READLINK(2)

NAAM
readlink, readlinkat- lees waarde van een symbolische koppeling

SAMENVATTING
#include <unistd.h>

ssize_t readlink(const char *padnaam, char *buf, size_t bufmaat);
#include <fcntl.h>           /* Definitie van AT_* constanten */
#include <unistd.h>

ssize_t readlinkat(int map_bes_ind, const char *padnaam,
                   char *buf, size_t bufmaat);

    Test Macro´s in glibc (zie feature_test_macros(7)):

readlink():
    _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L
        || /* Glibc versies <= 2.19: */ _BSD_SOURCE

readlinkat():
    Sinds glibc 2.10:
        _POSIX_C_SOURCE >= 200809L
    Vóór glibc 2.10:
        _ATFILE_SOURCE

BESCHRIJVING
readlink() places the contents of the symbolic link pathname in the buffer buf, which has size bufsiz. readlink() does not append a null byte to buf. It will (silently) truncate the contents (to a length of bufsiz characters), 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 pathname is 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 pathname is relative and dirfd is the special value AT_FDCWD, then pathname is interpreted relative to the current working directory of the calling process (like readlink()).

Als padnaam absoluut is, dan wordt mapbi genegeerd.

Since Linux 2.6.39, pathname can 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_PATH and O_NOFOLLOW flags).

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

EIND WAARDE
On success, these calls return the number of bytes placed in buf. (If the returned value equals bufsiz, then truncation may have occurred.) On error, -1 is returned and errno is set to indicate the error.

FOUTEN

EACCES
    Zoek toestemming werd geweigerd voor een deel van het pad voorvoegsel. (Zie ook path_resolution(7).)

EFAULT
    buf strekt zich uit voorbij de aan het proces toegewezen adres ruimte.

EINVAL
    bufmaat is niet positief.

EINVAL
    The named file (i.e., the final filename component of pathname) is not a symbolic link.

EIO Een In/Uit fout trad op terwijl er van het bestandsysteem gelezen werd.

ELOOP
    Teveel symbolische koppelingen werden tegengekomen bij het vertalen van de padnaam.

ENAMETOOLONG
    Een padnaam, of een deel van een padnaam, was te lang.

ENOENT
    Het genoemde bestand bestaat niet.

ENOMEM
    Onvoldoende kernelgeheugen voorhanden.

ENOTDIR
    Een deel van het pad-voorvoegsel is geen map.

De volgende extra fouten kunnen optreden voor readlinkat():

EBADF
    mapbi is geen geldige bestandindicator.

ENOTDIR
    padnaam is relatief en mapbi is een bestandsindicatoor die naar een bestand wijst die geen map is.

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

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

readlinkat(): POSIX.1-2008.

OPMERKINGEN
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() 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_MAX for the buffer size, as this constant is not guaranteed to be defined per POSIX if the system does not have such limit.

  Glibc-opmerkingen
On older kernels where readlinkat() is unavailable, the glibc wrapper function falls back to the use of readlink(). When pathname is a relative pathname, glibc constructs a pathname based on the symbolic link in /proc/self/fd that corresponds to the dirfd argument.

VOORBEELDEN
The following program allocates the buffer needed by readlink() dynamically from the information provided by lstat(2), falling back to a buffer of size PATH_MAX in cases where lstat(2) reports a size of zero.

    #include <sys/types.h>
    #include <sys/stat.h>
    #include <limits.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>
    int
    main(int argc, char *argv[])
    {
        struct stat sb;
        char *buf;
        ssize_t nbytes, bufsiz;
        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);
        }
        /* Add one to the link size, so that we can determine whether
           the buffer returned by readlink() was truncated. */
        bufsiz = sb.st_size + 1;
        /* Some magic symlinks under (for example) /proc and /sys
           report 'st_size' as zero. In that case, take PATH_MAX as
           a "good enough" estimate. */
        if (sb.st_size == 0)
            bufsiz = PATH_MAX;
        buf = malloc(bufsiz);
        if (buf == NULL) {
            perror("malloc");
            exit(EXIT_FAILURE);
        }
        nbytes = readlink(argv[1], buf, bufsiz);
        if (nbytes == -1) {
            perror("readlink");
            exit(EXIT_FAILURE);
        }
        printf("'%s' points to '%.*s'\n", argv[1], (int) nbytes, buf);
        /* If the return value was equal to the buffer size, then the
           the link target was larger than expected (perhaps because the
           target was changed between the call to lstat() and the call to
           readlink()). Warn the user that the returned target may have
           been truncated. */
        if (nbytes == bufsiz)
            printf("(Returned buffer may have been truncated)\n");
        free(buf);
        exit(EXIT_SUCCESS);
    }

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

COLOFON
Deze pagina is onderdeel van release 5.10 van het Linux man-pages-project. Een beschrijving van het project, informatie over het melden van bugs en de nieuwste versie van deze pagina zijn op https://www.kernel.org/doc/man-pages/ te vinden.

VERTALING
De Nederlandse vertaling van deze handleiding is geschreven door Jos Boersema <joshb@xs4all.nl> en Mario Blättermann <mario.blaettermann@gmail.com> Deze vertaling is vrije documentatie; lees de GNU General Public License Version 3 (https://www.gnu.org/licenses/gpl-3.0.html) of later over de Copyright-voorwaarden. Er is geen AANSPRAKELIJKHEID. Indien U fouten in de vertaling van deze handleiding zou vinden, stuur een e-mail naar debian-l10n-dutch@lists.debian.org.