MAKECONTEXT(3) Library Functions Manual MAKECONTEXT(3)

makecontext swapcontext — manipulate user contexts


#include <ucontext.h>

void makecontext(ucontext_t *ucp, void (*func)(void),
       int argc, ...);
int swapcontext(ucontext_t *restrict oucp,
       const ucontext_t *restrict ucp);

The makecontext () function shall modify the context specified by ucp, which has been initialized using getcontext(3). When this context is resumed using swapcontext () or setcontext(3), program execution shall continue by calling func, passing it the arguments that follow argc in the makecontext () call.

Before a call is made to makecontext (), the application shall ensure that the context being modified has a stack allocated for it. The application shall ensure that the value of argc matches the number of integer arguments passed to func; otherwise, the behavior is undefined.

The uc_link member is used to determine the context that shall be resumed when the context being modified by makecontext () returns. The application shall ensure that the uc_link member is initialized prior to the call to makecontext ().

The swapcontext () function shall save the current context in the context structure pointed to by and shall set the context to the context structure pointed to by ucp.

Upon successful completion, swapcontext () shall return 0. Otherwise, -1 shall be returned and set to indicate the error.

The swapcontext () function shall fail if:

The ucp argument does not have enough stack left to complete the operation.

The following example illustrates the use of makecontext ():

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


static ucontext_t ctx[3];


static void
f1 (void)
{
    puts("start f1");
    swapcontext(&ctx[1], &ctx[2]);
    puts("finish f1");
}


static void
f2 (void)
{
    puts("start f2");
    swapcontext(&ctx[2], &ctx[1]);
    puts("finish f2");
}


int
main (void)
{
    char st1[8192];
    char st2[8192];


    getcontext(&ctx[1]);
    ctx[1].uc_stack.ss_sp = st1;
    ctx[1].uc_stack.ss_size = sizeof st1;
    ctx[1].uc_link = &ctx[0];
    makecontext(&ctx[1], f1, 0);


    getcontext(&ctx[2]);
    ctx[2].uc_stack.ss_sp = st2;
    ctx[2].uc_stack.ss_size = sizeof st2;
    ctx[2].uc_link = &ctx[1];
    makecontext(&ctx[2], f2, 0);


    swapcontext(&ctx[0], &ctx[2]);
    return 0;
}

None.

None.

None.

exit(3) , getcontext(3) , sigaction(3) , sigprocmask(3) , the Base Definitions volume of IEEE Std 1003.1-2001 (“POSIX.1”), <ucontext.h>

First released in Issue 4, Version 2.

Moved from X/OPEN UNIX extension to BASE.

In the ERRORS section, the description of [ENOMEM] is changed to apply to swapcontext () only.

The DESCRIPTION is updated to avoid use of the term "must" for application requirements.

The keyword is added to the swapcontext () prototype for alignment with the ISO/IEC 9899:1999 (“ISO C99”) standard.

footer end

December 6, 2001 posix.fail