Read in a line using recursion and return a pointer to the string

Question

I need to create a char *read_line(void) function which reads in a character recursively so with each recursive call it creates a local char variable and when enter character has been read in, it can determine how much memory is needed to be allocated so it can eventually return a pointer to a string. The string must end with \0 terminator. I may only use one malloc call, I cannot use any global or static variables or any other containers. I may use any custom helper functions. Prohibited words: realloc;calloc;strcpy;strncpy;10 I know this is impractical, but it's a homework assignment.

Answer 1

The final call will need to do

char *str = malloc(i+1);
str[i] = 0;
return str;

Other calls will need to do

char *str = read_line_helper(...);
str[i] = ch;
return str;

That means that each call needs an i that's one more than its caller. This can easily be achieved by passing i+1 to the recursive function.

char *read_line_helper(size_t i) {
   char ch = ...;

   char *str;
   if (...) {
      str = malloc(i+1);
      str[i] = 0;
   } else {
      str = read_line_helper(i+1);
      str[i] = ch;
   }

   return str;
}

The first call takes 0.

char *read_line(void) {
   return read_line_helper(0);
}