CODE:

// rev_str.c
// for reverse string
#include <stdio.h>
#include <string.h>
#define STR_SIZE 32

void rev_str(char *str)
{
  int str_len = strlen(str);
  unsigned char ch; 
  for (int i = 0; i < str_len / 2; ++i) {
    ch = str[i];
    str[i] = str[str_len - i - 1]; 
    str[str_len - i - 1] = ch; 
  }
}

int main(void)
{
  char word[STR_SIZE];
  memset(word, '\0', STR_SIZE);

  strcpy(word, "dog");
  printf("%s\n", word);

  rev_str(word); // reverse string
  printf("%s\n", word);

  return 0;
}

// 実行結果
dog
god