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Date: Thu, 21 Jul 2005 22:52:00 -0000
From: Jean-Sebastien Legare <jslegare@gmail.com>
Reply-To: Jean-Sebastien Legare <jslegare@gmail.com>
To: gcc-help@gcc.gnu.org
Subject: Constructing function calls
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Hi

I have a special task I would like to accomplish regarding construction of
function calls, without (preferably) using __asm__ sections.

I would like to pass a block of data containing arguments to a function from
which I don't know the type nor number of arguments. The only thing I know=
=20
is the address of the function pointer and the size of the argument block
(number of 32bit words).

That is, I would like to fill this function:

void call_func( (void*)func(), void* args, int size) {

    1. push the contents of args to the stack;
    2. call func() with no args (func will recuperate them from the stack);
    3. return
}

For instance, I would like to be able to call :

void print3ints(int a, int b, int c) {
    printf(" %d %d %d\n",a,b,c);
}

by doing :

{
    int* args =3D malloc( 3 *sizeof(int));
    args[0] =3D 1; args[1] =3D 2; args[2] =3D 3;
    call_func(print3ints,args,3);
}


I looked at "__builtin_apply" but it requires calling "__builtin_apply_args"
beforehand,  which I cannot do.

I successfully managed to do that without using __asm__ on a x86.=20
I filled a local int array in call_func with the passed argument block and =
then=20
I called the passed function (stack bashing).

However, the method breaks when compiling in -O3 mode and it would certainly
not work on other systems where some arguments are passed inside
registers.

What are my best options ?