示例：读取当前进程的绝对路径文件名
傲慢的天鹅/proc/self/exe是指向当前进程的符号链接。
/proc/self
[root@localhost self]$ ll exe
lrwxrwxrwx.1根根根0a ug 810:48 exe--/bin/bash
=====================================
名字
SYNOPSIS
#include unistd.h
size _ tread link (const char * pathname，char *buf，size_t bufsiz )；
# include fcntl.h/* definition ofat _ * constants * /
#include unistd.h
size_treadlinkat(intdirFD，const char *pathname，
char *buf，size_t bufsiz；
featuretestmacrorequirementsforglibc (see feature _ test _ macros (7) ) :
_ x open _ source=500|_ POSIX _ c _ source=200112 l
|/* glibc版本=2.19: */_ BSD _ source
Since glibc 2.10:
_POSIX_C_SOURCE=200809L
Before glibc 2.10:
_ATFILE_SOURCE
描述顶部
buffer buf，which has size bufsiz. readlink () does not append a null
bytetobuf.itwill(silently ) truncatethecontents ) toalength
of bufsiz characters )，incasethebufferistoosmalltoholdallof
the contents。
readlinkat () )
thereadlinkat (系统语言操作系统xactlythesamewayas
readlink (，exceptforthedifferencesdescribedhere。
ifthepathnamegiveninpathnameisrelative，then it is interpreted
relativetothedirectoryreferredtobythefiledescriptordirfd
(ratherthanrelativetothecurrentworkingdirectoryofthecalling )
处理，as is done by readlink () for a relative pathname )。
ifpathnameisrelativeanddirfdisthespecialvalueat _ FD CWD，then
pathnameisinterpretedrelativetothecurrentworkingdirectoryof
技术阅读流程(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
The following additional errors can occur for readlinkat():
to a file other than a directory.
VERSIONS top
CONFORMING TO 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.