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[I'm learning C]: Learning to use ucontext

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Hello, I started to experimenting with ucontext to implement basic cooperative multithreading and stuccoed with this piece of code:

#include <stdlib.h>
#include <stdio.h>
#include <ucontext.h>

static ucontext_t routine_main;
static ucontext_t routine1;
static ucontext_t routine2;

void runner(int n) {
  fprintf(stderr, "runner %d\n", n);
  while(1) {
    fprintf(stderr, "runner %d\n", n);
    if (n == 1)
      swapcontext(&routine1, &routine2);
    else
      swapcontext(&routine2, &routine1);
  }
}

int main(int argc, char **argv) {
  char st1c[8192];
  char st2c[8192];

  if (getcontext(&routine1) == -1)
    abort();
  routine1.uc_link = &routine_main;
  routine1.uc_stack.ss_sp = st1c;
  routine1.uc_stack.ss_size = sizeof st1c;
  makecontext(&routine1, (void(*)(void))runner, 1, 1);

  if (getcontext(&routine2) == -1)
    abort();
  routine2.uc_link = &routine_main;
  routine2.uc_stack.ss_sp = st2c;
  routine2.uc_stack.ss_size = sizeof st2c;
  makecontext(&routine1, (void(*)(void))runner, 1, 2);

  fprintf(stderr, "start executing\n");
  if (swapcontext(&routine_main, &routine1) == -1)
    abort();
  fprintf(stderr, "exiting\n");
  return 0;
}

which doesn't work the way it should — it just constantly prints "start executing", which means it doesn't change execution context from routeine_main to routine1 but instead from routine_main to routine_main.

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[I'm learning C]: Learning to use ucontext Andrey Popp <8mayday@gmail.com> - 2012-01-29 08:06 -0800
  Re: [I'm learning C]: Learning to use ucontext Shao Miller <sha0.miller@gmail.com> - 2012-01-29 11:42 -0500
  Re: [I'm learning C]: Learning to use ucontext Barry Schwarz <schwarzb@dqel.com> - 2012-01-29 10:07 -0800
  Re: [I'm learning C]: Learning to use ucontext Jens Gustedt <jens.gustedt@loria.fr> - 2012-01-30 20:38 +0100
    Re: [I'm learning C]: Learning to use ucontext Uncle Steve <stevet810@gmail.com> - 2012-01-30 18:58 -0500
      Re: [I'm learning C]: Learning to use ucontext Keith Thompson <kst-u@mib.org> - 2012-01-30 17:05 -0800

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