portable coroutine implementation (sketch)

// portable coroutine implementation (sketch)
// note that Clang (LLVM) compiler requires -fno-stack-check option
#include <stdio.h>
#include <stdlib.h>
#include <setjmp.h>

jmp_buf contexts[2];
int cid = 0; // coroutine id

void printsp()
{
  int x;
  printf("stack pointer = %p\n", &x);
}

// execute the func on the specified stack
void exec_on_altstack(void *stkbase, size_t stksz, void (*func)())
{
  size_t diff = (void*)&diff - stkbase - stksz;
  // allocate a dummy local variable to change the stack pointer
  volatile char dummy[diff];
  dummy[0] = '\0';  // don't remove dummy, optimizer
  (*func)();
}

// context switch
void csw(int target) {
  if (_setjmp(contexts[cid]) == 0) {
    printf("# coroutine[%d] context saved\n", cid);
    cid = target;
    _longjmp(contexts[target], 1);
  } else {
    printf("# coroutine[%d] resumes\n", cid);
  }
}

void test() {
  printf("test started\n");
  printsp();
  csw(0);
  printf("test resumed\n");
  csw(0);
  printf("test resumed2\n");
  csw(0);
}

extern int main(int argc, char **argv)
{
  printsp();
  size_t stksz = 4096;
  void *stkbase = malloc(stksz);
  printf("allocated stack = %p to %p\n", stkbase, stkbase + stksz - 1);
  if (_setjmp(contexts[cid]) == 0) {
    cid = 1;
    exec_on_altstack(stkbase, stksz, test);
  } else {
    printf("main started\n");
    csw(1);
    printf("main resumed\n");
    csw(1);
    printf("main resumed2\n");
  }
  return 0;
}

/* executed on OSX 10.10.4

~/stacktest% ./a.out
stack pointer = 0x7fff5917492c
allocated stack = 0x7feb08804200 to 0x7feb088051ff
test started
stack pointer = 0x7feb088051bc
# coroutine[1] context saved
main started
# coroutine[0] context saved
# coroutine[1] resumes
test continued
# coroutine[1] context saved
# coroutine[0] resumes
main resumed
# coroutine[0] context saved
# coroutine[1] resumes
test continued2
# coroutine[1] context saved
# coroutine[0] resumes
main resumed2

*/