sef@kithrup.COM (Sean Eric Fagan) (06/01/91)
I've been playing with something, lately, and have been getting a core-dump,
and I'm not entirely sure why.
Basicly, gccv2.0 will have nested functions. When you take the address of a
nested function, it will take a code prototype (in binary form), and push it
on the stack, and then modify it appropriately. On the '386, the prototype
(called a "trampoline") is
mov $static, %ecx ; 0xb9 0x0000 0x0000
mov $function_addr, %eax ; 0xb8 0x0000 0x0000
jmp *%eax ; 0xff 0x40
The actual value of the address given as the function pointer is the stack
address where that code is; this sets up the frame pointers properly.
Now, the following code:
main() {
int foobar() {
return 34;
}
int (*fp)();
fp = foobar;
(*fp)();
}
will memory-fault at the 'jmp *%eax' instruction, and I'm not sure why. I
actually expected it to die when it tried executing code on the stack, but
that didn't seem to be a problem; it was when it tried to transfer out.
I've examined the reigster using adb and codeview, and, in both cases, %eax
holds the proper address. (Actually, it's quite fun. codeview will core
dump itself if I try to use it to step through the code.)
Any thoughts, people? Thanks in advance...
--
Sean Eric Fagan | "I made the universe, but please don't blame me for it;
sef@kithrup.COM | I had a bellyache at the time."
-----------------+ -- The Turtle (Stephen King, _It_)
Any opinions expressed are my own, and generally unpopular with others.sef@kithrup.COM (Sean Eric Fagan) (06/02/91)
In article <1991May31.204707.19339@kithrup.COM> sef@kithrup.COM (Sean Eric Fagan) writes: >I've been playing with something, lately, and have been getting a core-dump, >and I'm not entirely sure why. Thanks to Bruce Evans (evans@syd.dit.csiro.au), I do know why. >>> jmp *%eax ; 0xff 0x40 That should have been a '0xff 0xe0'. Whoever typed it into the '386 definition file did so incorrectly (actually, I was wrong, as well: it was '040', or 32 decimal). Changing it to 0xe0 causes it to work. Anyway, nested functions appear to work now. I'll make sure the change makes its way back to rms... -- Sean Eric Fagan | "I made the universe, but please don't blame me for it; sef@kithrup.COM | I had a bellyache at the time." -----------------+ -- The Turtle (Stephen King, _It_) Any opinions expressed are my own, and generally unpopular with others.