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, 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 al-
low 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 swapcon-
text()) 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 swapcon-
text() 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 since glibc 2.1.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
┌──────────────┬───────────────┬───────────────────────────────────────┐
│Interface │ Attribute │ Value │
├──────────────┼───────────────┼───────────────────────────────────────┤
│makecontext() │ Thread safety │ MT-Safe race:ucp │
├──────────────┼───────────────┼───────────────────────────────────────┤
│swapcontext() │ Thread safety │ MT-Safe race:oucp race:ucp │
└──────────────┴───────────────┴───────────────────────────────────────┘
STANDARDS
SUSv2, POSIX.1-2001. POSIX.1-2008 removes the specifications of make-
context() and swapcontext(), citing portability issues, and recommend-
ing that applications be rewritten to use POSIX threads instead.
NOTES
The interpretation of ucp->uc_stack is just as in sigaltstack(2), name-
ly, 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 di-
rection.
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. Howev-
er, 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 architec-
tures (e.g., x86-64).
EXAMPLES
The example program below demonstrates the use of getcontext(3), make-
context(), and swapcontext(). Running the program produces the follow-
ing 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);
}
SEE ALSO
sigaction(2), sigaltstack(2), sigprocmask(2), getcontext(3),
sigsetjmp(3)
Linux man-pages 6.03 2022-12-15 makecontext(3)
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