управлять пользовательским контекстом (manipulate user context)
Имя (Name)
makecontext, swapcontext - manipulate user context
Синопсис (Synopsis)
#include <ucontext.h>
void makecontext(ucontext_t *ucp, void (*func)(), int argc, ...);
int swapcontext(ucontext_t *restrict oucp,
const ucontext_t *restrict ucp);
Описание (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.
Возвращаемое значение (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.
Ошибки (Error)
ENOMEM Insufficient stack space left.
Версии (Versions)
makecontext() and swapcontext() are provided in glibc since
version 2.1.
Атрибуты (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 │
└──────────────┴───────────────┴─────────────────────────────────┘
Стандарты (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.
Примечание (Note)
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).
Примеры (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);
}
Смотри также (See also)
sigaction(2), sigaltstack(2), sigprocmask(2), getcontext(3),
sigsetjmp(3)
