Question

does anybody know a website or a paper where the sizes of C data types were compared on different machines? I’m interested in values of some ‘big’ machines like a System z or the like.

And:
Is there an upper bound of bytes that the biggest native datatype on any machine can have and is it always of the type complex long double?

Edit: I’m not sure about but does the SIMD register data also take advantage of the CPU’s cache? Data types that will be stored in a special unit and do not use the L1/L2/L cache are out of my interesst. Only the types {char, short, int, long, long long, float, double, long double, _Bool, void *} (and with _Complex) will be examined.

Answer 1

You mention “native” datatypes, but note that complex is not defined by the C spec and thus isn’t a native type. The native types for C are char,int,float, double, and void.

The size of a data type is generally determined by the underlying platform as well as the compiler. The C standard defines the minimum range for these types and defines a few relative relationships (long int must be at least as long as a regular int, etc). There’s no easy way to determine the absolute size of any type without testing it.

When working with a new platform and I don’t know the particular data type sizes, I write up a short app that dumps the result of sizeof for all the standard C types. There are also headers like stdint.h that give you data types that you can trust to be a certain size.

There is no upper bound as to the size of a data type. The C standard defines char to be “large enough to store any member of the execution character set”. This partially binds the size of the native types to the machine architecture, and so a theoretical machine that had execution characters up to 1MB in size would have sizeof(char) equal to 1MB. Practically speaking, you probably won’t find a machine where this is the case.