Reversing a String

reverse-string.c

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

inline void swap(char * str, int a, int b) {
    char * tmp = str[a];
    str[a] = str[b];
    str[b] = tmp;
}

#define INLINE_SWAP(str, a, b) {char * tmp = str[a]; str[a] = str[b]; str[b] = tmp;}

int main (int argc, char * const argv[])
{
    char * str = strdup("Hello World");

    const int len = strlen(str);
    int i;
    for(i = 0; i < len/2; i++)
      swap(str, i, len-(i+1));

    printf("%s\n", str);

    free(str);

    return 0;
}

About

A bit of C that reverses a string. It uses strdup so that we can modify the string.
I tried using a macro but there was no discernible difference in execution time.