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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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       information  about  reporting  bugs,  and  the  latest  version  of  this   page,   can   be   found   at
       https://www.kernel.org/doc/man-pages/.

GNU                                                2019-03-06                                     MAKECONTEXT(3)