longjmp to the callee

[Jerald Fijerald]

Hello.

I'm trying to find an elegant way to longjmp to the callee
(aka "generators for C")

So far I've come to this experimental program (gcc only as it uses
__builtin_frame_address, although we can do without it):

======================================================================
/* longjmp to the callee */

#include <setjmp.h>
#include <stdio.h>

jmp_buf F;
jmp_buf F2;

static inline char* bfr ()
{
return __builtin_frame_address (0);
}

char *top;

int foo()
{
int i;
for (i = 0; i < 5; i++) {
printf (".i=%i\n", i);
if (i == 2) {
if (!setjmp (F2)) {
top = bfr();
//printf ("Mytop=%p\n", top);
longjmp (F, 1);
} else {
printf ("resumed...\n");
}
}
printf ("i=%i\n", i);
}
return 0;
}

int bar ()
{
return foo ();
}

int potatoe ()
{
int i, j;
for (i = 0, j = 10; i < j; i++)
printf ("%i ", i);
printf ("\n");
return 132;
}

int main ()
{
int i, j;
char *thistop;
for (i = 0; i < 3; i++)
if (!setjmp (F)) {
printf ("trying...\n");
bar();
} else {
/* preserve the stack */
j=40;
char CAP1[j];
thistop = bfr();
char GAP2 [top-thistop];

printf ("caught...\n");

/* test that stack is not polluted */
potatoe ();

/* longjmp to the callee and resume */
longjmp (F2, 2);
}
return 0;
}
=============================================================

Notes:
longjmp already supports jumping to a callee. The problem
is that we have to preserve the stack (and thus the function
potatoe() which is supposed to polute it).
The solution I found is to use C99 variable-length arrays to
displace the stacktop beyond the region we want to preserve.

I'm looking for ideas/enhancements to make this more portable,
safe, fast, etc. It must have been done before, right?


jf

 

[Dave Vandervies]

Hello.

I'm trying to find an elegant way to longjmp to the callee
(aka "generators for C")

You can't. longjmp can only jump into stack[1] frames that are still
live.

If this is a "fun" project, you may have found an excuse to go learn
about functional languages that support continuations (which, at a first
glance, appears to be exactly what you're looking for).

If you can't change your tools, you might find something not entirely
unlike what you're looking for at
www.chiark.greenend.org.uk/~sgtatham/coroutines.html


dave

[1] That's the function-invocation stack, which need not be represented
with the "usual" block of memory that the hardware treats as a stack.
Just mentioning it so that the pedants don't jump on me for it.

 

[Eric Sosman]

Hello.

I'm trying to find an elegant way to longjmp to the callee
(aka "generators for C")

You can't. longjmp can only jump into stack[1] frames that are still
live.

If this is a "fun" project, you may have found an excuse to go learn
about functional languages that support continuations (which, at a first
glance, appears to be exactly what you're looking for).

If you can't change your tools, you might find something not entirely
unlike what you're looking for at
www.chiark.greenend.org.uk/~sgtatham/coroutines.html


dave

[1] That's the function-invocation stack, which need not be represented
with the "usual" block of memory that the hardware treats as a stack.
Just mentioning it so that the pedants don't jump on me for it.

Then I guess we'll have to longjmp() on you for it.