Inverting string in plain c

untitled

#include <stdio.h>   //for printf
#include <stdlib.h>  //for malloc,free
#include <string.h>  //for strlen

/*
    Sapmple for inverting sting in plain c
    Compile and run:
    >gcc test.c -o test.bin && ./test.bin
*/

char* testString = "INVERT ME";

char* invertString(char* s){
    int len = strlen(s) - 1;

    for(int i = 0; i <= len/2; i++){
        printf("%c  <==> %c \r\n",s[i], s[len - i]);
        if (s[i] != s[len - i]){
            s[i]       = s[i] ^ s[len - i];
            s[len - i] = s[i] ^ s[len - i];
            s[i]       = s[i] ^ s[len - i];
        }
    }
    return s;
}

int main(int argc, char* argv []){
    char *tmpString = malloc(256);
    strcpy(tmpString,testString);
    char* result =  invertString(tmpString);
    printf("SOURCE = %s \n",testString);
    printf("RESULT = %s \n",tmpString);
    free(tmpString);
    return 0;
}