/*
* Example of using the getcontext()/swapcontext() API. This API, provided by
* ucontext.h, is not part of POSIX anymore because of portability issues.
*
* Create two threads from the main function and start a predefined sequence of
* context switches.
*/
#include
#include
#include
/*
* 3 context objects (for the “main” thread and the two extra function threads)
*/
static ucontext_t ctx[3];
/*
* Context switching takes a snapshot of currently running @from’ context before
* stopping its execution, and resumes the execution of @to by restoring its
* saved context.
*/
static void context_switch(int from, int to)
{
swapcontext(&ctx[from], &ctx[to]);
}
/*
* Thread f1
*/
static void f1(void)
{
printf(“start f1\n”);
/* Context switch from f1 to f2 */
context_switch(1, 2);
printf(“finish f1\n”);
/* Context switch from f1 to main */
context_switch(1, 0);
assert(0); /* We should never come back here */
}
/*
* Thread f2
*/
static void f2(void)
{
printf(“start f2\n”);
/* Context switch from f2 to f1 */
context_switch(2, 1);
printf(“finish f2\n”);
/* Context switch from f2 to f1 */
context_switch(2, 1);
assert(0); /* We should never come back here */
}
#define STACK_SIZE 32768
typedef void (*func_t) ();
int main(int argc, char *argv[])
{
char stack1[STACK_SIZE], stack2[STACK_SIZE];
/*
* Initialize context object for thread f1
* – use stack1 as stack and fill out stack size
* – pass one argument (23) to thread f1
*/
getcontext(&ctx[1]);
ctx[1].uc_stack.ss_sp = stack1;
ctx[1].uc_stack.ss_size = STACK_SIZE;
makecontext(&ctx[1], (func_t)f1, 0);
/*
* Initialize context object for thread f2
* – use stack2 as stack and fill out stack size
* – pass one argument (42) to thread f2
*/
getcontext(&ctx[2]);
ctx[2].uc_stack.ss_sp = stack2;
ctx[2].uc_stack.ss_size = STACK_SIZE;
makecontext(&ctx[2], (func_t)f2, 0);
/* Before we start scheduling */
printf(“The beginning…\n”);
/*
* Start the predefined scheduling sequence
* – Context switch from main to f2
*/
context_switch(0, 2);
/* Back to main and quit */
printf(“The end…\n”);
return 0;
}