Reverse a string

reverse.c

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

char* reverse(char* in);
void swap(char* s, int a, int b);

int main(int argc, char** argv) {
  if (argv[1]) {
    printf("%s\n", reverse(argv[1]));
  } else {
    fprintf(stderr, "Please specify a word.\n");
  }
  return 0;
}

char* reverse(char* in) {
  int len = strlen(in);
  for (int i = 0; i < len / 2; i++) {
    swap(in, i, len - i - 1);
  }
  return in;
}

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