I was working on other project and found, for me, strange behaviour from strdup.

Lets say we have 3 simple files:

foo.h:

#define FOO_H

void example(char *a);

#endif //FOO_H 

foo.c:

#include <stdio.h>
#include <stdlib.h>
#include "foo.h"

void example(char *a) {
    if (a == NULL) {
        fprintf(stderr, "a is NULL");
        exit(1);
    }
    char *temp = strdup(a);
    printf("%s", temp);
}

bar.c

#include <stdio.h>
#include <stdlib.h>
#include "foo.h"

int main() {
    char *a = "Hello Worldn";
    example(a);
    return 0;
}

When I compile these, I get warning and when I run these, I get seg fault:

*foo.c:10:18: warning: implicit declaration of function ‘strdup’ [-Wimplicit-function-declaration]

foo.c:10:18: warning: initialization of ‘char ’ from ‘int’ makes pointer from integer without a cast [-Wint-conversion]

The solution is to not compile with c11 or put your own strdup declaration in the .h file.
So the question is, why is this happening.