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]

In article <aa************ **************@ posting.google. com>,
Jerald Fijerald <jf*@freemail.g r> wrote:

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
http://www.chiark.greenend.org.uk/~s...oroutines.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.

--
Dave Vandervies dj******@csclub .uwaterloo.ca
Yes, I'm a middle-aged fogy. Forty years ago I was a young fogy.
In fifteen years or so, I'll be an old fogy.
--Mike Andrews in the scary devil monastery

[Eric Sosman]

Dave Vandervies wrote:

In article <aa************ **************@ posting.google. com>,
Jerald Fijerald <jf*@freemail.g r> wrote:

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
http://www.chiark.greenend.org.uk/~s...oroutines.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.

--
Er*********@sun .com