Linux readlink

例子：读取当前进程的绝对路径文件名

int ret = readlink("/proc/self/exe", processBuf, sizeof(processBuf)-1);

其中/proc/self/exe是指向当前进程的符号链接。

[root@localhost self]$ pwd
/proc/self
[root@localhost self]$ ll exe
lrwxrwxrwx. 1 root root 0 Aug 8 10:48 exe -> /bin/bash

=========================================================================
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         top

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()).

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().
RETURN VALUE top

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.
ERRORS         top

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 (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 dirfd is not a valid file descriptor.

ENOTDIR
              pathname is relative and dirfd is a file descriptor referring
              to a file other than a directory.
VERSIONS         top

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

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

readlinkat(): POSIX.1-2008.
NOTES top

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.

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