系統調用 fchownat

fchownat 是linux kernel 2.6.16 以後添加的系統調用

linux kernel 2.6.16 新增了系列 at 系統調用( 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);

說明：

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().

這些 *at 系統調用相對與之前的系統調用增加了個檔案描述符參數（上面的 int dfd）

功能是相對與相對與此檔案描述符（目錄）的路徑而操作， 之前的系統調用的操作都是相對與進程當前位置的

當然，如果此檔案描述符未設定或者不是個有效目錄檔案描述符， *at 系統調用就和老版系統調用功能相同了

一個例子展示了 openat 系統調用的用法：

#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其實就是 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;}

該程式建立了檔案 /opt/openat

下段代碼則示範了 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;}