Coroutines in 10 lines of ANSI C

co.h

/**
 * Copyright (c) 2025 Chris Baker <mail.chris.baker@gmail.com>
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to
 * deal in the Software without restriction, including without limitation the
 * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
 * sell copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 **/

#ifndef __CO_H__
#define __CO_H__

/**
 * EXAMPLE:
 * 
 * typedef struct example {
 *   unsigned short co;
 * } example_t;
 * 
 * static void example_init(example_t *example) {
 *   example->co = 0;
 * }
 * 
 * static unsigned int example_next(example_t *example) {
 *   coroutine(&example->co) {
 *     yield(&example->co) return 1;
 *     yield(&example->co) return 2;
 *     yield(&example->co) return 3;
 *   }
 * 
 *   return 0;
 * }
 * 
 * int main(void) {
 *   example_t example;
 * 
 *   example_init(&example);
 *   assert(example_next(&example) == 1);
 *   assert(example_next(&example) == 2);
 *   assert(example_next(&example) == 3);
 *   assert(example_next(&example) == 0);
 *   assert(example_next(&example) == 0);
 *   assert(example_next(&example) == 0);
 * 
 *   return 0;
 * }
 *
 */

#define coroutine(co)         \
  switch (*(co))              \
    case 0:                   \
      if ((*(co) = 0)) {      \
        default: break;       \
      } else if (!(*(co))--)

#define yield(co)             \
  if (!(*(co) = __LINE__)) {  \
    case __LINE__:            \
    *(co) = 0;                \
    (*(co))--;                \
  } else

#endif /* __CO_H__ */