Why the program crashes when I am trying to use strcat() in for-loop?

Question

I have an issue with strcat. When I am trying to use it in for-loop it throws a reading access violation. There is a program as example:

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

printf("Enter count: ");
int count = 0;
scanf("%d", &count);
char* str = (char*)malloc(sizeof(char));
for(int i = 0; i < count; i++) str = strcat(str, "a");

It works fine with values in kind of 10 or 100 for count but it crashes when I use it with 1000 or bigger. Can someone tell me what's the problem?

Answer 1

Your code has several problems:

1. you only allocate space for one character
2. you never initialize the string before calling strcat
3. you forgot to include <stdlib.h>

You want this:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>         // you need this for malloc

int main()
{
  printf("Enter count: ");
  int count = 0;
  scanf("%d", &count);

  char* str = malloc(sizeof(char) * (count + 1));  // allocate count + 1 bytes, the +1 
                                                   // is for the string NUL terminator

  str[0] = 0;                    // initialize the string to an empty string

  for (int i = 0; i < count; i++)
    str = strcat(str, "a");

  printf("Result = %s\n", str);  // print the result
}

BTW the (char*) cast is not needed with malloc.