Just a simple C program that reverses a string in place.

reverse_string.c

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

void reverse(char str[]) {
  int len = strlen(str);

  int i;
  for(i = 0; i < (len / 2); i++) {
    char c = str[i];
    str[i] = str[len - i - 1];
    str[len - i - 1] = c;
  }
  
}

int main() {
  char str[] = "Please reverse me";

  printf("Original: [%s]\n", str);
  
  reverse(str);

  printf("Reversed: [%s]\n", str);
  
  return 0;
}