man/man2/link.2

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.\"     @(#)link.2	8.3 (Berkeley) 1/12/94
.\"
.Dd June 3, 2021
.Dt LINK 2
.Os BSD 4
.Sh NAME
.Nm link ,
.Nm linkat
.Nd make a hard file link
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fo link
.Fa "const char *path1"
.Fa "const char *path2"
.Fc
.Ft int
.Fo linkat
.Fa "int fd1" "const char *name1" "int fd2" "const char *name2" "int flag"
.Fc
.Sh DESCRIPTION
The
.Fn link
function call
atomically creates the specified directory entry (hard link)
.Fa path2
with the attributes of the underlying object pointed at by
.Fa path1 .
If the link is successful,
the link count of the underlying object is incremented;
.Fa path1
and
.Fa path2
share equal access and rights
to the
underlying object.
.Pp
If
.Fa path1
is removed, the file
.Fa path2
is not deleted and the link count of the
underlying object is
decremented.
.Pp
In order for the system call to succeed,
.Fa path1
must exist and both
.Fa path1
and
.Fa path2
must be in the same file system.
As mandated by POSIX.1,
.Fa path1
may not be a directory.
.Pp
.Fn link
will resolve and follow symbolic links contained within both
.Fa path1
and
.Fa path2 .
If the last component of
.Fa path1
is a symbolic link,
.Fn link
will point the hard link,
.Fa path2 ,
to the underlying object pointed to by
.Fa path1 ,
not to the symbolic link itself.
.Pp
The
.Fn linkat
system call is equivalent to
.Fa link
except in the case where either
.Fa name1
or
.Fa name2
or both are relative paths.
In this case a relative path
.Fa name1
is interpreted relative to
the directory associated with the file descriptor
.Fa fd1
instead of the current working directory and similarly for
.Fa name2
and the file descriptor
.Fa fd2 .
.Pp
Values for
.Fa flag
are constructed by a bitwise-inclusive OR of flags from the following
list, defined in
.In fcntl.h :
.Bl -tag -width indent
.It Dv AT_SYMLINK_FOLLOW
If
.Fa name1
names a symbolic link, a new link for the target of the symbolic link is
created.
.El
.Pp
If
.Fn linkat
is passed the special value
.Dv AT_FDCWD
in the
.Fa fd1
or
.Fa fd2
parameter, the current working directory is used for the respective
.Fa name
argument.
If both
.Fa fd1
and
.Fa fd2
have value
.Dv AT_FDCWD ,
the behavior is identical to a call to
.Fn link .
Unless
.Fa flag
contains the
.Dv AT_SYMLINK_FOLLOW
flag, if
.Fa name1
names a symbolic link, a new link is created for the symbolic link
.Fa name1
and not its target. On OS X, not assigning AT_SYMLINK_FOLLOW to
.Fa flag
may result in some file systems returning an error.
.Sh RETURN VALUES
Upon successful completion, a value of 0 is returned.  Otherwise,
a value of -1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn link
will fail and no link will be created if:
.Bl -tag -width Er
.\" ==========
.It Bq Er EACCES
A component of either path prefix denies search permission.
.\" ==========
.It Bq Er EACCES
The requested link requires writing in a directory with a mode
that denies write permission.
.\" ==========
.It Bq Er EACCES
The current process cannot access the existing file.
.\" ==========
.It Bq Er EDQUOT
The directory in which the entry for the new link
is being placed cannot be extended because the
user's quota of disk blocks on the file system
containing the directory has been exhausted.
.\" ==========
.It Bq Er EEXIST
The link named by
.Fa path2
already exists.
.\" ==========
.It Bq Er EFAULT
One of the pathnames specified
is outside the process's allocated address space.
.\" ==========
.It Bq Er EIO
An I/O error occurs while reading from or writing to
the file system to make the directory entry.
.\" ==========
.It Bq Er ELOOP
Too many symbolic links are encountered in translating one of the pathnames.
This is taken to be indicative of a looping symbolic link.
.\" ==========
.It Bq Er EMLINK
The file already has {LINK_MAX} links.
.\" ==========
.It Bq Er ENAMETOOLONG
A component of a pathname exceeds
.Dv {NAME_MAX}
characters, or an entire path name exceeded
.Dv {PATH_MAX}
characters.
.\" ==========
.It Bq Er ENOENT
A component of either path prefix does not exist, or is a dangling symbolic link.
.\" ==========
.It Bq Er ENOENT
The file named by
.Fa path1
does not exist, or is a dangling symbolic link.
.\" ==========
.It Bq Er ENOSPC
The directory in which the entry for the new link is being placed
cannot be extended because there is no space left on the file
system containing the directory.
.\" ==========
.It Bq Er ENOTDIR
A component of either path prefix is not a directory.
.\" ==========
.It Bq Er EPERM
The file named by
.Fa path1
is a directory.
.\" ==========
.It Bq Er EROFS
The requested link requires writing in a directory
on a read-only file system.
.\" ==========
.It Bq Er EXDEV
The link named by
.Fa path2
and the file named by
.Fa path1
are on different file systems.
.\" ==========
.It Bq Er EDEADLK
The file named by
.Fa path1
is a
.Dq dataless
file that must be materialized before being linked and the I/O policy of
the current thread or process disallows file materialization
.Po see
.Xr getiopolicy_np 3
.Pc .
.El
.Pp
In addition to the errors returned by the
.Fn link ,
the
.Fn linkat
system call may fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa name1
or
.Fa name2
argument does not specify an absolute path and the
.Fa fd1
or
.Fa fd2
argument, respectively, is neither
.Dv AT_FDCWD
nor a valid file descriptor open for searching.
.It Bq Er EINVAL
The value of the
.Fa flag
argument is not valid.
.It Bq Er ENOTSUP
.Fa flag
was not set to
.Dv AT_SYMLINK_FOLLOW (some file systems only).
.It Bq Er ENOTSUP
The underlying file system does not support this call.
.It Bq Er ENOTDIR
The
.Fa name1
or
.Fa name2
argument is not an absolute path and
.Fa fd1
or
.Fa fd2 ,
respectively, is neither
.Dv AT_FDCWD
nor a file descriptor associated with a directory.
.El
.Sh SEE ALSO
.Xr symlink 2 ,
.Xr unlink 2
.Sh STANDARDS
The
.Fn link
function is expected to conform to
.St -p1003.1-88 .
The
.Fn linkat
system call is expected to conform to POSIX.1-2008 .