Dynamically prompt for string without knowing string size

Question

In C, what is the best way of prompting and storing a string without wasted space if we cannot prompt for the string length. For example, normally I would do something like

Answer 1

From man scanf:

• An optional 'm' character. This is used with string conversions (%s, %c, %[), and relieves the caller of the need to allocate a corresponding buffer to hold the input: instead, scanf() allocates a buffer of sufficient size, and assigns the address of this buffer to the corresponding pointer argument, which should be a pointer to a char * variable (this variable does not need to be initialized before the call). The caller should subsequently free(3) this buffer when it is no longer required.

this however is a POSIX extension (as noted by fiddling_bits).

To be portable I think that in your usage case I would prepare a function like the following:

char *alloc_answer() {
  char buf[1000];
  fgets(buf,sizeof(buf),stdin);
  size_t l = strlen(buf);
  if (buf[l-1]=='\n') buf[l]=0; // remove possible trailing '\n'
  return strdup(buf);
}

even if this solution will break lines longer than 1000 characters (but it prevents buffer overflow, at least).

A fully featured solution would need to read input in chunks and realloc the buffer on every chunk...