Thread: How to input 16 base IP adress

I suggest clarifying the implications of "you can't use data structures and define your own functions" with your instructor: an array is a data structure and the main function would be "your own function". Having said that, my guess is that the restriction has more to do with defining structs in C and functions other than the main function.

I suggest that you start by hardcoding these test IP addresses, valid and invalid, as strings, then think of how you can validate them by analysing the content of these strings. If you can get this right, the rest is just a matter of implementing the interactive I/O to read a string.

Maybe using scanf with "%2hhX.%2hhX.%2hhX.%2hhX"? Example:

Code:

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

int main( void )
{
  unsigned char a[4];
  int converted;

  const static char ipstr[] = "AA.0.CC.DD";  // should pass.
  const static char ipstr2[] = "AA.256.0.10";  // should fail.

  if ( ( converted = sscanf( ipstr, "%2hhX.%2hhX.%2hhX.%2hhX", a, a+1, a+2, a+3 ) ) != 4) 
  {
    if ( converted == EOF )
      perror( "sscanf" );
    else
      fprintf( stderr, "error: Only %d octects converted.\n", converted );
    return EXIT_FAILURE;
  }

  printf("%hhu.%hhu.%hhu.%hhu\n", *a, *(a+1), *(a+2), *(a+3));

  if ( ( converted = sscanf( ipstr2, "%2hhX.%2hhX.%2hhX.%2hhX", a, a+1, a+2, a+3 ) ) != 4) 
  {
    if ( converted == EOF )
      perror( "sscanf" );
    else
      fprintf( stderr, "error: Only %d octects converted.\n", converted );
    return EXIT_FAILURE;
  }

  printf("%hhu.%hhu.%hhu.%hhu\n", *a, *(a+1), *(a+2), *(a+3));

  return EXIT_SUCCESS;
}

There is also inet_pton(), but it fails with non standard IPv4 or IPv6 addresses...

Sounds like your instructor wants you to validate character by character.