This is a simple example of "threads" implemented in userspace, which cooperatively hand off control to one another. The API is quite simple:

/*
 * Functions called to initialize, add tasks, and run the scheduler.
 */
void scheduler_init(void);
void scheduler_create_task(void (*func)(void*), void *arg);
void scheduler_run(void);

/*
 * Functions called by tasks to exit or hand off control.
 */
void scheduler_exit_current_task(void);
void scheduler_relinquish(void);

You can find the implementation of this API in scheduler.c and an example of using it in main.c.

This project involves platform-specific x86_64 assembly and thus only runs on these platforms. It is, however, quite easy to port to other architectures.

This project uses the Meson build system. Install meson and ninja packages if you don't already have those commands.

$ meson build
$ ninja -C build
$ build/main