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MAKECONTEXT(3) Linux Programmer's Manual MAKECONTEXT(3) NAME makecontext, swapcontext - manipulate user context SYNOPSIS #include <ucontext.h> void makecontext(ucontext_t *ucp, void (*func)(), int argc, ...); int swapcontext(ucontext_t *restrict oucp, const ucontext_t *restrict ucp); DESCRIPTION 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), setβ context(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. 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 to indicate the error. 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). ββββββββββββββββ¬ββββββββββββββββ¬ββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ βInterface β Attribute β Value β ββββββββββββββββΌββββββββββββββββΌββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ€ βmakecontext() β Thread safety β MT-Safe race:ucp β ββββββββββββββββΌββββββββββββββββΌββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ€ βswapcontext() β Thread safety β MT-Safe race:oucp race:ucp β ββββββββββββββββ΄ββββββββββββββββ΄ββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ 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, reβ gardless 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 arβ chitectures 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). EXAMPLES 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) GNU 2021-03-22 MAKECONTEXT(3)