MAKECONTEXT(3)              Linux Programmer's Manual             MAKECONTEXT(3)

       makecontext, swapcontext - manipulate user context

       #include <ucontext.h>

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

       int swapcontext(ucontext_t *oucp, const ucontext_t *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

       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 appropriately.

       ENOMEM Insufficient stack space left.

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

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

       │Interface     Attribute     Value                      │
       │makecontext() │ Thread safety │ MT-Safe race:ucp           │
       │swapcontext() │ Thread safety │ MT-Safe race:oucp race:ucp │
       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.

       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).

       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
           printf("func1: started\n");
           printf("func1: swapcontext(&uctx_func1, &uctx_func2)\n");
           if (swapcontext(&uctx_func1, &uctx_func2) == -1)
           printf("func1: returning\n");

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

       main(int argc, char *argv[])
           char func1_stack[16384];
           char func2_stack[16384];

           if (getcontext(&uctx_func1) == -1)
           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)
           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)

           printf("main: exiting\n");

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

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GNU                                2020-12-21                     MAKECONTEXT(3)