Undefined symbol "_clone" on OS X - c

Question

Code:
#include <stdio.h>
#include <sched.h>
#include <stdlib.h>
#include <sys/wait.h>
#define _GNU_SOURCE
void *stack_memory()
{
const int stackSize = 65536;
void* stack = (void*)malloc(stackSize);
if (stack == NULL) {
printf("%s\n", "Cannot allocate memory \n");
exit(EXIT_FAILURE);
}
return stack;
}
int jail(void *args)
{
printf("Hello !! - child \n");
return EXIT_SUCCESS;
}
int main()
{
printf("%s\n", "Hello, world! - parent");
clone(jail, stack_memory(), SIGCHLD, 0);
return EXIT_SUCCESS;
}
Error:
Undefined symbols for architecture x86_64: "_clone", referenced
from:
_main in docker-4f3ae8.o ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code
1 (use -v to see invocation)

Answer 1

Linux doesn't prefix symbols with a leading _ so you're not using Linux.
But the clone(2) system call is Linux-specific, according to its man page.
clone() is Linux-specific and should not be used in programs intended
to be portable.
Probably you're using OS X or something. And you're compiling as C, so calling an un-declared function isn't a compile-time error (just a big warning). This is why it's a linker error instead of a compile-time error (and you ignored compiler warnings.)
And BTW, #define _GNU_SOURCE after including header files is pointless. You have to define feature-request macros before including headers to get them to define prototypes for GNU-only functions in cases where that's not already the default.