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From: antispam@fricas.org (Waldek Hebisch)
Newsgroups: comp.lang.c
Subject: Re: else ladders practice
Date: Fri, 22 Nov 2024 12:51:27 -0000 (UTC)
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Bart <bc@freeuk.com> wrote:
> 
>   int main(void) {
>      int a;
>      int* p = 0;
>      a = *p;
>  }
> 
> Here's what happens with my C compiler when told to interpret it:
> 
>   c:\cx>cc -i c
>   Compiling c.c to c.(int)
>   Error: Null ptr access
> 
> Here's what happens with gcc:
> 
>   c:\cx>gcc c.c
>   c:\cx>a
>   <crashes>
> 
> Is there some option to insert such a check with gcc? I've no idea; most 
> people don't.

I would do

gcc -g c.c
gdb a.out
run

and gdb would show me place with bad access.  Things like bound
checking array access or overflow checking makes a big difference.
Null pointer access is reliably detected by hardware so no big
deal.  Say what you 'cc' will do with the following function:

int
foo(int n) {
    int a[10];
    int i;
    int res = 0;
    for(i = 0; i <= 10; i++) {
        a[i] = n + i;
    }
    for(i = 0; i <= 10; i++) {
        res += a[i];
    }
    res;
}

Here gcc at compile time says:

foo.c: In function ‘foo’:
foo.c:15:17: warning: iteration 10 invokes undefined behavior [-Waggressive-loop-optimizations]
   15 |         res += a[i];
      |                ~^~~
foo.c:14:18: note: within this loop
   14 |     for(i = 0; i <= 10; i++) {
      |                ~~^~~~~

Of course, there are also cases like

void
bar(int n, int a[n]) {
    int i;
    for(i = 0; i <= n; i++) {
        a[i] = i;
    }
}

which are really wrong, but IIUC C standard considers them OK.
Still, good compiler should have an option to flag them either
at compile time or at runtime.

-- 
                              Waldek Hebisch