loesung-3.c

#include <stdio.h>

int read_hex (char c)
{
  if (c >= '0' && c <= '9')
    return c - '0';
  else if (c >= 'A' && c <= 'F')
    return c - 'A' + 10;
  else if (c >= 'a' && c <= 'f')
    return c - 'a' + 10;
  else
    {
      fprintf (stderr, "invalid hex digit '%c'\n", c);
      return 0;
    }
}

int main (void)
{
  char buffer[100];
  fgets (buffer, 100, stdin);
  for (char *p = buffer; p[0] && p[1]; p += 2)
    {
      char c = 16 * read_hex (p[0]) + read_hex (p[1]);
      printf ("%c", c);
    }
  printf ("\n");
}