT_EMPTY_PATHwas specified in flags, oldpathis an empty string, and olddirfdrefers to a directory.
VERSIONSlinkat() was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4.
CONFORMING TOlink(): SVr4, 4.3BSD, POSIX.1-2001 (but see NOTES), POSIX.1-2008.
NOTESHard links, as created by link(), cannot span filesystems. Use symlink(2) if this is required.
POSIX.1-2001 says that link() should dereference oldpathif it is a symbolic link. However, since kernel 2.0, Linux does not do so: if oldpathis a symbolic link, then newpathis created as a (hard) link to the same symbolic link file (i.e., newpathbecomes a symbolic link to the same file that oldpathrefers 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 oldpathis dereferenced if it is a symbolic link. For precise control over the treatment of symbolic links when creating a link, use linkat().
Glibc notesOn older kernels where linkat() is unavailable, the glibc wrapper function falls back to the use of link(), unless the AT_SYMLINK_FOLLOWis specified. When oldpathand newpathare relative pathnames, glibc constructs pathnames based on the symbolic links in /proc/self/fdthat correspond to the olddirfdand newdirfdarguments.
BUGSOn 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.
SEE ALSOln(1), open(2), rename(2), stat(2), symlink(2), unlink(2), path_resolution(7), symlink(7)
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