c - How to determine integer types that are twice the width as `int` and `unsigned`? -

values of intermediate multiplication typically need twice number of bits inputs.

 // example int foo(int a, int b, int carry, int rem) {   int2x c;  // type twice wide @ `int`   c = (int2x)a * b + carry;   return (int) (c % rem); } 

considering potential padding, (which appears limit sizeof() usefulness) , non-2`s complement integers (which limits bit dibbling), ...

does following create needed type?
if not, how code @ least reasonable solution, if not entirely portable?

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#include <limits.h> #include <stdint.h>  #if long_max/2/int_max - 2 == int_max   typedef long int2x;   typedef unsigned long unsigned2x; #elif llong_max/2/int_max - 2 == int_max   typedef long long int2x;   typedef unsigned long long unsigned2x; #elif intmax_max/2/int_max - 2 == int_max   typedef intmax_t int2x;   typedef uintmax_t unsigned2x; #else   #error int2x/unsigned2x not available #endif 

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[edit]
qualify:"always", if long, long long , intmax_t, not work ok #error.
want know if @ least 1 of long, long long, or intmax_t work, int2x correctly typed?

notes: above assumes xxx_max odd power-of-2 minus 1. maybe assumption? above works on @ least 2 platforms, hardly great portability test.

the assumption *_max constants of form (2^n)-1 valid. see 6.2.6 representations of types, , particularly 6.2.6.2 integer types, representations of unsigned integer types , positive values of signed integer types defined pure binary, yielding maximum 1 less power of two.