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NAME

       makecontext, swapcontext - manipulate user context

SYNOPSIS

       #include <ucontext.h>

       void makecontext(ucontext_t *ucp, void (*func)(), int argc, ...);

       int swapcontext(ucontext_t *oucp, const ucontext_t *ucp);

DESCRIPTION

       In  a  System  V-like environment, one has the type ucontext_t defined in <ucontext.h> 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.

       For the type and the first two functions, see getcontext(3).

       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.

RETURN VALUE

       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 appropriately.

ERRORS

       ENOMEM Insufficient stack space left.

VERSIONS

       makecontext() and swapcontext() are provided in glibc since version 2.1.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────┬───────────────┬────────────────────────────┐
       │InterfaceAttributeValue                      │
       ├──────────────┼───────────────┼────────────────────────────┤
       │makecontext() │ Thread safety │ MT-Safe race:ucp           │
       ├──────────────┼───────────────┼────────────────────────────┤
       │swapcontext() │ Thread safety │ MT-Safe race:oucp race:ucp │
       └──────────────┴───────────────┴────────────────────────────┘

CONFORMING TO

       SUSv2,  POSIX.1-2001.   POSIX.1-2008  removes  the  specifications  of  makecontext()  and
       swapcontext(), citing portability issues, and recommending that applications be  rewritten
       to use POSIX threads instead.

NOTES

       The  interpretation  of  ucp->uc_stack  is  just as in sigaltstack(2), namely, this struct
       contains the start and length of a memory area to be used as the stack, regardless of  the
       direction of growth of the stack.  Thus, it is not necessary for the user program to worry
       about this direction.

       On architectures where int and pointer types are the same size (e.g., x86-32,  where  both
       types  are  32  bits),  you  may be able to get away with passing pointers as arguments to
       makecontext() following argc.  However, doing this is not guaranteed to  be  portable,  is
       undefined  according  to the standards, and won't work on architectures where pointers are
       larger than ints.  Nevertheless, starting with version 2.8, glibc makes  some  changes  to
       makecontext(), to permit this on some 64-bit architectures (e.g., x86-64).

EXAMPLE

       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 <ucontext.h>
       #include <stdio.h>
       #include <stdlib.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("func1: started\n");
           printf("func1: swapcontext(&uctx_func1, &uctx_func2)\n");
           if (swapcontext(&uctx_func1, &uctx_func2) == -1)
               handle_error("swapcontext");
           printf("func1: returning\n");
       }

       static void
       func2(void)
       {
           printf("func2: started\n");
           printf("func2: swapcontext(&uctx_func2, &uctx_func1)\n");
           if (swapcontext(&uctx_func2, &uctx_func1) == -1)
               handle_error("swapcontext");
           printf("func2: returning\n");
       }

       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("main: swapcontext(&uctx_main, &uctx_func2)\n");
           if (swapcontext(&uctx_main, &uctx_func2) == -1)
               handle_error("swapcontext");

           printf("main: exiting\n");
           exit(EXIT_SUCCESS);
       }

SEE ALSO

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

COLOPHON

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