Data structure prototype

[mce]

Hello,

I got a third party prototypes declaration as follow:

typedef struct mydata{

 DWORD Length; //length of data field in bytes
 BYTE Data[1]; //  the data itself, variable length

}MYDATA

My question is why the Data is an array of 1???, if it is supposed to be of variable length, of value Length., as stated in the comments??

[Dave McLelland]

Because 1 is the smallest dimension for the array that allocates space for it. The way these things work is that the struct is declared with an array of size 1 (as the last member). When the memory for the struct is allocated it can be bigger than the structs declared size allowing access to further array elements.

eg. MYDATA *p = malloc(sizeof(MYDATA) + 100;
now the array indexes could range from 0 to 100.

The Length field is used to convey the number of elements that are accesible in the array.

[mce]

the following achieve the same purpose, why not use the following style instead?

typedef struct mydata{

 DWORD Length; //length of data field in bytes
 BYTE * Data; //  pointer to data

}MYDATA

[Paul McKenzie]

Search google for "struct hack". You will find many hits and lots of information on why this strange way of setting up a structure is done.

Regards,

Paul McKenzie

[Ajay Vijay]

May be just because BYTE* pointer occupies sizeof(WORD) bytes but BYTE occupies only 1. Further, due to different word sizes, sizeof structure does not remain constant - for compatibility between platforms, this may be imposed.

[Dave McLelland]

Also changing the type from BYTE to BYTE* moves the actual data out of the struct.

[vicodin451]

Originally posted by Ajay Vijay
BYTE* pointer occupies sizeof(WORD) bytes

sizeof( WORD ) = 2 bytes
sizeof( BYTE* /* or any other pointer on Win32 */ ) = 4 bytes
sizeof( BYTE** /* or any other pointer on Win32 */ ) = 4 bytes

[Ajay Vijay]

Sorry, but I meant with word size as per the platform and not the WORD datatype. In other words what 'int' occupies (Generally) on various OSs.