NAME
makecontext, swapcontext - manipulate user context
SYNOPSIS
#include <ucontext.h>
I void makecontext(ucontext_t * ucp , void * func (),
I int argc , ...);
I int swapcontext(ucontext_t * oucp , ucontext_t * ucp );
DESCRIPTION
In a System V-like environment, one has the type
ucontext_t defined in
<ucontext.h>
and the four functions
getcontext(2),
setcontext(2),
R makecontext ()
and
R 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
R makecontext ()
function modifies the context pointed to
by
ucp (which was obtained from a call to
getcontext(2)).
Before invoking
R 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
R swapcontext ())
the function
func is called,
and passed the series of integer
(int)
arguments that follow
R argc ;
the caller must specify the number of these arguments in
R argc .
When this function returns, the successor context is activated.
If the successor context pointer is NULL, the thread exits.
The
R 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,
R swapcontext ()
does not return.
(But we may return later, in case oucp is
activated, in which case it looks like
R swapcontext ()
returns 0.)
On error,
R swapcontext ()
returns -1 and
sets errno appropriately.
ERRORS
ENOMEM
Insufficient stack space left.
CONFORMING TO
SUSv2, POSIX.1-2001.
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.
EXAMPLE
The example program below demonstrates the use of
getcontext(2),
R makecontext (),
and
R 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 die(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)
die("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)
die("swapcontext");
printf("func2: returning\n");
}
int
main(int argc, char *argv[])
{
char func1_stack[16384];
char func2_stack[16384];
if (getcontext(&uctx_func1) == -1)
die("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)
die("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)
die("swapcontext");
printf("main: exiting\n");
exit(EXIT_SUCCESS);
}
SEE ALSO
ČLÁNKY DO MAILU