RE: Segmentation fault Causes

["Ajay Bansal" <Ajay_Bansal@infosys.com>]

John

Can you please tell me how to use this fprintf(stderr... . ) function?

Do I need to give it at many different places in the code??? If that is
the case, what's is the difference between putting many different
"cerr<<................"???

-Ajay


-----Original Message-----
From: J.C.Wren [mailto:jcwren@jcwren.com] 
Sent: Monday, February 24, 2003 10:52 AM
To: 'Daniel Bolgheroni'; gcc-help@gcc.gnu.org
Subject: RE: Segmentation fault Causes


	Most commonly it's caused by trying to write to a pointer that
either is uninitialized, or NULL.

int main (int argc, char **argv)
{
   char *p = NULL;
   *p = 'A';
}

will cause a segfault.  Also, if 'p' isn't assigned an explicit value,
it will have whatevers on the stack at that point.  Another thing that
will cause a segfault is writing passed the end of an array:

int main (int argc, char **argv)
{
   char buffer [16];
   memset (buffer, 0, 64);
}

will segfault.

	I would suggest compiling your program with the -W and -Wall
switches ('gcc -W -Wall myprog.c -o myprog').  Fix any and all warnings.
GCC is very good about warning about usage of uninitialized variables.

	If you want to see how far you're getting in your program use
'fprintf (stderr, "Got to line %d\n", __LINE__);'.  This will show how
far you've got before the segfault occurs.  You must write to stderr and
not stdout because of buffering.  When the program segfaults, unbuffered
I/O will NOT be written to the console.  This is why people often make
the mistake of saying 'printf ("Got to line %d\n", __LINE__);' and
thinking they really didn't get that far.  In reality, it did get that
far, but the I/O was lost when the buffer remained unflushed after the
segfault.  There are some exceptions to this rule, but it's not worth
pursuing at this point (just so someone does say "but if you set
unbuffered I/O mode, that doesn't apply!").

	--John


> -----Original Message-----
> From: gcc-help-owner@gcc.gnu.org [mailto:gcc-help-owner@gcc.gnu.org]On
> Behalf Of Daniel Bolgheroni
> Sent: Sunday, February 23, 2003 23:55
> To: gcc-help@gcc.gnu.org
> Subject: Segmentation fault Causes
>
>
> What are the causes that a program can give an error "Segmentation 
> fault (core dumped)"?
>
> I'm compiling a program I'm writing but I getting some of this error 
> messages executing it. I looked into gcc FAQ and gcc 2.95 manual, but 
> I found no references to this problem.
>
> I'm using the following version of gcc:
>
> gcc version 2.95.3 20010315 (release) (NetBSD nb3)
>
> Thank you.