int link(const char *path1, const char *path2);
The link() function shall create a new link (directory entry) for the existing file, path1.
The path1 argument points to a pathname naming an existing file. The path2 argument points to a pathname naming the new directory entry to be created. The link() function shall atomically create a new link for the existing file and the link count of the file shall be incremented by one.
If path1 names a directory, link() shall fail unless the process has appropriate privileges and the implementation supports using link() on directories.
Upon successful completion, link() shall mark for update the st_ctime field of the file. Also, the st_ctime and st_mtime fields of the directory that contains the new entry shall be marked for update.
If link() fails, no link shall be created and the link count of the file shall remain unchanged.
The implementation may require that the calling process has permission to access the existing file.
Upon successful completion, 0 shall be returned. Otherwise, -1 shall be returned and errno set to indicate the error.
The link() function shall fail if:
The link() function may fail if:
The following sections are informative.
The following example shows how to create a link to a file named /home/cnd/mod1 by creating a new directory entry named /modules/pass1.
#include <unistd.h> char *path1 = "/home/cnd/mod1"; char *path2 = "/modules/pass1"; int status; ... status = link (path1, path2);
In the following program example, the link() function links the /etc/passwd file (defined as PASSWDFILE) to a file named /etc/opasswd (defined as SAVEFILE), which is used to save the current password file. Then, after removing the current password file (defined as PASSWDFILE), the new password file is saved as the current password file using the link() function again.
#include <unistd.h> #define LOCKFILE "/etc/ptmp" #define PASSWDFILE "/etc/passwd" #define SAVEFILE "/etc/opasswd" ... /* Save current password file */ link (PASSWDFILE, SAVEFILE); /* Remove current password file. */ unlink (PASSWDFILE); /* Save new password file as current password file. */ link (LOCKFILE,PASSWDFILE);
Some implementations do allow links between file systems.
Linking to a directory is restricted to the superuser in most historical implementations because this capability may produce loops in the file hierarchy or otherwise corrupt the file system. This volume of IEEE Std 1003.1-2001 continues that philosophy by prohibiting link() and unlink() from doing this. Other functions could do it if the implementor designed such an extension.
Some historical implementations allow linking of files on different file systems. Wording was added to explicitly allow this optional behavior.
The exception for cross-file system links is intended to apply only to links that are programmatically indistinguishable from "hard" links.
symlink() , unlink() , the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>