makecontext, swapcontext - manipulate user context

#include <ucontext.h>
void makecontext(ucontext_t *ucp, void (*func)(), int argc, ...);
int swapcontext(ucontext_t *restrict oucp,
 const ucontext_t *restrict ucp);

In a System V-like environment, one has the type ucontext_t (defined in <ucontext.h> and described in getcontext(3)) and the four functions getcontext(3), setcontext(3), makecontext(), and swapcontext() that allow user-level context switching between multiple threads of control within a process.

The makecontext() function modifies the context pointed to by ucp (which was obtained from a call to getcontext(3)). Before invoking makecontext(), the caller must allocate a new stack for this context and assign its address to ucp->uc_stack, and define a successor context and assign its address to ucp->uc_link.

When this context is later activated (using setcontext(3) or swapcontext()) the function func is called, and passed the series of integer (int) arguments that follow argc; the caller must specify the number of these arguments in argc. When this function returns, the successor context is activated. If the successor context pointer is NULL, the thread exits.

The swapcontext() function saves the current context in the structure pointed to by oucp, and then activates the context pointed to by ucp.

When successful, swapcontext() does not return. (But we may return later, in case oucp is activated, in which case it looks like swapcontext() returns 0.) On error, swapcontext() returns -1 and sets errno to indicate the error.

The example program below demonstrates the use of getcontext(3), makecontext(), and swapcontext(). Running the program produces the following output:

$ ./a.out
main: swapcontext(&uctx_main, &uctx_func2)
func2: started
func2: swapcontext(&uctx_func2, &uctx_func1)
func1: started
func1: swapcontext(&uctx_func1, &uctx_func2)
func2: returning
func1: returning
main: exiting

Program source

#include <stdio.h>
#include <stdlib.h>
#include <ucontext.h>
static ucontext_t uctx_main, uctx_func1, uctx_func2;
#define handle_error(msg) \
    do { perror(msg); exit(EXIT_FAILURE); } while (0)
static void
func1(void)
{
    printf("%s: started\n", __func__);
    printf("%s: swapcontext(&uctx_func1, &uctx_func2)\n", __func__);
    if (swapcontext(&uctx_func1, &uctx_func2) == -1)
        handle_error("swapcontext");
    printf("%s: returning\n", __func__);
}
static void
func2(void)
{
    printf("%s: started\n", __func__);
    printf("%s: swapcontext(&uctx_func2, &uctx_func1)\n", __func__);
    if (swapcontext(&uctx_func2, &uctx_func1) == -1)
        handle_error("swapcontext");
    printf("%s: returning\n", __func__);
}
int
main(int argc, char *argv[])
{
    char func1_stack[16384];
    char func2_stack[16384];
    if (getcontext(&uctx_func1) == -1)
        handle_error("getcontext");
    uctx_func1.uc_stack.ss_sp = func1_stack;
    uctx_func1.uc_stack.ss_size = sizeof(func1_stack);
    uctx_func1.uc_link = &uctx_main;
    makecontext(&uctx_func1, func1, 0);
    if (getcontext(&uctx_func2) == -1)
        handle_error("getcontext");
    uctx_func2.uc_stack.ss_sp = func2_stack;
    uctx_func2.uc_stack.ss_size = sizeof(func2_stack);
    /* Successor context is f1(), unless argc > 1 */
    uctx_func2.uc_link = (argc > 1) ? NULL : &uctx_func1;
    makecontext(&uctx_func2, func2, 0);
    printf("%s: swapcontext(&uctx_main, &uctx_func2)\n", __func__);
    if (swapcontext(&uctx_main, &uctx_func2) == -1)
        handle_error("swapcontext");
    printf("%s: exiting\n", __func__);
    exit(EXIT_SUCCESS);
}

sigaction(2), sigaltstack(2), sigprocmask(2), getcontext(3), sigsetjmp(3)