makecontext(3) Library Functions Manual makecontext(3)
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 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.
ERRORSENOMEM Insufficient stack space left.
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 │
└───────────────┴───────────────┴───────────────────────────────────────┘
STANDARDS
None.
HISTORY
glibc 2.1. SUSv2, POSIX.1-2001. Removed in POSIX.1-2008, 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.,
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 glibc 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 <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\n", __func__);
printf("%s: swapcontext(&uctx_func1, &uctx_func2)\n", __func__);
if (swapcontext(&uctx_func1, &uctx_func2) == -1)
err(EXIT_FAILURE, "swapcontext");
printf("%s: returning\n", __func__);
}
static void
func2(void)
{
printf("%s: started\n", __func__);
printf("%s: swapcontext(&uctx_func2, &uctx_func1)\n", __func__);
if (swapcontext(&uctx_func2, &uctx_func1) == -1)
err(EXIT_FAILURE, "swapcontext");
printf("%s: returning\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)\n", __func__);
if (swapcontext(&uctx_main, &uctx_func2) == -1)
err(EXIT_FAILURE, "swapcontext");
printf("%s: exiting\n", __func__);
exit(EXIT_SUCCESS);
}
SEE ALSOsigaction(2), sigaltstack(2), sigprocmask(2), getcontext(3), sigsetjmp(3)
Linux man-pages 6.17 2026-02-08 makecontext(3)