#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
char* string_reverse_aux(char* s1, char* s2, size_t count, size_t length) {
  if (count == length) {
    s2[count] = '\0';
    return s2;
  } else {
    s2[count] = s1[length - 1 - count];
    return string_reverse_aux(s1, s2, count + 1, length);
  }
}
 
char* string_reverse(char* s) {
  int n = strlen(s);
  char* _s = (char*)malloc(sizeof(char) * (n + 1));
  return string_reverse_aux(s, _s, 0, n);
}
 
int main(void) {
  char* s = "abcdefgh";
  printf("%s\n%s\n", s, string_reverse(s));
  return EXIT_SUCCESS;
}
 

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