Thread: Using sizeof( long ) at compile time?

Hi,
I was wondering if there's a way to do something like this in C:

Code:

#if sizeof( long ) != 4
#  error "sizeof( long ) != 4"
#endif

I'm trying to test how many bytes are in a long, int, short... I have a function that will only run on machines where short = 2 bytes, int & long = 4 bytes. If the code is ever compiled on a system where that's not true, I need to generate a compile error so the person porting the code can take the appropriate steps to fix it.

Originally Posted by matsp

My best suggestion would be to use <limits.h>, which contains "LONG_BIT", which should be 32 in your case.

Mats

Is LONG_BIT a standard constant? I don't see it in MSVC++ 8.

Maybe I should just do this:

Code:

#if UINT_MAX != 0xFFFFFFFF
#  error "ints aren't 4 bytes here!"
#endif

A good optimizing compiler can do it at compile time even if you use code instead of the preprocessor.

Code:

if(sizeof(long) == 4)
{
    do_something();
}
else
{
    do_something_else();
}

The sizeof(long) folds to the constant 4, then the constant comparison 4 == 4 folds to the constant 1, then the compiler sees if(1), and optimizes the other case away.

Of course this doesn't work if you wanted to, say, declare a certain variable only if sizeof(long) == 4.

Originally Posted by brewbuck

A good optimizing compiler can do it at compile time even if you use code instead of the preprocessor.

But that wouldn't issue a compile error, which is what I want.

Basically, in my code I say the htons()/ntohs()... functions should be used.
If, for some reason, those functions don't exist on the platform being ported to, they can use my own version of those functions. But if sizeof( short ) != 2 or sizeof( long ) != 4, they need to make some changes to get it to work.

Originally Posted by cpjust

But that wouldn't issue a compile error, which is what I want.

Code:

if(sizeof(long) != 4)
{
    some_function_that_doesnt_exist();
}

This will create a link failure if sizeof(long) != 4. Again, depends on the compiler optimizing it to oblivion. I don't 100% recommend it, but it's an option.

Sorry but,
when you type:

Code:

long long var_name;

or:

Code:

long int var_name;

what doesn't that mean?

This is part of the standard integer types (well, at least in the C99 version). This is the complete list

• char
• short int
• int
• long int
• long long int

or, if you prefer,

• char
• short
• int
• long
• long long

Both lists are equivalent.