minix/lib/libc/posix/_ucontext.c

#include <lib.h>
#include <machine/stackframe.h>
#include <ucontext.h>
#include <signal.h>
#include <stdarg.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdint.h>
#include <stdio.h>

_PROTOTYPE( void ctx_start, (void (*)(void), int, ...)			);

/*===========================================================================*
 *				setuctx					     *
 *===========================================================================*/
PUBLIC int setuctx(const ucontext_t *ucp)
{
  int r;

  if (ucp == NULL) {
	errno = EFAULT;
	return(-1);
  }

  if (!(ucp->uc_flags & UCF_IGNSIGM)) {
	/* Set signal mask */
	if ((r = sigprocmask(SIG_SETMASK, &ucp->uc_sigmask, NULL)) == -1)
		return(r);
  }

  if (!(ucp->uc_flags & UCF_IGNFPU)) {
	if ((r = setmcontext(&(ucp->uc_mcontext))) == -1)
		return(r);
  }

  return(0);
}


/*===========================================================================*
 *				getuctx					     *
 *===========================================================================*/
PUBLIC int getuctx(ucontext_t *ucp) 
{
  int r;

  if (ucp == NULL) {
	errno = EFAULT;
	return(-1);
  }

  if (!(ucp->uc_flags & UCF_IGNSIGM)) {
	/* Get signal mask */
	if ((r = sigprocmask(0, NULL, &ucp->uc_sigmask)) == -1)
		return(r);
  }

  if (!(ucp->uc_flags & UCF_IGNFPU)) {
	if ((r = getmcontext(&(ucp->uc_mcontext))) != 0)
		return(r);
  }

  return(0);
}


/*===========================================================================*
 *				makecontext				     *
 *===========================================================================*/
PUBLIC void makecontext(ucontext_t *ucp, void (*func)(void), int argc, ...)
{
  va_list ap;
  unsigned int *stack_top, *stack_guard;

  /* There are a number of situations that are erroneous, but we can't actually
     tell the caller something is wrong, because this is a void function.
     Instead, mcontext_t contains a magic field that has to be set
     properly before it can be used. */
  if (ucp == NULL) {
	return;
  } else if ((ucp->uc_stack.ss_sp == NULL) || 
	     (ucp->uc_stack.ss_size < MINSIGSTKSZ)) {
	ucp->uc_mcontext.mc_magic = 0; 
	ucp->uc_mcontext.mc_p_reg.sp = 0;
	return;
  }

  if (ucp->uc_mcontext.mc_magic == MCF_MAGIC) {
#if (_MINIX_CHIP == _CHIP_INTEL)
	/* The caller provides a pointer to a stack that we can use to run our
	   context on. When the context starts, control is given to a wrapped 
	   start routine, which calls a function and cleans up the stack
	   afterwards. The wrapper needs the address of that function on the
	   stack.
	   The stack will be prepared as follows:
		func()       - start routine
		arg1         - first argument
		...
		argn         - last argument
		ucp          - context, esp points here when `func' returns
	   _ctx_start pops the address of `func' from the stack and calls it. 
	   The stack will then be setup with all arguments for `func'. When
	   `func' returns, _ctx_start cleans up the stack such that ucp is at
	   the top of the stack, ready to be used by resumecontext.
	   Resumecontext, in turn, checks whether another context is ready to
	   be executed (i.e., uc_link != NULL) or exit(2)s the process. */

	/* Find the top of the stack from which we grow downwards. */
	stack_top = (unsigned int *) ((uintptr_t ) ucp->uc_stack.ss_sp +
						   ucp->uc_stack.ss_size);

	/* Align the arguments to 16 bytes (we might lose a few bytes of stack
	   space here).*/
	stack_top = (unsigned int *) ((uintptr_t) stack_top & ~0xf);
	
	/* Make room for 'func', the `func' routine arguments, and ucp. */
	stack_top -= (1 + argc + 1);

	/* Adjust the machine context to point to the top of this stack and the
	   program counter to the context start wrapper. */
	ucp->uc_mcontext.mc_p_reg.fp = 0; /* Clear frame pointer */
	ucp->uc_mcontext.mc_p_reg.sp = (reg_t) stack_top;
	ucp->uc_mcontext.mc_p_reg.pc = (reg_t) ctx_start;

	*stack_top++ = (uintptr_t) func;

	/* Copy arguments to the stack. */
	va_start(ap, argc);
	while (argc-- > 0) {
		*stack_top++ = va_arg(ap, uintptr_t);
	}
	va_end(ap);

	/* Store ucp on the stack */
	*stack_top = (uintptr_t) ucp;

	/* Set ESI to point to the base of the stack where ucp is stored, so
	   that the wrapper function knows how to clean up the stack after
	   calling `func' (i.e., how to adjust ESP). */
	ucp->uc_mcontext.mc_p_reg.si = (reg_t) stack_top;
	

	/* If we ran out of stack space, invalidate stack pointer. Eventually,
	   swapcontext will choke on this and return ENOMEM. */
	if (stack_top == ucp->uc_stack.ss_sp)
		ucp->uc_mcontext.mc_p_reg.sp = 0;

#else
# error "Unsupported platform"
#endif
  }	
}


/*===========================================================================*
 *				swapcontext				     *
 *===========================================================================*/
PUBLIC int swapcontext(ucontext_t *oucp, const ucontext_t *ucp)
{
  int r;
  unsigned int *stack_guard;

  if ((oucp == NULL) || (ucp == NULL)) {
	errno = EFAULT;
	return(-1);
  }

  if (ucp->uc_mcontext.mc_p_reg.sp == 0) {
	/* No stack space. Bail out. */
	errno = ENOMEM;
	return(-1);
  } 

  oucp->uc_flags &= ~UCF_SWAPPED;
  r = getcontext(oucp);
  if ((r == 0) && !(oucp->uc_flags & UCF_SWAPPED)) {
	oucp->uc_flags |= UCF_SWAPPED;
	r = setcontext(ucp);
  }

  return(r);
}


/*===========================================================================*
 *				resumecontext				     *
 *===========================================================================*/
PUBLIC void resumecontext(ucontext_t *ucp)
{
  if (ucp->uc_link == NULL) exit(0);

  /* Error handling? Where should the error go to? */
  (void) setcontext((const ucontext_t *) ucp->uc_link);

  exit(1); /* Never reached */
}