#iec

Due to the nature of C17 being an ISO/IEC document, it is not freely available for download, however the final working draft version of it is freely available, and there should be no significant changes between it and final. Link here.

Now, in what might seem like a very odd choice, we’re going to instantly skip to section 6 of the standard, not only that we’re skipping directly to 6.2.5 (page 31), because this is what defines the fundamental types of the language. Paragraphs 2, 4 and 6 are the important ones right now. From 2 we learn that the type _Bool (just bool in C2X) is at least large enough to hold the values 0 or 1 (hence its name, bool, short for boolean). Then we come to paragraph 4, which states there are 5 standard signed integer types: char, short, int, long, and long long (there’s also a statement about extended types but that can be ignored for the moment). Finally we hit paragraph 6, which simply states that all signed integer types have an unsigned equivalent that can be written by simply attaching the word unsigned to one of the types (ex. unsigned int), and that these 5 unsigned types combined with _Bool make up the set of unsigned types, and that any unsigned type takes up the same amount of storage/memory space as its corresponding signed type.

Moving back slightly to paragraph 5 we learn that signed char and char take the same amount of memory (and as such so does unsigned char), it also states that the int type should be the natural operating size of whatever machine architecture the code is being written for (so long as it satisfies a minimum value requirement). Moving forward to paragraph 9 we learn that, as you would expect, the signed types can store negative and positive values, and that the unsigned types can only store positive (or zero) values. We also learn that by definition unsigned values can not overflow, instead their value simply wraps around the range, however this is often referred to as overflow regardless.

Yet further down (paragraph 15, next page) we learn that char, signed char, and unsigned char are all different types, however char must match either char or unsigned char (the reasoning for this is that a compiler is free to choose the most efficient match for processing).

So with all that jumble out of the way what did we actually learn? Well: there exist 5 fundamental integer types: char, short, int, long, and long long, and that signed and unsigned versions of each exist, we also know there’s an unsigned type, _Bool that can represent the values 0 and 1. Together these make up the integer types of C.

Beyond integers there’s also floating point types, float, double, and long double, but those are a much more complicated subject that are outside the scope of this post. There’s also the void type, which has confusing wording that basically boils down to “you can’t use this, it represents nothing” (which is why functions with no return value have their return type marked void). And finally that you can make arrays out of these types (except void), make structure types that contain any number of these types (except void), make union types (complicated subject, think a box that can store only one type (except void) at a time), you can declare functions that return any type, you can have a pointer to any type (including void in order to represent a pointer to an unknown type), and finally that any type can be made atomic (safe for access from multiple threads).

Most of that statement can be freely ignored for now, it’s a mess that we’ll (hopefully) get into later. The important bit to know is that you can build other types out of the fundamental types.

Moving beyond that section and into 6.2.6 we learn from 6.2.6.1 Paragraphs 2-4 that objects (any type’s representation in memory basically) is made out of a continuous sequence of bytes. Moving to 6.2.6.2 we get to the representation of integer types, which is pretty much what you would expect for unsigned values (an unsigned integer of size n value bits can store 2n values from 0 to 2n-1), signed integer types in C17 list three different representations, however C2X is reducing that down to just twos’ complement (a signed integer of size n bits can store from -2n-1 to 2n-1-1, negative zeros are not possible).

So far: there exist integer types, and these store values in the expected way, but what are the range of values they must store?

Section 5.2.4.2.1 (page 20), paragraph 1 states that all implementations of the language must have the integer types be able to represent at least the values in the ranges listed below, meaning an implementation is free to expand the range if it sees fit (like how many do with int). From here we learn that a char must be at least 8 bits, and as such a signed char must be able to represent -127 (-128 in twos’ complement) to 127, and that an unsigned char is 0 to 255. Helpfully the actual (minimum) number of bits for each type is specified in a comment next to the definition. short is 16 bits/2 bytes, int is also 16 bits/2 bytes (again often expanded to 32 bits/4 bytes), long is 32 bits/4 bytes, and long long is 64 bits/8 bytes. _Bool due to its nature as a boolean type is not mentioned (actually any values placed into an object of type _Bool will become 0 if the value is 0 or 1 otherwise).

In summary: 6 types, _Bool (0 or 1), char (at least 8 bits), short (at least 16 bits), int (at least 16 bits, often 32), long (at least 32 bits), and long long (at least 64 bits).