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
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