C99 problem with strdup

segfault.c

/*
 * $ gcc -std=c99 -o segfault segfault.c -lpthread
 * segfault.c: In function ‘thread_fn’:
 * segfault.c:7:5: warning: implicit declaration of function ‘strdup’ [-Wimplicit-function-declaration]
 * segfault.c:7:17: warning: initialization makes pointer from integer without a cast [enabled by default]
 * $ ./segfault
 * Segmentation fault
 */

#include <stdio.h>
#include <string.h>
#include <pthread.h>

static void *thread_fn(void *arg)
{
    char *str = strdup("a string literal");
    printf("here's a string: %s\n", str);
}

int main()
{
    pthread_t thread;
    pthread_create(&thread, NULL, thread_fn, NULL);
    pthread_join(thread, NULL);

    return 0;
}

I came here by searching "strdup c99".

One of the solutions is to append -D_GNU_SOURCE to the gcc command line or prepending #define _GNU_SOURCE to the beginning of the file.

Faced a similar problem.
This solved my issue.