I am fairly new to C and I am trying to write a simple program to concatenate two strings. While running it on my Linux box, I get the following exception:

test.c:12:10: error: conflicting types for ‘strcat’

Can you please help me understand what I am missing here:

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

void main() {
  // string concatenation
  char str1[] = {'S', 'h'};
  char str2[] = {'X', 'y'};
  strcat(str1, str2);
}

void strcat(char str1[], char str2[]) {
  int index;
  int str1_length = strlen(str1);
  for(index=0; index<strlen(str2); index++) {
    str1[index + str1_length] = str2[index];
  }
}

```