Given:

struct mystruct myinstance;

Which is better?

sizeof(struct mystruct)
sizeof(myinstance)

Normally this doesn’t matter all that much. However, I believe the first is the better way. The reason for this is that if you use the instance variable in the sizeof you can later introduce a nasty bug should you decide that the instance should no longer exist on the stack. That is changing the first line to:

struct mystruct *myinstance;

The second form of sizeof() given above will now return the size of the pointer not the size of the structure. Boom. Nasty bug.