Segmentation fault while using strcat(num, num) to concatenate string to itself

Question

I used strcat function to concatenate "1" and "1" so that the result is "11" but I'm getting a segmentation fault.

My code is below:

#include <stdio.h>
#include <string.h>
int main(){
  char num[10] = "1";
  strcat(num, num);
  printf("%s", num);
}

Answer 1

Considering the most likely implementation of strcat , don't do it. In general, it's a bad idea to pass a function the same parameter for both input and output. In fact, let's put this a little better. Unless the function says its defined, it's not defined, so don't do it.

Try this:

size_t n = strlen(num);
memmove(num + n, num, n + 1);

memmove is a nice function that's designed for overlapping inputs and outputs.

Answer 2

You can't use strcat to concatenate two strings which overlap, which includes the case when they are the same string.

• https://en.cppreference.com/w/c/string/byte/strcat : "The behavior is undefined if the strings overlap."

• C17 7.24.3.1 (2): "If copying takes place between objects that overlap, the behavior is undefined."

You'll have to write your concatenation routine by hand, or think about a solution based on memcpy .

Answer 3

You need two char[] one as source and other for destination, where destination array must be initialized and have enough space to accommodate source length.

#define DEST_SIZE 40

int main(){
  char num[10] = "1";
  char dest[DEST_SIZE] = "1"; //Destination with null characters after "1"
  strcat(dest,num);
  printf(dest);
  return 0;
}

For detailed explanation about different scenarios: here