Conflicting types for 'reverse_string' function

Question

I was using this SO question as part of a program that needs to reverse a string. The problem I am having is that I cannot seem to get the function to work. Here is the code I have:

int main(int argc, char *argv[]){
  char *test = "Testing";
  fputs(test, stdout);
  fputs(reverse_string(test), stdout);
}

char* reverse_string(char *str){
  char temp;
  size_t len = strlen(str) - 1;
  size_t i;
  size_t k = len;
  for(i = 0; i < (len +1)/2; i++){
    temp = str[k];
    str[k] = str[i];
    str[i] = temp;
    k--;
  }  
  return str;
}

I am getting an error that there is conflicting types for 'reverse_string'

Edit: For anyone wondering here is the code that works. See @chux's answer for an explanation.

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
char* reverse_string(char *str){
  char temp;
  size_t len = strlen(str) - 1;
  size_t i;
  size_t k = len;
  for(i = 0; i < (len +1)/2; i++){
    temp = str[k];
    str[k] = str[i];
    str[i] = temp;
    k--;
  }
  return str;
}
int main(int argc, char *argv[]){
  char test[] = "Testing";
  fputs(test, stdout);
  fputs(reverse_string(test), stdout);
}

Answer 1

You can not pass a const char * to a char *

char *test = "Testing";
fputs(reverse_string(test), ... // bad, attempting to change constant data.
// bad as  reverse_string() is assumed to return int, but fputs() expects char *

char* reverse_string(char *str) { // Bad, there's now a function conflict

Instead

char* reverse_string(char *str);  // Need to declare/define function first

char test[] = "Testing";
fputs(reverse_string(test), ... // good

[Edit]
You problem was well identified (missing function declaration) by others. My suggestion takes care of the next problem. In C, a missing declaration of a function will assume int reverse_string(...) which does not match char* reverse_string(char *str).

[Edit]
As @Shafik Yaghmou suggests, modifying a string literal char *test = "Testing" will result in undefined behavior. Hence the char test[] = "Testing" which initializes test with "Testing\0", but may be modified.

[Edit]
@GreenAsJade correctly points out OP's original error message is due to the assumed int reverse_string(...) supplying an int to s in int fputs(const char * s, FILE * stream);

char *test1 = "Testing" is not the same thing as char test2[] = "Testing". test1 becomes a char * with the size of a pointer. The initial pointer value is to a string "Testing" located elsewhere in memory. test2 is a char array with size 8: length of "Testing" + 1 for '\0'. The array test2 is initialized with 'T', 'e', ... '\0' etc.