swapcontext() — Save and restore user context

The <ucontext.h> header file defines the ucontext_t type as a structure that includes the following members:

mcontext_t       uc_mcontext     A machine-specific representation
                                 of the saved context.
ucontext_t      *uc_link         Pointer to the context that will
                                 be resumed when this context returns.
sigset_t         uc_sigmask      The set of signals that are blocked
                                 when this context is active.
stack_t          uc_stack        The stack used by this context.

Special behavior for XPLINK-compiled C/C++: Restrictions concerning setjmp.h and ucontext.h:

1. All XPLINK programs compiled with the V2R10 or later C compilers that are to run with Language Environment V2R10 or later libraries and use the jmp_buf, sigjmp_buf or ucontext_t types must not be compiled with C headers from Language Environment V2R9 or earlier.
2. Non-XPLINK functions compiled with any level of Language Environment headers must not define jmp_buf, sigjmp_buf or ucontext_t data items and pass them to XPLINK functions that call getcontext(), longjmp(), _longjmp(), setjmp(), _setjmp(), setcontext(), sigsetjmp(), or swapcontext() with these passed-in data items.
3. When __XPLINK__ is defined, the Language Environment V2R10 and later headers define a larger jmp_buf, sigjmp_buf or ucontext_t area that is required by setjmp(), getcontext(), and related functions when they are called from an XPLINK routine. If __XPLINK__ is not defined, the Language Environment V2R10 and later headers define a shorter jmp_buf, sigjmp_buf or ucontext_t area. The Language Environment headers before V2R10 also define the shorter version of these data areas. If an XPLINK function calls setjmp(), getcontext() or similar functions with a short jmp_buf, sigjmp_buf or ucontext_t area, a storage overlay or program check may occur when the C library tries to store past the end of the passed-in (too short) data area.

This example uses two contexts. It creates the first, fcontext, in main with the getcontext() statement, and invokes the function func. It invokes the function with the swapcontext() statement, saving the context at that point in the second context, mcontext. The function returns to the point of the swapcontext() using the setcontext() statement and specifying mcontext as the context.

/* This example shows the usage of swapcontext().                    */

#define _XOPEN_SOURCE_EXTENDED 1
#include <stdlib.h>
#include <stdio.h>
#include <ucontext.h>
#include <errno.h>

 #ifdef _LP64
  #define STACK_SIZE 2097152+16384 /* large enough value for AMODE 64 */
 #else
  #define STACK_SIZE 16384  /* AMODE 31 addressing*/
 #endif
void func(int);

ucontext_t fcontext,mcontext;
int x = 0;

int main(void) {

  int  value = 1;

  getcontext(&fcontext);
  if ((fcontext.uc_stack.ss_sp = (char *) malloc(STACK_SIZE)) != NULL) {
    fcontext.uc_stack.ss_size = STACK_SIZE;
    fcontext.uc_stack.ss_flags = 0;
			errno = 0;
    makecontext(&fcontext,func,1,value);
			if (errno != 0){
				perror("Error reported by makecontext()");
				return -1;    /* Error occurred exit */
	}
  	}
  else {
    perror("not enough storage for stack");
    abort();
  }
  printf("context has been built\n");
  swapcontext(&mcontext,&fcontext);
  if (!x) {
    perror("incorrect return from swapcontext");
    abort();
  }
  else {
    printf("returned from function\n");
  }

}

void func(int arg) {

  printf("function called with value %d\n",arg);
  x++;
  printf("function returning to main\n");
  setcontext(&mcontext);

}

Output

context has been built
function called with value 1
function returning to main
returned from function