Why call sizeof operator with two arguments?

Question

I recently came across some code that looked like:

if(sizeof(var,2) == 4) { ... }

(where var is a type)

I was quite surprised to see what appeared to be two arguments to the sizeof operator. A quick scan of the ISO/ANSI C99 standard did not yield any secrets. I couldn't come up with any reading of the grammar that allowed a comma there.

Searching Google Code, I was able to find an example of this syntax in some PPC code.

Is this some PPC-specific syntax? What does it mean?

EDIT: It turns out that both what I was looking at--as well as the linked code--is syntax specific to the WindRiver Diab compiler:

sizeof(type, int-const):

If int-const is 0 sizeof returns the size in bytes of type.

If int-const is 1 sizeof returns the alignment of type.

If int-const is 2 sizeof returns an integer constant designating the type of type. Look up "sizeof operator" in the Diab C/C++ User's Guide for values.

Wow, they've really overloaded the meaning of the sizeof operator.

EDIT2: Full documentation is here: http://www.vxdev.com/docs/vx55man/diab5.0ppc/c-additi.htm#3001432

Answer 1

It looks to me like a simple application of the comma operator, which evaluates its first argument, throws away the result, then evaluates its second argument.

In this case, it's determining whether the literal 2 has size 4. The "var" part is irrelevant.

Answer 2

Looks like a red herring. My guess is that you are accidentally using the comma operator and sizeof is being applied to the last value.

Answer 3

As mentioned, the comma operator is being applied and sizeof is returning the size of an integer literal. Offhand this looks like an error on the author's part, but there could be some sinister coding happening.

sizeof expressions are not evaluated so they can be used for a number of tricky things. One example is to provide a reference for an otherwise unreferenced variable without causing the compiler to generate any code. See this article on creating a better assert macro for an example. Alexandrescu has some other examples of sizeof trickery in Modern C++ Design, if memory serves. It's possible, but not likely, that one of these non-obvious usages is intended.

Whatever the usage, if it's not commented in this situation then it's clearly not worth the trade-off in readability and should be changed.

Answer 4

On further research, I discovered that this is behavior specific to the WindRiver Diab compiler. Please see the EDIT in the question for details.