Question

I am working with a binary file structure. The code example for reading the data is in C, and I need to read it in Delphi. I hasten to add I have no C programming experience.

Given the following

typedef struct {
uchar ID, DataSource;
ushort ChecksumOffset;
uchar Spare, NDataTypes;
ushort Offset [256];
} HeaderType; 

...

typedef struct {
ushort ID;
...
ushort DistanceToBin1Middle,TransmitLength;
} FixLeaderType;

...

HeaderType *HdrPtr;
FixLeaderType *FLdrPtr;

unsigned char RcvBuff[8192];
void DecodeBBensemble( void )
{
unsigned short i, *IDptr, ID;
FLdrPtr = (FixLeaderType *)&RcvBuff [ HdrPtr->Offset[0] ];
if (FLdrPtr->NBins > 128)
FLdrPtr->NBins = 32;

...

The bit I am having difficulty following is this:

FLdrPtr = (FixLeaderType *)&RcvBuff [ HdrPtr->Offset[0] ];

From the little I understand, [ HdrPtr->Offset[0] ]; would be returning the value of the first Offset array item from the HeaderType struct pointed to by HdrPtr? So equivalent to HdrPtr^.Offset[0] ?

Then &RcvBuff [ HdrPtr->Offset[0] ]; should be returning the memory address containing the value of the RcvBuff array item indexed, so equivalent to @RecBuff[HdrPtr^.Offset[0]] ?

Then I get lost with (FixLeaderType *).. . Could someone please help explain exactly what is being referenced by FldrPtr ?

Answer 1

The bits of code that are relevant are

FixLeaderType *FLdrPtr; 
unsigned char RcvBuff[8192]; 

FLdrPtr = (FixLeaderType *)&RcvBuff [ HdrPtr->Offset[0] ]; 

1. FldPtr is of type FixLeaderType *, or pointer to FixLeaderType.
2. RcvBuff is an array of char.
3. HdrPtr->Offset[0] resolves to an ushort value, so RcvBuff [ HdrPtr->Offset[0] ] yields a char value.
4. The & means that instead of getting the value of the char the address of the value is returned. Note that this means that it is of type char *.
5. The char * type is the wrong type to assign to FldPtr. The (FixLeaderType *) converts the type so that it is valid. This is called a cast operation.