系统调用 fchownat

fchownat 是linux kernel 2.6.16 以后添加的系统调用

linux kernel 2.6.16 新增了系列 at 系统调用( openat, linkat ..... )

原型：include/linux/syscalls.h

asmlinkage long sys_fchownat(int dfd, const char __user *filename, uid_t user, gid_t group, int flag);

说明：

The fchownat() function sets the owner ID and  group  ID  of     the named  file  in the same manner as chown(). If, however,     the path argument is relative, the path is resolved relative     to  the  fildes  argument  rather  than  the current working     directory.  If the fildes argument  has  the  special  value     FDCWD,  the  path  path  resolution  reverts back to current     working directory relative.  If the flag argument is set  to     SYMLNK,  the  function behaves like lchown() with respect to     symbolic links. If the path argument is absolute, the fildes     argument  is  ignored.   If  the  path  argument  is  a null     pointer, the function behaves like fchown().

这些 *at 系统调用相对与之前的系统调用增加了个文件描述符参数（上面的 int dfd）

功能是相对与相对与此文件描述符（目录）的路径而操作， 之前的系统调用的操作都是相对与进程当前位置的

当然，如果此文件描述符未设置或者不是个有效的目录文件描述符， *at 系统调用就和老版系统调用功能相同了

一个例子展示了 openat 系统调用的用法：

#include <stdlib.h>#include <sys/types.h>#include <sys/stat.h>#include <unistd.h>#include <errno.h>#include <fcntl.h>int main(){    int dfd,ffd;    dfd = open("/home", O_RDONLY|00200000); // 00200000其实就是 O_DIRECTORY    if(dfd < 1)    {        perror("open home");        return -1;    }    ffd = openat(dfd, "../opt/openat", O_CREAT, 777);    if(ffd < 1)    {        perror("open new file");        return -1;    }    return 0;}

该程序创建了文件 /opt/openat

下段代码则演示了 unlinkat 的用法：

#include <stdlib.h>#include <sys/types.h>#include <sys/stat.h>#include <unistd.h>#include <errno.h>#include <fcntl.h>int main(){    int dfd,ffd;    dfd = open("/opt/cc", O_RDONLY|00200000);    if(dfd < 1)    {        perror("open home");        return -1;    }    //ffd = openat(dfd, "../opt/deny/openat", O_CREAT, 777);    ffd = unlinkat(dfd,"../deny/openat", 0);    if(ffd  != 0)    {        perror("unlinkat new file");        return -1;    }    return 0;}