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PROLOG |NAME |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |EXAMPLES |APPLICATION USAGE |RATIONALE |FUTURE DIRECTIONS |SEE ALSO |COPYRIGHT | |
LINK(3P) POSIX Programmer's ManualLINK(3P)This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
link, linkat — link one file to another file
#include <unistd.h> int link(const char *path1, const char *path2); #include <fcntl.h> int linkat(intfd1, const char *path1, intfd2, const char *path2, intflag);
Thelink() function shall create a new link (directory entry) for the existing file,path1. Thepath1 argument points to a pathname naming an existing file. Thepath2 argument points to a pathname naming the new directory entry to be created. Thelink() function shall atomically create a new link for the existing file and the link count of the file shall be incremented by one. Ifpath1 names a directory,link() shall fail unless the process has appropriate privileges and the implementation supports usinglink() on directories. Ifpath1 names a symbolic link, it is implementation-defined whetherlink() follows the symbolic link, or creates a new link to the symbolic link itself. Upon successful completion,link() shall mark for update the last file status change timestamp of the file. Also, the last data modification and last file status change timestamps of the directory that contains the new entry shall be marked for update. Iflink() 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. Thelinkat() function shall be equivalent to thelink() function except that symbolic links shall be handled as specified by the value offlag (see below) and except in the case where eitherpath1 orpath2 or both are relative paths. In this case a relative pathpath1 is interpreted relative to the directory associated with the file descriptorfd1 instead of the current working directory and similarly forpath2 and the file descriptorfd2. If the access mode of the open file description associated with the file descriptor is not O_SEARCH, the function shall check whether directory searches are permitted using the current permissions of the directory underlying the file descriptor. If the access mode is O_SEARCH, the function shall not perform the check. Values forflag are constructed by a bitwise-inclusive OR of flags from the following list, defined in<fcntl.h>: AT_SYMLINK_FOLLOW Ifpath1 names a symbolic link, a new link for the target of the symbolic link is created. Iflinkat() is passed the special value AT_FDCWD in thefd1 orfd2 parameter, the current working directory shall be used for the respectivepath argument. If bothfd1 andfd2 have value AT_FDCWD, the behavior shall be identical to a call tolink(), except that symbolic links shall be handled as specified by the value offlag. If the AT_SYMLINK_FOLLOW flag is clear in theflag argument and thepath1 argument names a symbolic link, a new link is created for the symbolic linkpath1 and not its target.
Upon successful completion, these functions shall return 0. Otherwise, these functions shall return -1 and seterrno to indicate the error.
These functions shall fail if:EACCESA component of either path prefix denies search permission, or the requested link requires writing in a directory that denies write permission, or the calling process does not have permission to access the existing file and this is required by the implementation.EEXISTThepath2 argument resolves to an existing directory entry or refers to a symbolic link.ELOOPA loop exists in symbolic links encountered during resolution of thepath1 orpath2 argument.EMLINKThe number of links to the file named bypath1 would exceed {LINK_MAX}.ENAMETOOLONG The length of a component of a pathname is longer than {NAME_MAX}.ENOENTA component of either path prefix does not exist; the file named bypath1 does not exist; orpath1 orpath2 points to an empty string.ENOENTorENOTDIR Thepath1 argument names an existing non-directory file, and thepath2 argument contains at least one non-<slash> character and ends with one or more trailing <slash> characters. Ifpath2 without the trailing <slash> characters would name an existing file, an[ENOENT]error shall not occur.ENOSPCThe directory to contain the link cannot be extended.ENOTDIR A component of either path prefix names an existing file that is neither a directory nor a symbolic link to a directory, or thepath1 argument contains at least one non-<slash> character and ends with one or more trailing <slash> characters and the last pathname component names an existing file that is neither a directory nor a symbolic link to a directory, or thepath1 argument names an existing non-directory file and thepath2 argument names a nonexistent file, contains at least one non-<slash> character, and ends with one or more trailing <slash> characters.EPERMThe file named bypath1 is a directory and either the calling process does not have appropriate privileges or the implementation prohibits usinglink() on directories.EROFSThe requested link requires writing in a directory on a read-only file system.EXDEVThe link named bypath2 and the file named bypath1 are on different file systems and the implementation does not support links between file systems.EXDEVpath1 refers to a named STREAM. Thelinkat() function shall fail if:EACCESThe access mode of the open file description associated withfd1 orfd2 is not O_SEARCH and the permissions of the directory underlyingfd1 orfd2, respectively, do not permit directory searches.EBADFThepath1 orpath2 argument does not specify an absolute path and thefd1 orfd2 argument, respectively, is neither AT_FDCWD nor a valid file descriptor open for reading or searching.ENOTDIR Thepath1 orpath2 argument is not an absolute path andfd1 orfd2, respectively, is a file descriptor associated with a non-directory file. These functions may fail if:ELOOPMore than {SYMLOOP_MAX} symbolic links were encountered during resolution of thepath1 orpath2 argument.ENAMETOOLONG The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic link produced an intermediate result with a length that exceeds {PATH_MAX}. Thelinkat() function may fail if:EINVALThe value of theflag argument is not valid.The following sections are informative.Creating a Link to a File The following example shows how to create a link to a file named/home/cnd/mod1by 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);Creating a Link to a File Within a Program In the following program example, thelink() function links the/etc/passwdfile (defined asPASSWDFILE) to a file named/etc/opasswd(defined asSAVEFILE), which is used to save the current password file. Then, after removing the current password file (defined asPASSWDFILE), the new password file is saved as the current password file using thelink() 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. Ifpath1 refers to a symbolic link, application developers should uselinkat() with appropriate flags to select whether or not the symbolic link should be resolved.
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 POSIX.1‐2017 continues that philosophy by prohibitinglink() andunlink() 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. The purpose of thelinkat() function is to link files in directories other than the current working directory without exposure to race conditions. Any part of the path of a file could be changed in parallel to a call tolink(), resulting in unspecified behavior. By opening a file descriptor for the directory of both the existing file and the target location and using thelinkat() function it can be guaranteed that the both filenames are in the desired directories. The AT_SYMLINK_FOLLOW flag allows for implementing both common behaviors of thelink() function. The POSIX specification requires that ifpath1 is a symbolic link, a new link for the target of the symbolic link is created. Many systems by default or as an alternative provide a mechanism to avoid the implicit symbolic link lookup and create a new link for the symbolic link itself. Earlier versions of this standard specified only thelink() function, and required it to behave likelinkat() with the AT_SYMLINK_FOLLOW flag. However, historical practice from SVR4 and Linux kernels hadlink() behaving likelinkat() with no flags, and many systems that attempted to provide a conforminglink() function did so in a way that was rarely used, and when it was used did not conform to the standard (e.g., by not being atomic, or by dereferencing the symbolic link incorrectly). Since applications could not rely onlink() following links in practice, thelinkat() function was added taking a flag to specify the desired behavior for the application.
None.
rename(3p),symlink(3p),unlink(3p) The Base Definitions volume of POSIX.1‐2017,fcntl.h(0p),unistd.h(0p)
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online athttp://www.opengroup.org/unix/online.html . Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, seehttps://www.kernel.org/doc/man-pages/reporting_bugs.html .IEEE/The Open Group 2017LINK(3P)Pages that refer to this page:unistd.h(0p), link(1p), ln(1p), fstatvfs(3p), open(3p), rename(3p), symlink(3p), unlink(3p)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |