link.3p (12057B)
- '\" et
- .TH LINK "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
- .\"
- .SH PROLOG
- 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.
- .\"
- .SH NAME
- link, linkat
- \(em link one file to another file
- .SH SYNOPSIS
- .LP
- .nf
- #include <unistd.h>
- .P
- int link(const char *\fIpath1\fP, const char *\fIpath2\fP);
- .P
- #include <fcntl.h>
- .P
- int linkat(int \fIfd1\fP, const char *\fIpath1\fP, int \fIfd2\fP,
- const char *\fIpath2\fP, int \fIflag\fP);
- .fi
- .SH DESCRIPTION
- The
- \fIlink\fR()
- function shall create a new link (directory entry) for the existing file,
- .IR path1 .
- .P
- The
- .IR path1
- argument points to a pathname naming an existing file. The
- .IR path2
- argument points to a pathname naming the new directory entry to be
- created. The
- \fIlink\fR()
- function shall atomically create a new link for the existing file and
- the link count of the file shall be incremented by one.
- .P
- If
- .IR path1
- names a directory,
- \fIlink\fR()
- shall fail unless the process has appropriate privileges and the
- implementation supports using
- \fIlink\fR()
- on directories.
- .P
- If
- .IR path1
- names a symbolic link, it is implementation-defined whether
- \fIlink\fR()
- follows the symbolic link, or creates a new link to the symbolic
- link itself.
- .P
- Upon successful completion,
- \fIlink\fR()
- 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.
- .P
- If
- \fIlink\fR()
- fails, no link shall be created and the link count of the file shall
- remain unchanged.
- .P
- The implementation may require that the calling process has permission
- to access the existing file.
- .P
- The
- \fIlinkat\fR()
- function shall be equivalent to the
- \fIlink\fR()
- function except that symbolic links shall be handled as specified by
- the value of
- .IR flag
- (see below) and except in the case where either
- .IR path1
- or
- .IR path2
- or both are relative paths. In this case a relative path
- .IR path1
- is interpreted relative to the directory associated with the file
- descriptor
- .IR fd1
- instead of the current working directory and similarly for
- .IR path2
- and the file descriptor
- .IR fd2 .
- 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.
- .P
- Values for
- .IR flag
- are constructed by a bitwise-inclusive OR of flags from the following
- list, defined in
- .IR <fcntl.h> :
- .IP AT_SYMLINK_FOLLOW 6
- .br
- If
- .IR path1
- names a symbolic link, a new link for the target of the symbolic link
- is created.
- .P
- If
- \fIlinkat\fR()
- is passed the special value AT_FDCWD in the
- .IR fd1
- or
- .IR fd2
- parameter, the current working directory shall be used for the respective
- .IR path
- argument. If both
- .IR fd1
- and
- .IR fd2
- have value AT_FDCWD, the behavior shall be identical to a call to
- \fIlink\fR(),
- except that symbolic links shall be handled as specified by the value of
- .IR flag .
- .P
- If the AT_SYMLINK_FOLLOW flag is clear in the
- .IR flag
- argument and the
- .IR path1
- argument names a symbolic link, a new link is created for the symbolic
- link
- .IR path1
- and not its target.
- .SH "RETURN VALUE"
- Upon successful completion, these functions shall return 0. Otherwise,
- these functions shall return \-1 and set
- .IR errno
- to indicate the error.
- .br
- .SH ERRORS
- These functions shall fail if:
- .TP
- .BR EACCES
- A 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.
- .TP
- .BR EEXIST
- The
- .IR path2
- argument resolves to an existing directory entry or refers to a symbolic
- link.
- .TP
- .BR ELOOP
- A loop exists in symbolic links encountered during resolution of the
- .IR path1
- or
- .IR path2
- argument.
- .TP
- .BR EMLINK
- The number of links to the file named by
- .IR path1
- would exceed
- {LINK_MAX}.
- .TP
- .BR ENAMETOOLONG
- .br
- The length of a component of a pathname is longer than
- {NAME_MAX}.
- .TP
- .BR ENOENT
- A component of either path prefix does not exist; the file named by
- .IR path1
- does not exist; or
- .IR path1
- or
- .IR path2
- points to an empty string.
- .TP
- .BR ENOENT " or " ENOTDIR
- .br
- The
- .IR path1
- argument names an existing non-directory file, and the
- .IR path2
- argument contains at least one non-\c
- <slash>
- character and ends with one or more trailing
- <slash>
- characters. If
- .IR path2
- without the trailing
- <slash>
- characters would name an existing file, an
- .BR [ENOENT]
- error shall not occur.
- .TP
- .BR ENOSPC
- The directory to contain the link cannot be extended.
- .TP
- .BR ENOTDIR
- A component of either path prefix names an existing file that is neither
- a directory nor a symbolic link to a directory, or the
- .IR path1
- argument contains at least one non-\c
- <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 the
- .IR path1
- argument names an existing non-directory file and the
- .IR path2
- argument names a nonexistent file, contains at least one non-\c
- <slash>
- character, and ends with one or more trailing
- <slash>
- characters.
- .TP
- .BR EPERM
- The file named by
- .IR path1
- is a directory and either the calling process does not have appropriate
- privileges or the implementation prohibits using
- \fIlink\fR()
- on directories.
- .TP
- .BR EROFS
- The requested link requires writing in a directory on a read-only file
- system.
- .TP
- .BR EXDEV
- The link named by
- .IR path2
- and the file named by
- .IR path1
- are on different file systems and the implementation does not support
- links between file systems.
- .TP
- .BR EXDEV
- .IR path1
- refers to a named STREAM.
- .P
- The
- \fIlinkat\fR()
- function shall fail if:
- .TP
- .BR EACCES
- The access mode of the open file description associated with
- .IR fd1
- or
- .IR fd2
- is not O_SEARCH and the permissions of the directory underlying
- .IR fd1
- or
- .IR fd2 ,
- respectively, do not permit directory searches.
- .TP
- .BR EBADF
- The
- .IR path1
- or
- .IR path2
- argument does not specify an absolute path and the
- .IR fd1
- or
- .IR fd2
- argument, respectively, is neither AT_FDCWD nor a valid file descriptor
- open for reading or searching.
- .TP
- .BR ENOTDIR
- The
- .IR path1
- or
- .IR path2
- argument is not an absolute path and
- .IR fd1
- or
- .IR fd2 ,
- respectively, is a file descriptor associated with a non-directory file.
- .P
- These functions may fail if:
- .TP
- .BR ELOOP
- More than
- {SYMLOOP_MAX}
- symbolic links were encountered during resolution of the
- .IR path1
- or
- .IR path2
- argument.
- .TP
- .BR ENAMETOOLONG
- .br
- 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}.
- .br
- .P
- The
- \fIlinkat\fR()
- function may fail if:
- .TP
- .BR EINVAL
- The value of the
- .IR flag
- argument is not valid.
- .LP
- .IR "The following sections are informative."
- .SH EXAMPLES
- .SS "Creating a Link to a File"
- .P
- The following example shows how to create a link to a file named
- .BR /home/cnd/mod1
- by creating a new directory entry named
- .BR /modules/pass1 .
- .sp
- .RS 4
- .nf
- #include <unistd.h>
- .P
- char *path1 = "/home/cnd/mod1";
- char *path2 = "/modules/pass1";
- int status;
- \&...
- status = link (path1, path2);
- .fi
- .P
- .RE
- .SS "Creating a Link to a File Within a Program"
- .P
- In the following program example, the
- \fIlink\fR()
- function links the
- .BR /etc/passwd
- file (defined as
- .BR PASSWDFILE )
- to a file named
- .BR /etc/opasswd
- (defined as
- .BR SAVEFILE ),
- which is used to save the current password file. Then, after removing
- the current password file (defined as
- .BR PASSWDFILE ),
- the new password file is saved as the current password file using the
- \fIlink\fR()
- function again.
- .sp
- .RS 4
- .nf
- #include <unistd.h>
- .P
- #define LOCKFILE "/etc/ptmp"
- #define PASSWDFILE "/etc/passwd"
- #define SAVEFILE "/etc/opasswd"
- \&...
- /* Save current password file */
- link (PASSWDFILE, SAVEFILE);
- .P
- /* Remove current password file. */
- unlink (PASSWDFILE);
- .P
- /* Save new password file as current password file. */
- link (LOCKFILE,PASSWDFILE);
- .fi
- .P
- .RE
- .SH "APPLICATION USAGE"
- Some implementations do allow links between file systems.
- .P
- If
- .IR path1
- refers to a symbolic link, application developers should use
- \fIlinkat\fR()
- with appropriate flags to select whether or not the symbolic link should
- be resolved.
- .SH RATIONALE
- 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\(hy2017
- continues that philosophy by prohibiting
- \fIlink\fR()
- and
- \fIunlink\fR()
- from doing this. Other functions could do it if the implementor designed
- such an extension.
- .P
- Some historical implementations allow linking of files on different file
- systems. Wording was added to explicitly allow this optional behavior.
- .P
- The exception for cross-file system links is intended to apply only to
- links that are programmatically indistinguishable from ``hard'' links.
- .P
- The purpose of the
- \fIlinkat\fR()
- 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 to
- \fIlink\fR(),
- resulting in unspecified behavior. By opening a file descriptor for the
- directory of both the existing file and the target location and using the
- \fIlinkat\fR()
- function it can be guaranteed that the both filenames are in the desired
- directories.
- .P
- The AT_SYMLINK_FOLLOW flag allows for implementing both common behaviors
- of the
- \fIlink\fR()
- function. The POSIX specification requires that if
- .IR path1
- 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.
- .P
- Earlier versions of this standard specified only the
- \fIlink\fR()
- function, and required it to behave like
- \fIlinkat\fR()
- with the AT_SYMLINK_FOLLOW flag. However, historical practice from SVR4
- and Linux kernels had
- \fIlink\fR()
- behaving like
- \fIlinkat\fR()
- with no flags, and many systems that attempted to provide a conforming
- \fIlink\fR()
- 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 on
- \fIlink\fR()
- following links in practice, the
- \fIlinkat\fR()
- function was added taking a flag to specify the desired behavior
- for the application.
- .SH "FUTURE DIRECTIONS"
- None.
- .SH "SEE ALSO"
- .IR "\fIrename\fR\^(\|)",
- .IR "\fIsymlink\fR\^(\|)",
- .IR "\fIunlink\fR\^(\|)"
- .P
- The Base Definitions volume of POSIX.1\(hy2017,
- .IR "\fB<fcntl.h>\fP",
- .IR "\fB<unistd.h>\fP"
- .\"
- .SH COPYRIGHT
- 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 at
- http://www.opengroup.org/unix/online.html .
- .PP
- 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, see
- https://www.kernel.org/doc/man-pages/reporting_bugs.html .