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makecontext

Section: C Library Functions (3)
Updated: 202-0-08
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NAME

makecontext, swapcontext - manipulate user context

LIBRARY

Standard C library (libc,~-lc)

SYNOPSIS

#include <ucontext.h>
void makecontext(ucontext_t *ucp, typeof(void (int arg0, ...)) *func,
                 int argc, ...);
int swapcontext(ucontext_t *restrict oucp,
                 const ucontext_t *restrict ucp);

DESCRIPTION

In a System -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 use-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.

ATTRIBUTES

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

Interface	Attribute	Value
makecontext()	Thread safety	M-Safe race:ucp
swapcontext()	Thread safety	M-Safe race:oucp race:ucp

STANDARDS

None.

HISTORY

glibc 2.1. SUSv2, POSIX.-2001. Removed in POSIX.-2008, 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., x8-32, 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 glibc 2.8, glibc makes some changes to makecontext(), to permit this on some 6-bit architectures (e.g., x8-64).

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 <err.h> #include <stdio.h> #include <stdlib.h> #include <ucontext.h> static ucontext_t uctx_main, uctx_func1, uctx_func2; static void func1(void) {
    printf("%s: started[rs]n", __func__);
    printf("%s: swapcontext(&uctx_func1, &uctx_func2)[rs]n", __func__);
    if (swapcontext(&uctx_func1, &uctx_func2) == -1)
        err(EXIT_FAILURE, "swapcontext");
    printf("%s: returning[rs]n", __func__); } static void func2(void) {
    printf("%s: started[rs]n", __func__);
    printf("%s: swapcontext(&uctx_func2, &uctx_func1)[rs]n", __func__);
    if (swapcontext(&uctx_func2, &uctx_func1) == -1)
        err(EXIT_FAILURE, "swapcontext");
    printf("%s: returning[rs]n", __func__); } int main(int argc, char *argv[]) {
    char func1_stack[16384];
    char func2_stack[16384];
    if (getcontext(&uctx_func1) == -1)
        err(EXIT_FAILURE, "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)
        err(EXIT_FAILURE, "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("%s: swapcontext(&uctx_main, &uctx_func2)[rs]n", __func__);
    if (swapcontext(&uctx_main, &uctx_func2) == -1)
        err(EXIT_FAILURE, "swapcontext");
    printf("%s: exiting[rs]n", __func__);
    exit(EXIT_SUCCESS); }

Index

Program source

SEE ALSO