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Segmentation Fault

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Join Date: May 2005

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vicky_dev

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Junior Poster in Training

Segmentation Fault

Mar 25th, 2006

Hi all, what exactly is segmentation fault or access violation and exactly when does it occur?

Why does the following program work correctly :

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
 
#include <stdio.h>
#include <conio.h>
 
int main( )
{
    int a[5];
    a[20] = 20;
    a[-20] = 30;
    printf("%d %d", a[20], a[-20] );
    getch( );
    return 0;
}
 
#include <stdio.h>
#include <conio.h>

int main( )
{
    int a[5];
    a[20] = 20;
    a[-20] = 30;
    printf("%d %d", a[20], a[-20] );
    getch( );
    return 0;
}

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Join Date: Sep 2004

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Narue

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Senior Bitch

Re: Segmentation Fault

Mar 25th, 2006

>what exactly is segmentation fault or access violation and exactly when does it occur?
A segmentation fault is when you access memory outside of your address space and the OS politely asks you not to do that.

>Why does the following program work correctly
Undefined behavior can do anything, including work the way you expect it to. Don't rely on a single implementation to tell you what works and what doesn't. We have a standard document for that.

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Join Date: Dec 2005

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Salem

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Void main'ers are DOOMed

Re: Segmentation Fault

Mar 25th, 2006

> Why does the following program work correctly
There is a big difference between "produces expected output" and "works correctly". Your program is not correct, even though it seems to pass the "works for me" test.

When run with some diagnostic software, I get

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 C Syntax (Toggle Plain Text) 
$ valgrind ./a.out
==20047== Memcheck, a memory error detector for x86-linux.
==20047== Copyright (C) 2002-2005, and GNU GPL'd, by Julian Seward et al.
==20047== Using valgrind-2.4.0, a program supervision framework for x86-linux.
==20047== Copyright (C) 2000-2005, and GNU GPL'd, by Julian Seward et al.
==20047== For more details, rerun with: -v
==20047==
==20047== Invalid write of size 4
==20047==    at 0x804838F: main (in ./a.out)
==20047==  Address 0x52BFE7E4 is just below %esp.  Possibly a bug in GCC/G++
==20047==   v 2.96 or 3.0.X.  To suppress, use: --workaround-gcc296-bugs=yes
20 30==20047==
==20047== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 13 from 1)
==20047== malloc/free: in use at exit: 0 bytes in 0 blocks.
==20047== malloc/free: 0 allocs, 0 frees, 0 bytes allocated.
==20047== For counts of detected errors, rerun with: -v
==20047== No malloc'd blocks -- no leaks are possible.
 
$ valgrind ./a.out
==20047== Memcheck, a memory error detector for x86-linux.
==20047== Copyright (C) 2002-2005, and GNU GPL'd, by Julian Seward et al.
==20047== Using valgrind-2.4.0, a program supervision framework for x86-linux.
==20047== Copyright (C) 2000-2005, and GNU GPL'd, by Julian Seward et al.
==20047== For more details, rerun with: -v
==20047==
==20047== Invalid write of size 4
==20047==    at 0x804838F: main (in ./a.out)
==20047==  Address 0x52BFE7E4 is just below %esp.  Possibly a bug in GCC/G++
==20047==   v 2.96 or 3.0.X.  To suppress, use: --workaround-gcc296-bugs=yes
20 30==20047==
==20047== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 13 from 1)
==20047== malloc/free: in use at exit: 0 bytes in 0 blocks.
==20047== malloc/free: 0 allocs, 0 frees, 0 bytes allocated.
==20047== For counts of detected errors, rerun with: -v
==20047== No malloc'd blocks -- no leaks are possible.