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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 ints. 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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