C atoi and strrev (for fun)

atoi_strrev.c

/* Test:
 *     gcc -o atoi atoi.c
 *     ./atoi 12345 123465
 * Output:
 *     Number: 12345          Reverse:           54321           54321           54321
 *     Number: 123456         Reverse:          654321          654321          654321
 */

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

int atoi(const char *a) {
  int num = 0;
  do {
    num = (num * 10) + (*a - '0');
  } while (*++a);
  return num;
}

char *strrev(char *s) {
  char *b = s;
  char *e = b + strlen(s) - 1;
  while (e - b > 1) {
    char temp = *b;
    *b++ = *e;
    *e-- = temp;
  }
  return s;
}

char *strrev2(char *s) {
  int len = strlen(s);
  char *e = s + len / 2;
  char *b = e - !(len % 2);
  while (*e) {
    char temp = *b;
    *b-- = *e;
    *e++ = temp;
  }
  return s;
}

char *strrev3(char *s) {
  int len = strlen(s);
  for (int i=0; i < len/2; i++) {
    char temp = s[i];
    s[i] = s[len - i - 1];
    s[len - i - 1] = temp;
  }
  return s;
}

typedef char *(*strrev_fn_t)(char *);

char *check_strrev(strrev_fn_t fn, const char *s) {
  char temp[100];
  strcpy(temp, s);
  return fn(temp);
}

int main(int argc, const char *argv[]) {
  strrev_fn_t strrev_fns[] = {strrev, strrev2, strrev3, 0};
  int strrev_fns_len = sizeof(strrev_fns)/sizeof(strrev_fn_t);

  if (argc < 2) {
    fprintf(stderr, "Usage: %s integer\n", argv[0]);
    return 0;
  }

  for (int i=1; i < argc; i++) {
    // with an index
    printf("Number: %-15d", atoi(argv[i]));
    printf("Reverse:");
    for (int j=0; j < strrev_fns_len - 1; j++)
      printf(" %15s", check_strrev(strrev_fns[j], argv[i]));
    printf("\n");

    // with pointers
    printf("Number: %-15d", atoi(argv[i]));
    printf("Reverse:");
    strrev_fn_t *strrev_fn = strrev_fns;
    do {
      printf(" %15s", check_strrev(*strrev_fn, argv[i]));
    } while (*++strrev_fn);
    printf("\n");
  }

  return 0;
}