C 警告 : implicit declaration of function ‘fchmod’

Question

我有一个使用 fchmod 的函数 createFile:

int createFile(char *pFileName) {
   int ret;

   if ((ret = open(pFileName, O_RDWR | O_CREAT | O_TRUNC)) < 0)
      errorAndQuit(2);

   fchmod(ret, S_IRUSR | S_IWUSR);
   return ret;
}

在我的文件顶部，我有以下内容:

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

编译时:编译器吐出:

warning: implicit declaration of function ‘fchmod’

我包含了所有正确的文件，但收到此警告。该程序运行良好，即使有警告。

Answer 1

巧合的是，feature_test_macros(7) 联机帮助页直接回答了您的问题:

Specification of feature test macro requirements in manual pages
   When a function requires that a feature test macro is
   defined, the manual page SYNOPSIS typically includes a note
   of the following form (this example from the chmod(2) manual
   page):

          #include <sys/stat.h>

          int chmod(const char *path, mode_t mode);
          int fchmod(int fd, mode_t mode);

      Feature Test Macro Requirements for glibc (see
      feature_test_macros(7)):

          fchmod(): _BSD_SOURCE || _XOPEN_SOURCE >= 500

   The || means that in order to obtain the declaration of
   fchmod(2) from <sys/stat.h>, either of the following macro
   definitions must be made before including any header files:

          #define _BSD_SOURCE
          #define _XOPEN_SOURCE 500     /* or any value > 500 */

   Alternatively, equivalent definitions can be included in the
   compilation command:

          cc -D_BSD_SOURCE
          cc -D_XOPEN_SOURCE=500        # Or any value > 500