What does this line of C code mean?

Question

I'm converting some C code to Delphi. Can someone please explain to me what this line means?

nResult = ( (pBuffer[ 0 ] << 8) & 0xFF00 )
 | ( pBuffer[ 1 ] & 0x00FF );

Here is the rest of the code for context:

USHORT UTIL_htons( USHORT hostshort )
{
 PUCHAR pBuffer;
 USHORT nResult;

 nResult = 0;
 pBuffer = (PUCHAR )&hostshort;

 nResult = ( (pBuffer[ 0 ] << 8) & 0xFF00 )
  | ( pBuffer[ 1 ] & 0x00FF );

 return( nResult );
}

USHORT UTIL_ntohs( USHORT netshort )
{
 return( UTIL_htons( netshort ) );
}
ULONG UTIL_htonl( ULONG hostlong )
{
 PUCHAR pBuffer;
 ULONG nResult;
 UCHAR c, *pResult;

 pBuffer = (PUCHAR )&hostlong;

 if( !pBuffer )
 {
  return( 0L );
 }

 pResult = (UCHAR * )&nResult;

 c = ((UCHAR * )pBuffer)[ 0 ];
 ((UCHAR * )pResult)[ 0 ] = ((UCHAR * )pBuffer)[ 3 ];
 ((UCHAR * )pResult)[ 3 ] = c;

 c = ((UCHAR * )pBuffer)[ 1 ];
 ((UCHAR * )pResult)[ 1 ] = ((UCHAR * )pBuffer)[ 2 ];
 ((UCHAR * )pResult)[ 2 ] = c;

 return( nResult );
}
ULONG UTIL_ntohl( ULONG netlong )
{
 return( UTIL_htonl( netlong ) );
}

Thanks in advance Bojan

Answer 1

Apparently (all those defines in uppercase make it hard reading) the functions are swapping the internal byte ordering of values that occupy 2 or 4 bytes. For example:

UTIL_htons(0x1234); /* returns 0x3412 */
UTIL_htonl(0x12345678); /* returns 0x78563412 */

I have no idea how to write them in Delphi ...

Hopefully they are already written and the library Delphi uses has them with some name or other. Check your documentation.

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Edit

nResult = ( (pBuffer[ 0 ] << 8) & 0xFF00 ) | ( pBuffer[ 1 ] & 0x00FF );

in this line

pBuffer[0] is the first element of the array pBuffer
pBuffer[0] << 8 is shifting that value 8 bits to the left (0x12 becomes 0x1200)
(...) & 0xFF00 is redundant: it resets the rightmost 8 bits

In pBuffer[1] & 0x00FF only the rightmost 8 bits are kept (so 0x1234 becomes 0x0034)

The other operation | is a bitwise or

( ... & 0xFF00) | ( ... & 0xFF00) is the leftmost 8 bits of the first part and the rightmost 8 bits of the second part.

---

Edit: hto* / *toh naming

The functions htonl, htons, ntohl, ntohs in C are used to convert values between host and network byte order.

The byte ordering is not necessarily different (network byte order is big-endian), so the first part of the functions should check if the host byte ordering is little- or big-endian before doing the swaps ... or the check was done previously in the program that uses the functions you posted.

Answer 2

You do not have to translate them, just include WinSock, it has all four of them!