c - Taking string input in char pointer

Question

#include<stdio.h>
#include<string.h>
#include<stdlib.h>

int main(){
    char *s;
    printf("enter the string : ");
    scanf("%s", s);
    printf("you entered %s
", s);
    return 0;
}

When I provide small inputs of length up to 17 characters (for example "aaaaaaaaaaaaaaaaa") the program works perfectly fine but on providing inputs of larger lengths, it gives me a runtime error saying "main.c has stopped working unexpectedly".

Is there some problem with my compiler (codeblocks) or my pc (windows 7)? Or is it somehow related to the input buffer of C?

Answer 1

It's undefined behaviour as the pointer is uninitialized. There's no problem with your compiler but your code has problem :)

Make s point to valid memory before storing data in there.

---

To manage buffer overflow, you can specify the length in the format specifier:

scanf("%255s", s); // If s holds a memory of 256 bytes
// '255' should be modified as per the memory allocated.

GNU C supports an non-standard extension with which you don't have to allocate memory as allocation is done if %as is specified but a pointer to pointer should be passed:

#include<stdio.h>
#include<stdlib.h>  

int main() {
  char *s,*p;

  s = malloc(256);
  scanf("%255s", s); // Don't read more than 255 chars
  printf("%s", s);

  // No need to malloc `p` here
  scanf("%as", &p); // GNU C library supports this type of allocate and store.
  printf("%s", p);
  free(s);
  free(p); 
  return 0;
}