Manpages - makecontext.3
Table of Contents
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), *setcontext*(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).
| Interface | Attribute | Value |
| *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 /int/s. 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)
COLOPHON
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