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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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makecontext(3) Library Functions Manual makecontext(3)
makecontext, swapcontext - manipulate user context
Standard C library (libc, -lc)
#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);
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.
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.
ENOMEM Insufficient stack space left.
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 │
└───────────────┴───────────────┴────────────────────────────────┘
None.
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.
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).
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 <stdio.h>
#include <stdlib.h>
#include <ucontext.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("%s: started\n", __func__);
printf("%s: swapcontext(&uctx_func1, &uctx_func2)\n", __func__);
if (swapcontext(&uctx_func1, &uctx_func2) == -1)
handle_error("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)
handle_error("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)
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("%s: swapcontext(&uctx_main, &uctx_func2)\n", __func__);
if (swapcontext(&uctx_main, &uctx_func2) == -1)
handle_error("swapcontext");
printf("%s: exiting\n", __func__);
exit(EXIT_SUCCESS);
}
sigaction(2), sigaltstack(2), sigprocmask(2), getcontext(3),
sigsetjmp(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 makecontext(3)
Pages that refer to this page: sigaltstack(2), getcontext(3), signal(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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