Question

Linux3.2.0

Given the following source:

#include <stdio.h>
#include <sched.h>

int main(void)
{
        printf("%i \n", CLONE_PTRACE);
        return 0;
}

I get:

test.c: In function ‘main’:
test.c:6:18: error: ‘CLONE_PTRACE’ undeclared (first use in this function)

Why?

Answer 1

You need to #define _GNU_SOURCE one way or another to get those constants. (See the clone(2) man page.)

With your code saved as t.c:

$ gcc -std=c99 t.c
t.c: In function ‘main’:
t.c:6:25: error: ‘CLONE_PTRACE’ undeclared (first use in this function)
t.c:6:25: note: each undeclared identifier is reported only once for each function it appears in
$ gcc -std=c99 -D_GNU_SOURCE t.c
$ ./a.out 
8192