Manpage of READLINK

READLINK

Section: Linux Programmer's Manual (2)
Updated: 2016-03-15
Index
 

NAME

readlink, readlinkat - read value of a symbolic link  

SYNOPSIS

#include <unistd.h>ssize_t readlink(const char *pathname, char *buf, size_t bufsiz);#include <fcntl.h>           /* Definition of AT_* constants */
#include <unistd.h>ssize_t readlinkat(int dirfd, const char *pathname,                   char *buf, size_t bufsiz);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

readlink():

_XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L
    || /* Glibc versions <= 2.19: */ _BSD_SOURCE

readlinkat():

Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_ATFILE_SOURCE
 

DESCRIPTION

readlink() places the contents of the symbolic link pathnamein the buffer buf, which has size bufsiz. readlink() does not append a null byte to buf. It will truncate the contents (to a length of bufsizcharacters), in case the buffer is too small to hold all of the contents.  

readlinkat()

The readlinkat() system call operates in exactly the same way as readlink(), except for the differences described here.

If the pathname given in pathnameis relative, then it is interpreted relative to the directory referred to by the file descriptor dirfd(rather than relative to the current working directory of the calling process, as is done by readlink() for a relative pathname).

If pathnameis relative and dirfdis the special value AT_FDCWD, then pathnameis interpreted relative to the current working directory of the calling process (like readlink()).

If pathnameis absolute, then dirfdis ignored.

Since Linux 2.6.39, pathnamecan be an empty string, in which case the call operates on the symbolic link referred to by dirfd(which should have been obtained using open(2) with the O_PATHand O_NOFOLLOWflags).

See openat(2) for an explanation of the need for readlinkat().  

RETURN VALUE

On success, these calls return the number of bytes placed in buf. On error, -1 is returned and errnois set to indicate the error.  

ERRORS

EACCES
Search permission is denied for a component of the path prefix. (See also path_resolution(7).)
EFAULT
bufextends outside the process's allocated address space.
EINVAL
bufsizis not positive.
EINVAL
The named file (i.e., the final filename component of pathname) is not a symbolic link.
EIO
An I/O error occurred while reading from the filesystem.
ELOOP
Too many symbolic links were encountered in translating the pathname.
ENAMETOOLONG
A pathname, or a component of a pathname, was too long.
ENOENT
The named file does not exist.
ENOMEM
Insufficient kernel memory was available.
ENOTDIR
A component of the path prefix is not a directory.

The following additional errors can occur for readlinkat():

EBADF
dirfdis not a valid file descriptor.
ENOTDIR
pathnameis relative and dirfdis a file descriptor referring to a file other than a directory.
 

VERSIONS

readlinkat() was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4.  

CONFORMING TO

readlink(): 4.4BSD (readlink() first appeared in 4.2BSD), POSIX.1-2001, POSIX.1-2008.

readlinkat(): POSIX.1-2008.  

NOTES

In versions of glibc up to and including glibc 2.4, the return type of readlink() was declared as int. Nowadays, the return type is declared as ssize_t, as (newly) required in POSIX.1-2001.

Using a statically sized buffer might not provide enough room for the symbolic link contents. The required size for the buffer can be obtained from the stat.st_sizevalue returned by a call to lstat(2) on the link. However, the number of bytes written by readlink() and readlinkat() should be checked to make sure that the size of the symbolic link did not increase between the calls. Dynamically allocating the buffer for readlink() and readlinkat() also addresses a common portability problem when using PATH_MAXfor the buffer size, as this constant is not guaranteed to be defined per POSIX if the system does not have such limit.  

Glibc notes

On older kernels where readlinkat() is unavailable, the glibc wrapper function falls back to the use of readlink(). When pathnameis a relative pathname, glibc constructs a pathname based on the symbolic link in /proc/self/fdthat corresponds to the dirfdargument.  

EXAMPLE

The following program allocates the buffer needed by readlink() dynamically from the information provided by lstat(), making sure there's no race condition between the calls.
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int
main(int argc, char *argv[])
{
    struct stat sb;
    char *linkname;
    ssize_t r;

    if (argc != 2) {
        fprintf(stderr, "Usage: %s <pathname>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    if (lstat(argv[1], &sb) == -1) {
        perror("lstat");
        exit(EXIT_FAILURE);
    }

    linkname = malloc(sb.st_size + 1);
    if (linkname == NULL) {
        fprintf(stderr, "insufficient memory\n");
        exit(EXIT_FAILURE);
    }

    r = readlink(argv[1], linkname, sb.st_size + 1);

    if (r == -1) {
        perror("readlink");
        exit(EXIT_FAILURE);
    }

    if (r > sb.st_size) {
        fprintf(stderr, "symlink increased in size "
                        "between lstat() and readlink()\n");
        exit(EXIT_FAILURE);
    }

    linkname[r] = aq\0aq;

    printf("aq%saq points to aq%saq\n", argv[1], linkname);

    free(linkname);

    exit(EXIT_SUCCESS);
}
 

SEE ALSO

readlink(1), lstat(2), stat(2), symlink(2), realpath(3), path_resolution(7), symlink(7)


 

Index

NAME
SYNOPSIS
DESCRIPTION
readlinkat()
RETURN VALUE
ERRORS
VERSIONS
CONFORMING TO
NOTES
Glibc notes
EXAMPLE
SEE ALSO

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Time: 22:27:42 GMT, June 20, 2016