makecontext, swapcontext — manipulate user context
#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.
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 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(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 <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); }