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Setcontext

From Wikipedia, the free encyclopedia

The correct title of this article is setcontext. The initial letter is capitalized due to technical restrictions.

setcontext is one of a family of C library functions (the others being getcontext, makecontext and swapcontext) used for context control. setcontext et al allow the implementation in C of advanced control flow patterns such as iterators and coroutines.

setcontext et al can be viewed as a more advanced version of setjmp/longjmp; where the latter just allows a single non-local jump up the stack, setcontext allows the creation of multiple cooperative threads of control, each with their own stack.

1 Usage
2 Specification
3 Example
4 See also
5 External links

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Usage

The four function calls, along with the types ucontext_t and mcontext_t, are defined in <ucontext.h>.

The ucontext_t type stores execution state, including all registers and flags, the instruction pointer, and the stack pointer.

See below for an example of usage.

setcontext(const ucontext_t *ucp): transfer control to the context in ucp. Does not return.
getcontext(ucontext_t *ucp): store the current context into ucp. Will appear to return both after the initial call and whenever the context in ucp is jumped to; unfortunately, getcontext does not provide a return value to distinguish the cases, so the programmer must use a flag variable (defined volatile, and not a register variable, for obvious reasons).
makecontext(ucontext_t *ucp, void *func(), int argc, ...): set up an alternate thread of control, to be stored in ucp (which should have been initialised with getcontext first). ucp.uc_stack should contain an appropriately sized stack; SIGSTKSZ is adequate. ucp will begin control at the entry point to func, which will be called with argc arguments as supplied. When func terminates, control will pass to ucp.uc_link.
swapcontext(ucontext_t *oucp, ucontext_t *ucp): transfer control to ucp, saving the current execution state into oucp.

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Specification

setcontext et al are described in the Single Unix Specification, version 2. Not all Unix-like operating systems have the functions, a fact which has possibly led to their being little-used.

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Example

#include <stdio.h>
#include <stdlib.h>
#include <ucontext.h>

static void 
loop_iter (ucontext_t *my_ucp, ucontext_t *loop_ucp, int *yield_i) {
        int i;
        for (i = 0; i < 10; ++i) {
                *yield_i = i;
                swapcontext (my_ucp, loop_ucp);
        }
} 
 
int 
main (void) {
        ucontext_t main_uc, loop_uc, iterator_uc;
        char iterator_stack[SIGSTKSZ];
        volatile int has_run;
        volatile int yield_i;
        volatile const char *yield_path;

        iterator_uc.uc_link = &main_uc;
        iterator_uc.uc_stack.ss_sp = iterator_stack;
        iterator_uc.uc_stack.ss_size = sizeof iterator_stack;
        getcontext (&iterator_uc);

        makecontext (&iterator_uc, (void (*) (void)) loop_iter, 
                        3, &iterator_uc, &loop_uc, &yield_i);
        has_run = 0;
        getcontext (&main_uc);
        if (!has_run) {
                has_run = 1;
                while (1) {
                        swapcontext (&loop_uc, &iterator_uc);
                        printf ("%d\n", yield_i);
                }
         }

        return 0;
}

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