Segmentation Fault with pointer arithmetics C++

Question

I'm trying to learn some manupulations of pointer arithmetics in C++. The code written below throws me a Segmentation Fault. I could not comprehend how the program tries to access unallocated memory to cause a segmentation fault.

C++ Code (myarray.cc)

#include<iostream>
using namespace std;

int main(int argc, char** argv)
{
  int * pointer_s3_1_a;
  int * pointer_s3_1_a2;
  int value_s3_1_a, value_s3_1_a2 ;

  *pointer_s3_1_a=100;
  cout<<"pointer_s3_1_a, *pointer_s3_1_a "<<pointer_s3_1_a<<' '<<*pointer_s3_1_a<<endl;
  value_s3_1_a=*pointer_s3_1_a++; 
  cout<<"value_s3_1_a, pointer_s3_1_a, *pointer_s3_1_a "<<
    value_s3_1_a<<' '<<pointer_s3_1_a<<' '<<*pointer_s3_1_a<<endl;

  cout<<"pointer_s3_1_a2, *pointer_s3_1_a2 "<<pointer_s3_1_a2<<' '<<*pointer_s3_1_a2<<endl;

  *pointer_s3_1_a2=100; //Runtime error |** Segmentation fault (core dumped) **|

  return 0;
}

I'm running the program in Ubuntu 12.04 with g++ compiler. Running apt-cache policy g++ on terminal gave me the following output.

g++: Installed: 4:4.6.3-1ubuntu5 Candidate: 4:4.6.3-1ubuntu5
Version table: * 4:4.6.3-1ubuntu5 0 500 http://archive.ubuntu.com/ubuntu/ precise/main i386 Packages 100 /var/lib/dpkg/status

int * pointer_s3_1_a; ... *pointer_s3_1_a=100; is UB. Please PLEASE learn C++ using a book. Don't throw code in and expect it to work. — Luchian Grigore
– Luchian Grigore, Commented Jul 3, 2013 at 13:37

juanchopanza · Accepted Answer · 2013-07-03 13:47:40Z

6

Here, you declare a pointer that points to nowhere in particular:

int * pointer_s3_1_a;

Here, you attempt to set the value of the thing it points to to 100:

*pointer_s3_1_a=100;

This is undefined behaviour, and could cause a segmentation violation (although it doesn't have to, many wrong things could happen, and not all as noticeable as a segmentation fault. You just got lucky).

You must first make your pointer point to somewhere valid. For example,

int n = 42;
pointer_s3_1_a = &n; // pointer points to n
*pointer_s3_1_a=100; // set the value of the thing it points to (n) to 100

edited Jul 3, 2013 at 13:47

answered Jul 3, 2013 at 13:38

juanchopanza

228k35 gold badges421 silver badges493 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

trojanfoe · Accepted Answer · 2013-07-03 13:39:45Z

2

You cannot write data to uninitialized areas of memory:

int * pointer_s3_1_a;  // NOT Initialized (possibly 0)!!!
*pointer_s3_1_a=100;   // Undefined behaviour

answered Jul 3, 2013 at 13:39

trojanfoe

123k23 gold badges219 silver badges249 bronze badges

2 Comments

Lightness Races in Orbit Over a year ago

It's very unlikely to be 0, actually. And a page fault is only one possible outcome.

trojanfoe Over a year ago

@LightnessRacesinOrbit OK, fixed comments.

Marco Bonifazi · Accepted Answer · 2013-07-03 13:44:38Z

2

you have declared pointer_s3_1_a and pointer_s3_1_a2 as pointers, but there's actually no memory to point to, as you have missed to allocate/create them. You should do:

int* pointer_s3_1_a = new int();
*pointer_s3_1_a = 100;

...

int* pointer_s3_1_a2 = new int();
*pointer_s3_1_a2 = 100;

answered Jul 3, 2013 at 13:44

Marco Bonifazi

7132 gold badges10 silver badges11 bronze badges

Collectives™ on Stack Overflow

Segmentation Fault with pointer arithmetics C++

3 Answers 3

Comments

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related