прочитать значение символьной ссылки (read value of a symbolic link)
Имя (Name)
readlink, readlinkat - read value of a symbolic link
Синопсис (Synopsis)
#include <unistd.h>
ssize_t readlink(const char *restrict pathname, char *restrict buf,
size_t bufsiz);
#include <fcntl.h> /* Definition of AT_* constants */
#include <unistd.h>
ssize_t readlinkat(int dirfd, const char *restrict pathname,
char *restrict buf, size_t bufsiz);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
readlink():
_XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L
|| /* Glibc <= 2.19: */ _BSD_SOURCE
readlinkat():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_ATFILE_SOURCE
Описание (Description)
readlink() places the contents of the symbolic link pathname in
the buffer buf, which has size bufsiz. readlink() does not
append a terminating 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()).
If pathname is absolute, then dirfd is ignored.
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().
Возвращаемое значение (Return value)
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.
Ошибки (Error)
EACCES Search permission is denied for a component of the path
prefix. (See also path_resolution(7).)
EBADF (readlinkat()) pathname is relative but dirfd is neither
AT_FDCWD nor a valid file descriptor.
EFAULT buf extends outside the process's allocated address space.
EINVAL bufsiz is 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.
ENOTDIR
(readlinkat()) pathname is relative and dirfd is a file
descriptor referring to a file other than a directory.
Версии (Versions)
readlinkat() was added to Linux in kernel 2.6.16; library support
was added to glibc in version 2.4.
Стандарты (Conforming to)
readlink(): 4.4BSD (readlink() first appeared in 4.2BSD),
POSIX.1-2001, POSIX.1-2008.
readlinkat(): POSIX.1-2008.
Примечание (Note)
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 notes
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.
Примеры (Examples)
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);
}
/* Print only 'nbytes' of 'buf', as it doesn't contain a terminating
null byte ('\0'). */
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);
}
Смотри также (See also)
readlink(1), lstat(2), stat(2), symlink(2), realpath(3),
path_resolution(7), symlink(7)
