CODEWITHSUNDEEP
cdata-structuressizeof
stdcall
stdcall

Reputation: 28880

accessing members of structure with sizeof

I've got this code, which I don't understand why it doesn't compile:

typedef struct 
{
  uint32_t serial_number;
  uint32_t ieee_address[6];
} FACTORY_CONFIG; 

...
// Unlock flash (only the portion we write on)
    error = FLASHD_Unlock ( writeaddress, writeaddress + sizeof ( FACTORY_CONFIG.serial_number ), 0, 0 );

When I run it, I get this error:

Error[Pe018]: expected a ")"

When I change the

FACTORY_CONFIG.serial_number

to

FACTORY_CONFIG

, it compiles and everything works. I'm not sure, can I check the size of a type inside a structure ?

Upvotes: 1

Views: 205

Answers (3)

Abhishek Chandel
Abhishek Chandel

Reputation: 1354

You need to create the object first. The struct you are creating is only a type. Do like this:

sizeof (((FACTORY_CONFIG *)0)->serial_number)

Upvotes: 0

Jens Gustedt
Jens Gustedt

Reputation: 78903

What you seem to want, ist to interpret your writeaddress as if it were pointing to an object of your struct and then access the address of the ieee_address member.

The way such things are meant to be done in C is

((FACTORY_CONFIG*)writeaddress)->ieee_address

that's it.

The method you were using is very dangerous because you don't know how your compiler will layout the struct in memory. If you have to, there is the offsetof macro to get the exact postion of a field.

BTW, having type alias in all caps is against common coding style and against everybody's habits. Usually all caps names are reserved for macros.

Upvotes: 0

Eli Bendersky
Eli Bendersky

Reputation: 273456

You can't just access members of types in C like that. You can, however, take sizeof from actual objects. And since sizeof is a compile-time construct, these objects don't even have to be valid. So the following will work:

sizeof(((FACTORY_CONFIG *)0)->serial_number)

If you use this a lot, or just for readability, you could make a macro out of it.

Upvotes: 3

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