c - Segmentation fault: but why?

Question

#include <stdio.h>
int main()
{
 int* ptr;
 *ptr = 5;
 printf("%d", &ptr);
 return 0;
} 

This was asked in a coding interview, what should be the output?

I am confused between runtime error, compilation error and segmentation fault.

Anyone who can explain why will the answer be segmentation fault?

question from:https://stackoverflow.com/questions/65868462/segmentation-fault-but-why

Answer 1

Your code invokes undefined behavior because the pointer was left uninitialized. You need to initialize the pointer first before inserting into it anything:

int *ptr = malloc(sizeof(int) * REQ_SIZE);

After this, you will no longer get a segfault. Note that you need to put a dereference operator instead of & to print the containing value of the integer pointer.

Here's a demo.