segmentation fault (core dumped) for more than two pointers

Question

I am writing a program to just check what pointers do. But I get a segmentation fault(core dumped) message when I seem to have more than two pointers. I have checked for solutions online but the segmentation faults that are dealt with seem to be completely different to this problem.

#include<stdio.h>

int main()
{
  int x=3,y=3;
  int *p1, *p2, *p3;

   p1=&x;
  *p2=x;
   p3=&x;

  printf("%d\n",*p1);
  printf("%d\n",*p2);
  printf("%p\n",p1);
  printf("%p\n",p2);
  printf("%p\n",p3);



 return 0;
}

But if I have just two pointers nothing seems to be wrong. Like if I comment out all the sections related to one pointer(except declaring and initializing) it seems to work fine.

Also I am using ubuntu 16.04(if it is somehow important).

Can anyone please explain why this is happening.

Jeegar Patel · Answer 1 · 2016-11-03T06:45:36.107

3

  *p2=x;

As p2 is pointing NULL and you are trying store x value at NULL location

is causing problem

For debuging such crash gdb is your best friend.

copied your code in test.c

jeegar@jeegarp:~$ gcc -g test.c 
jeegar@jeegarp:~$ ./a.out 
Segmentation fault (core dumped)
jeegar@jeegarp:~$ gdb ./a.out 
GNU gdb (Ubuntu 7.10-1ubuntu2) 7.10
Copyright (C) 2015 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./a.out...done.
(gdb) r
Starting program: /home/jeegar/a.out 

Program received signal SIGSEGV, Segmentation fault.
0x0000000000400612 in main () at test.c:10
10    *p2=x;
(gdb)

So After OP's Update in Question, Update in Answer :)

int *p1, *p2, *p3;

  *p2=x;

So taking uninitialised pointer and dereferencing that pointer is Undefined behaviour. You should not do this.

You should assign p2 with some valid address of memory and then only you can write some value on tht pointer pointed address.

edited Nov 03 '16 at 06:45

answered Nov 03 '16 at 06:27

Jeegar Patel

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if i comment out the section of p3 it works fine. So I don't think it has anyhting to do with that. – Krishna Chaitanya Nov 03 '16 at 06:30
I was about to paste the same thing... – zapstar Nov 03 '16 at 06:34
After commenting this p3=&x; also You will get the same crash. @KrishnaChaitanya Please see my update with gdb – Jeegar Patel Nov 03 '16 at 06:36
Sorry. I just edited the question. Please look at it now @Jeegar Patel – Krishna Chaitanya Nov 03 '16 at 06:41
3

@KrishnaChaitanya Look up *undefined behaviour*. Understand that one possible manifestation of undefined behaviour, among an infinite amoint of possible manifestations, is "working like the programmer expected". – n. m. could be an AI Nov 03 '16 at 06:42
@KrishnaChaitanya see my updated answer after your updated in question. – Jeegar Patel Nov 03 '16 at 06:45
Ok @Jeegar Patel But it works if i comment out the p3 part (just the printf and p3=&x parts) I get an output like this 3 3 0x7ffe8f2927d0 0x7ffe8f2928d0 – Krishna Chaitanya Nov 03 '16 at 06:51
1

@KrishnaChaitanya On my system, After commenting out p3 part still i get the same crash. Thats why its said Undefined behaviour. Dear Friend please google about this term. – Jeegar Patel Nov 03 '16 at 06:54
1

p2 is not pointing at NULL, its value is indeterminate and you can't know what value it has. – Lundin Nov 03 '16 at 07:34

segmentation fault (core dumped) for more than two pointers

1 Answers1