从2.6.16版本开始，GNU/Linux引入opeant系统调用：

#define _XOPEN_SOURCE 700 /* Or define _POSIX_C_SOURCE >= 200809 */
#include <fcntl.h>
int openat(int  dirfd , const char * pathname , int  flags , ... /* mode_t  mode */);

Returns file descriptor on success, or –1 on error
同open相比，多了一个dirfd参数。关于它的用法，参考以下解释：

If pathname specifies a relative pathname, then it is interpreted
relative to the directory referred to by the open file descriptor
dirfd, rather than relative to the process’s current working
directory.

If pathname specifies a relative pathname, and dirfd contains the
special value AT_FDCWD , then pathname is interpreted relative to the
process’s current working directory (i.e., the same behavior as
open(2)).

If pathname specifies an absolute pathname, then dirfd is ignored.

总结起来，如果pathname是绝对路径，则dirfd参数没用。如果pathname是相对路径，并且dirfd的值不是AT_FDCWD，则pathname的参照物是相对于dirfd指向的目录，而不是进程的当前工作目录；反之，如果dirfd的值是AT_FDCWD，pathname则是相对于进程当前工作目录的相对路径，此时等同于open。参考kernel代码则一目了然：

引入openat（及其它at结尾的函数）有以下两个原因：

First, openat() allows an application to avoid race conditions that
could occur when using open(2) to open files in directories other than
the current working directory. These race conditions result from the
fact that some component of the directory prefix given to open(2)
could be changed in parallel with the call to open(2). Such races can
be avoided by opening a file descriptor for the target directory, and
then specifying that file descriptor as the dirfd argument of
openat().

Second, openat() allows the implementation of a per-thread “current
working directory”, via file descriptor(s) maintained by the
application. (This functionality can also be obtained by tricks based
on the use of /proc/self/fd/dirfd, but less efficiently.)

引入openat是方便一个进程内的各线程可拥有不同的当前目录，传统的chdir会影响整个进程，而使用openat只需要每个线程在初始化时打开一个目录(调用open)，然后就可以以openat在“当前目录”操作文件了，如：

int dirfd = open("/tmp"); // 相当于chdir到“/tmp”
int filefd = openat(dirfd, "myfile"); // 在/tmp目录下打开“myfile”文件

用法示例：

#include <stdio.h>  
#include <sys/stat.h>  
#include <fcntl.h>  
#include <stdlib.h>  
#include <unistd.h>  

void creat_at(char *dir_path, char *relative_path)  
{  
    int dir_fd;  
    int fd;  
    int flags;  
    mode_t mode;  

    dir_fd = open(dir_path, O_RDONLY);  //fd参数是通过打开相对路径名所在的目录来获取。
    if (dir_fd < 0)   
    {  
        perror("open");  
        exit(EXIT_FAILURE);  
    }  

    flags = O_CREAT | O_TRUNC | O_RDWR;  
    mode = 0640;  //-rw-r-----
    fd = openat(dir_fd, relative_path, flags, mode);  
    if (fd < 0)   
    {  
        perror("openat");  
        exit(EXIT_FAILURE);  
    }  

    write(fd, "HELLO", 5);  

    close(fd);  
    close(dir_fd);  
}  

int main()  
{  
    creat_at("../03.文件IO", "log.txt");  
    return 0;  
}

借用dirfd，将DIR*转换成int类型的文件描述符

#include <sys/types.h>  
#include <sys/stat.h>  
#include <fcntl.h>  
#include <dirent.h>  
#include <stdio.h>  
#include <unistd.h>  

int main()  
{  
    DIR *dir;  
    int dirfd2;  
    int fd;  
    int n;  

    dir = opendir("../03.文件IO");  
    if(NULL == dir)  
    {  
        perror("open dir error");  
        return -1;  
    }  
    dirfd2 = dirfd(dir);  
    if(-1 == dirfd2)  
    {  
        perror("dirfd error");  
        return -1;  
    }  

    fd = openat(dirfd2,"output.log",O_CREAT|O_RDWR|O_TRUNC, \
　　　　　　　　　　　　　　　　　　　　　　S_IRWXU|S_IRWXG|S_IRWXO);  
    if(-1 == fd)  
    {  
        perror("opeat error");  
        return -1;  
    }  
    n = write(fd,"Hello world!\n",15);  

    close(fd);  
    closedir(dir);  

    return 0;  

}