readlink (2)

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NAME

readlink - read value of a symbolic link

SYNOPSIS

#include <unistd.h>

ssize_t readlink(const char *path, char *buf, size_t bufsiz);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

readlink():

_BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED || _POSIX_C_SOURCE >= 200112L

DESCRIPTION

readlink() places the contents of the symbolic link path in 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 bufsiz characters), in case the buffer is too small to hold all of the contents.

RETURN VALUE

On success, readlink() returns the number of bytes placed in buf. On error, -1 is returned and errno is set to indicate the error.

ERRORS

EACCES

Search permission is denied for a component of the path prefix. (See also path_resolution(7).)

EFAULT

buf extends outside the process's allocated address space.

EINVAL

bufsiz is not positive.

EINVAL

The named file 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.

CONFORMING TO

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

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_size value returned by a call to lstat(2) on the link. However, the number of bytes written by readlink() 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() also addresses a common portability problem when using PATH_MAX for the buffer size, as this constant is not guaranteed to be defined per POSIX if the system does not have such limit.

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("lstat");
        exit(EXIT_FAILURE);
    }

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

    linkname[r] = '\0';

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

    exit(EXIT_SUCCESS);
}

SEE ALSO

readlink(1), lstat(2), readlinkat(2), stat(2), symlink(2), path_resolution(7), symlink(7)

COLOPHON

This page is part of release 3.54 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

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