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MAKECONTEXT(3) Linux Programmer's Manual MAKECONTEXT(3)
makecontext, swapcontext - manipulate user context
#include <ucontext.h>
void makecontext(ucontext_t *ucp, void (*func)(), int argc, ...);
int swapcontext(ucontext_t *oucp, ucontext_t *ucp);
In a System V-like environment, one has the type ucontext_t defined in
<ucontext.h> and the four functions getcontext(2), setcontext(2),
makecontext() and swapcontext() that allow user-level context switching
between multiple threads of control within a process.
For the type and the first two functions, see getcontext(2).
The makecontext() function modifies the context pointed to by ucp (which was
obtained from a call to getcontext(2)). 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(2) 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 appropriately.
ENOMEM Insufficient stack space left.
makecontext() and swapcontext() are provided in glibc since version 2.1.
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.
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).
The example program below demonstrates the use of getcontext(2),
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
#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);
}
getcontext(2), sigaction(2), sigaltstack(2), sigprocmask(2), sigsetjmp(3)
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GNU 2009-03-31 MAKECONTEXT(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface