Mail Archives: djgpp/1999/05/11/20:15:28
El día Tue, 11 May 1999 10:19:16 +0100, "Ya'qub" <rick AT nct-active DOT com>
escribió:
>Greetings,
> With all due respect, this explanation does not seem very convincing.
>Surely, the compiler *should* complain when you do such a thing as
A C compiler usually doesn't know if you will overrun an array
(sometimes it is just impossible to know this at compile time), but
even if the compiler knew, it would assume that you really *want* to
do that. And indeed, sometimes it is done! I think I've seen some
zero-length arrays in structures, where the size of the array isn't
actually known until run time, and for some reason an equivalent
pointer to <type> is not appropiate.
Now you could have runtime checks for array bounds, uninitialized
memory, etc., but the C language was designed to trust the programmer
so that you can freely do almost whatever you want. It is up to you if
your code crashes and your computer burns. If you want those checks,
just go for Pascal or Java.
>overrunning an array. I hoped that the compiler would help such a poor
>programmer as myself in catching such bugs which I suppose could cause quite
>a lot of headache to locate. Can anybody else confirm that gcc makes such
>assumptions that we know what we are doing and let us carry on regardless.
Not just gcc; almost every C compiler will.
>Also, out of curiosity, if you do overrun an array and the program has
>compiled without any warning or error, is there anything that can be said
>about its execution? i.e. do we know what is going to happen when the
>program tries to access an element from the array beyond its length. Thanks
>Regards,
According to ANSI, the behaviour is undefined. That means that
anything could happen. If you overwrite some system data or
unitialized memory, probably your program will crash with a GPF, core
dump, system exception or whatever. OTOH, if you just overwrite *your*
data, your program could even run normally, and the bug would go
unnoticed!
For example, try this: it will probably work (I haven't tried it), but
then, that doesn't mean it is correct!
#include <stdio.h>
int main()
{
int array1[10];
int array2[10];
array1[15] = 23;
printf("array1[15] = %i\n", array1[15]);
return 0;
}
Regards,
GUILLE
----
Guillermo Rodriguez Garcia
XXguille AT XXiies DOT XXes (ya sabes :-)
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