Implicit Function Declaration in C UNIX

Question

The following code displays a warning about the implicit declaration of the getpgid function. I only know his warning, but his class and professor want us to consider warnings as errors. So help please.

I also included the corresponding header file, so I have no idea what is wrong:

#include <unistd.h>

pid_t pid, pgid;

if ((pgid = getpgid(pid)) < 0) {
      app_error("Failure to get process group ID");
}

Answer 1

On the man page :

Requirements for function check macros for glibc (see feature_test_macros(7)):

getpgid():
      _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
      || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L

Answer 2

Assume all elision: pid_t is undefined. You need both

#include <sys/types.h>  
#include <unistd.h>  

Otherwise, you will not get what you think you are getting.

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#include <unistd.h>
#include <stdio.h>
int main() {
     pid_t pid, pgid;
     if((pgid = getpgid(pid)) < 0) {
          puts("Oops.");
     }
     return 0;
}

:
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Answer 3

/ , . , . , - .

, getpgid , , POSIX getpgrp(void). ( ), .

Answer 4

"getpgid", -