Pointer initialisation gives segmentation fault

Question

I wrote a C program as follows:

CASE 1

int *a; /* pointer variable declaration */

int b; /* actual variable declaration */

*a=11;

a=&b;/* st         


        

Answer 1

int *a; // a pointer variable that can hold a memory address of a integer value.

In case 1,

  *a = 10; // here you have asigned 10 to unknown memory address;

It shows segmentation fault because of assigning value to a memory address that is not defined. Undefined behaviour.

In case 2,

a=&b; // assigning a proper memory address to a.
*a=11;// assigning value to that address

Consider this example:

#include
int main()
{
   int *a,b=10;
   printf("\n%d",b);
   a=&b;
   *a=100;
   printf("-->%d",b);
}

Output: 10-->100

Here this is how it works.

         b        // name
     ----------
     +   10   +   // value
     ---------- 
        4000      // address 

Asuuming memory location of b is 4000.

a=&b => a=4000;
*a=100 => *(4000)=100 => valueat(4000) => 100

After manipulation it looks like this.

         b        // name
     ----------
     +  100   +   // value
     ---------- 
        4000      // address