makecontext, swapcontext - manipulate user context
Standard C library (
libc,
-lc)
#include <ucontext.h>
void makecontext(ucontext_t *ucp, void (*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.
makecontext() and
swapcontext() are provided since glibc 2.1.
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 |
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 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
#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)