Translating C pointer code example to Delphi pointer syntax

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 ?

torak · Accepted Answer · 2010-04-27 13:56:58Z

4

The bits of code that are relevant are

FixLeaderType *FLdrPtr; 
unsigned char RcvBuff[8192]; 

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

FldPtr is of type FixLeaderType *, or pointer to FixLeaderType.
RcvBuff is an array of char.
HdrPtr->Offset[0] resolves to an ushort value, so RcvBuff [ HdrPtr->Offset[0] ] yields a char value.
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 *.
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.

answered Apr 27, 2010 at 13:56

torak

5,83223 silver badges25 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

HMcG Over a year ago

Thank you! The typecast operation was the bit I was missing. That clears things up enough for me to continue. Cheers.

AHiismak Over a year ago

"RcvBuff is an array of char" - Correct in Delphi 2007 and older. But in Delphi 2009 and newer it should instead be: "RcvBuff is an array of AnsiChar". Or alternatively: "RcvBuff is an array of Byte". This change is due to the fact that in Delphi 2007 and older: char = alias that means AnsiChar. But in Delphi 2009 and newer: char = alias that means WideChar.

fdhsfdhdfsfd · Accepted Answer · 2010-04-27 13:43:07Z

3

i think you should read those like:

* = pointer to

& = address of

that makes things a lot easier

answered Apr 27, 2010 at 13:43

fdhsfdhdfsfd

311 bronze badge

Collectives™ on Stack Overflow

Translating C pointer code example to Delphi pointer syntax

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related