Re: Crash following optimisation goes away when inserting printf

[Jean-David Hsu]

Martin Franke wrote:
> Hi everybody,
>
> First post, so thanks a lot for reading, and, hopefully, replying!
>
> When turning on optimisation (gcc 3.4.2, O2/Os -fno-strict-aliasing)
> when compiling our multi-platform software (C/C++ mix, target now is an
> embedded MIPS based platform) it segfaults, while it runs without a
> problem when turning optimisations off. I still get quite a few compiler
> warnings, so it is probably best to completely resolve them first.
>
> Still, there is something that strikes me as odd, and I would like to
> kindly ask if someone could shed some light on this. The crash occurs in
> the initialisation routine of a certain module. Now, when inserting a
> printf("bla") right at start-up in main() the crash goes away, removing
> it makes it re-appear, so it is perfectly reproducible. The printf is in
> a different object file than where the crash happens and quite a lot of
> code is executed between the printf and the crash.
>
> So my questions are:
> - Is this a side-effect to be expected?
> - How can the additional printf in one object file can have a
> side-effect on the code in another object file? (Each source file is
> compiled and optimised separately, so I guess it must happen during
> linkage, but that's where my understanding hits the wall.)
>
> Thanks again!
>
> Martin

printf segfault
printf does have side-effects and C functions share the same stack.
Here is a short code I wrote where removing a call to printf triggers a segfault.
Also try commenting out the line.
This code is implementation specific and uses undefined behaviors.
Runs on gcc 4.0.2 no optimization

#include <stdio.h>

/* Side-effects of printf */

int main (void){
        /* A dirty hack to display the stack. Implementation specific. Does work on most compilers tough */
        printf("Stack looks like before printf:\n%p\n%p\n");
        /* Here 2 args passed to printf because the call to sprintf below overwrites one of them on the stack. Try the code with and without this line */
        printf ("%d", 0, 0);
        printf("Stack looks like after printf:\n%p\n%p\n");
        char buffer[20];
        sprintf (buffer,"%d");
        int* p = -1;
        /* The following assignment to produce a segfault but anycode would do really */
        (!atoi (buffer))?*p=0:printf ("\nRan fine\n");;
        return 0;
}

JD