Question On Windows DCB struct

[freddie1]

I don't understand what I'm seeing here in the DCB struct used in Serial Communication. Here is the struct ...

typedef struct _DCB 
{
    DWORD DCBlength;              // sizeof(DCB)                     
    DWORD BaudRate;               // Baudrate at which running       
    DWORD fBinary: 1;             // Binary Mode (skip EOF check)    
    DWORD fParity: 1;             // Enable parity checking          
    DWORD fOutxCtsFlow:1;         // CTS handshaking on output       
    DWORD fOutxDsrFlow:1;         // DSR handshaking on output       
    DWORD fDtrControl:2;          // DTR Flow control                
    DWORD fDsrSensitivity:1;      // DSR Sensitivity              
    DWORD fTXContinueOnXoff: 1;   // Continue TX when Xoff sent 
    DWORD fOutX: 1;               // Enable output X-ON/X-OFF        
    DWORD fInX: 1;                // Enable input X-ON/X-OFF         
    DWORD fErrorChar: 1;          // Enable Err Replacement          
    DWORD fNull: 1;               // Enable Null stripping           
    DWORD fRtsControl:2;          // Rts Flow control                
    DWORD fAbortOnError:1;        // Abort all reads and writes on Error 
    DWORD fDummy2:17;             // Reserved                        
    WORD wReserved;               // Not currently used              
    WORD XonLim;                  // Transmit X-ON threshold         
    WORD XoffLim;                 // Transmit X-OFF threshold        
    BYTE ByteSize;                // Number of bits/byte, 4-8        
    BYTE Parity;                  // 0-4=None,Odd,Even,Mark,Space    
    BYTE StopBits;                // 0,1,2 = 1, 1.5, 2               
    char XonChar;                 // Tx and Rx X-ON character        
    char XoffChar;                // Tx and Rx X-OFF character       
    char ErrorChar;               // Error replacement char          
    char EofChar;                 // End of Input character          
    char EvtChar;                 // Received Event character        
    WORD wReserved1;              // Fill for now.                   
} DCB, *LPDCB;

What in the world is the meaning of the colon symbol followed by a number right after most of the early members, e.g., DWORD fBinary: 1;? I don't know what that means! In all my years of C coding I've never seen anything like that! I just checked the list of C operators and the colon symbol by itself isn't even an operator! At least not that I know of or ever saw. At best, its half of the scope resolution operator '::' or half of the conditional operator '?:'.

Also, I instantiated one of these in a simple console program and used the sizeof operator to printf its size, i.e. ...

DCB dcb;
printf("sizeof(DCB) = %d\n", sizeof(DCB));

What do you think I got? There are 16 DWORDs, each of which should be 4 bytes for 64 bytes total. Then there are 4 WORDs for 2 bytes each which brings up up to 72 bytes. Then there are 8 BYTEs or chars for 8 more which brings us up to 80 bytes. Instead of 80 bytes returned by sizeof(DCB) I'm getting 28!!!

What in the world is going on? I feel I'm in for a real lesson here! I can honestly say this is one of the strangest things I've ever seen! I'm completely bewildered! Help!

[guestgulkan]

They are bit fields.
In the case of DWORD fDummy2:17; - this takes 17 bits.

If you add up all those bitfields - you will see that tey add up to 32.
They are 14 of those parameters - all show as being DWORD and would normally
take up 4 *14 = 56 bytes.
But they only take up 4 bytes.

(80 - 56) +4 = 28;

[freddie1]

Ah! Thanks guestgulkan. Never thought of that. I've used them in other languages, but never in C. I should be able to wrap my head around that now.

That syntax must be part of the definition of structs then? I was searching for C operators, and it doesn't show up there. When I first saw that I was thinking of the shift operators, which I don't use much, but that wasn't part of it.