Is 'long unsigned' as valid as 'unsigned long' in C?

Question

A question was recently asked about whether ULL or LLU was valid for specifying unsigned long long constants in C. I know they're both valid but I was thinking that ULL would be preferable since it matched the type unsigned long long.

However, I'm not so certain now. The gcc compiler allowed the following without complaint:

int main(void) {
    unsigned int a = 1;
    int unsigned b = 2;
    return 0;
}

So my question is: are int unsigned, and other variations like long long unsigned, valid types according to the standard?

Answer 1

The ISO C11 standard states in 6.2.5 Types:

There are five standard signed integer types, designated as signed char, short int, int, long int, and long long int.

For each of the signed integer types, there is a corresponding (but different) unsigned integer type (designated with the keyword unsigned) that uses the same amount of storage (including sign information) and has the same alignment requirements.

However there's no mandate in that section as to the order in which the base type and unsigned modifier appears.

The controlling section is later in the standard, 6.7.2 Type specifiers, paraphrased here:

Type specifiers are void, char, short, int, long, float, double, signed, unsigned, _Bool, _Complex, <struct-or-union-specifier>, <enum-specifier>, and <typedef-name>.

At least one type specifier shall be given in the declaration specifiers in each declaration, and in the specifier-qualifier list in each struct declaration and type name. Each list of type specifiers shall be one of the following multisets (delimited by commas, when there is more than one multiset per item); the type specifiers may occur in any order, possibly intermixed with the other declaration specifiers.

It then goes on to list all the multisets, such as unsigned long, or unsigned long int.

But the important phrase there is the type specifiers may occur in any order, meaning that all of these are valid for that multiset:

unsigned long
long unsigned

unsigned long int
unsigned int long
long unsigned int
long int unsigned
int unsigned long
int long unsigned