The fixed size int types int32_t and friends weren't introduced until C99. Microsoft held out until 2013 before it put <inttypes.h> into Visual Studio!

So there has been a really long time in C's evolution where we haven't had fixed size types which has been a super annoying mess of #ifdefs in portable code.

The variable size ints have allowed some super weird architectures though. I remember looking at the datasheet for the Motorola 56000 DSP and noting that the C compiler set char = short = int = 24 bits! That was because the hardware could not address anything smaller than 24 bits. I think long could be 48 bits.

I would agree if overflow on those types wasn't undefined behaviour, or unpredictable

Plus char could be signed or unsigned!

Per the C standard `sizeof(char)` is always 1, regardless of how many bits it has.

> At that time, 36 years ago, the C flexible integer sizes were already obsolete.

This is a highly ignorant comment. You're confusing the fact that you only had to work with a single target architecture with the whole concept of multiple processor architectures being somehow obsolete, as if there was a sudden law of nature that forced every single computer, being full blown HPC stuff or small microcontrollers used in embedded applications.

Take a look at arduino. They still have 16-bit models out there. Also noteworthy, it seems some DSPs also have ints larger than 32 bits.

`int` being 32-bits on amd64 was the correct decision, even in hindsight. If you are familiar with the ISA you will understand this. Existing 32-bit code just worked on the 64-bit chip, because the instruction encodings for 32-bit are unchanged. The 64-bit instructions are basically "opt-in", by placing a REX prefix on them - that makes them more expensive to encode and uses more instruction cache. Even today, compilers will emit 32-bit versions of instructions when the upper 32-bits are not needed, because it's cheaper.

When amd64 was released, x86 was almost ubiquitous on desktops and ran the majority of servers - most of the software used by the world could continue being used. If AMD had not gone through this effort to make it backward compatible, it's likely IA64 would've won and we wouldn't have this debate. Hard to understate the importance of not breaking things.

If you were designing a greenfield 64-bit ISA, then yes, it might make sense to have `int` be 64-bits, but it was definitely not, and still is not a mistake that it's 32-bits on amd64.

On RISC-V for example, it's questionable. The RV32 ecosystem is tiny and almost irrelevant - if they decided to break things for RV64 it wouldn't be a big problem - probably better to fix any problems early rather than hold baggage to run software that never existed - though it's much easier to port software to RV64 if `int` is still 32-bits.

> All relevant platforms

Don't make me tap the sign: the majority of processors running C are weird little dirtbag chips of 16 bits or less sprinkled by the dozen.

> No, I don't believe for a second that any future platform will change their size.

ILP64 (wherein int is 64 bits) exists. It's not very popular, but it exists; e.g. ICC supports it. So it happened in the past once already; it may again happen in the future. In any case, predicting the future is very hard, you really shouldn't be doing this.

> IEEE memory representation of float and double

Wait, what? I'm fairly certain that a) IEEE does not mandate the in-memory representation, and b) ARM actually uses big-endian byte order for floats/doubles when storing them in memory.

> always behave exactly the same (leaving out strange edge cases such as denormals)

So not always, but please pretend so? Yeah, no, thank you.

> platforms have very long agreed on twos-complement for negative integers. This even made it into the standard at some point, I believe.

Only in C23. It was explicitly rejected for C++ 23 (and C++ 26 too, I believe).

> but because everyone of course did the obvious

No, not everyone did the obvious. That's why it took so long to standardize because divergent implementations existed.

> There are much more guarantees modern C++ code can (and should) rely on.

As long as you only use only GCC (or Clang) exclusively, yes, you can. Otherwise, no, you can't and shan't.

    uint16_t x = 0xFFFF;
    uint16_t y = 0xFFFF;
    uint16_t z = (uint16_t)((x + 0U) * (y + 0U));

That is way too much to remember

This kind of flexibility is a purely historical thing.

On modern computers, it is impossible to write correct C programs that are agnostic about the true size in bits of the "flexible" types char, short, int, long and long long.

If your program must depend on assumptions about the size in bits of the integer types, those assumptions must be made explicit, by using types like int16_t, int32_t etc.

Writing correct programs that are agnostic about the integer sizes is possible only in programming languages that allow the programmer to install an integer overflow handler even if the CPU does not generate a hardware exception for that, in which case the compiler must insert appropriate overflow checking instructions that would invoke the installed handler when necessary.

This problem did not exist on old computers, where there were hardware exceptions for integer overflows, so even in C you could install a signal handler for SIGFPE, which would also be invoked by integer overflows.

    #include <limits.h>
    #if INT_MIN == 32767 && INT_MIN >= 2147483647L
    # error too small
    #else
    # error just right
    #endif

C survived because UNIX carried it.

Not sure why you would need to invent an new programming language when we're still strictly talking about data types. You just introduce a new data type for the hardware if that's what's necessary. You don't need a whole language.

This is why I think so many C developers have no clue what they are doing. They just take whatever decision was made in C as gospel instead of thinking of everything being up for negotiation.

Is that really that much of a big deal, though? Before stdint, if one needed that level of accuracy, one would do your own equivalent of stdinit by hand, and adjust those definitions when porting to another compiler/platform. The same goes for your local boolean type.

I think the only real annoyance is that each programmer/team did it with their own convention (I32, INT32, i32, int32, WORD, Word bool, BOOL, Bool, etc., etc.); standardizing helps with putting everyone on the same page more than it helps porting. It doesn't prevent people from reverse-typedef-ing standard names to local "dialectal" names, though.

But I also think that one should only rarely use raw integer types, in an ideal world; the elephant in the room is that typedef is kind of the second "billion dollars mistake" [1]. C is a weakly typed language and there's no practical way to undo it (besides transpilation), so there's double no point to leave behind raw types.

[1] For those not too familiar with C, typedef defines a "type alias", not a type: https://en.cppreference.com/cpp/language/typedef

    typedef typeof(sizeof(0)) size_t;
    typedef typeof(nullptr) nullptr_t;
    typedef typeof((void*)0-(void*)0) ptrdiff_t;
    typedef typeof(L'\0') wchar_t;
    typedef typeof(u8'\0') char8_t;
    typedef typeof(u'\0') char16_t;
    typedef typeof(U'\0') char32_t;

C99 kinda fixed that with the `(u)int_leastN_t` types (which are hardly used in practice though). And shame that it took Microsoft 16 years to even start supporting C99 though so we were basically forced to keep using our own custom integer typedefs long after the C standard had fixed the issue.

I thought most implementations of C have stdint (intN_t, leastN_t and fastN_t etc).

On x86-64, you need an extra prefix to do 64-bit operations (while 64-bit addressing is the default), so it's a question of "are you sure you need the 64 bits and 32 isn't enough?"

One a modern computer, the ideal integer type is usually smaller than 64 bits. Even though the arithmetic units handle 64 bits in one op, 64 bits take up twice the memory bandwidth, and half as many fits in a cache-line, and given that a modern computer is usually bound by memory access, smaller is better. So you can argue that both new and old computers prefer 32 bit over 64, but for different reasons.

The added compatibility between 32 and 64 bit systems, is another reason. Also if you choose int to be 64 bits, what would you make short? 16 or 32 bits? It makes much more sense to keep int 32 bits, and reserve long / long long for 64 bits.

Memory addresses being 64 bits due to needing to address more than 4 GiB memory doesn't mean most integer instructions operate most efficiently with 64 bits. Instructions for 32 bit integers are still more efficient than 64 bit, whereas 16 bit operands require a prefix byte meaning they're less compact and cache efficient (on AMD64 anyway).

> Shouldn't they be 64 bits on most modern systems then?

Arguably so, but then one would lose the ability to natively name 16-bit integer types because "short" would be 32 bits.

An earlier comment addresses x86-64. AArch64 (pedantically, the A64 instruction set used for AArch64's 64-bit execution mode) is similar, in that addresses are 64 bits wide but ALU instructions typically encode a width bit, called "sf", that selects either 32- or 64-bit data registers and arithmetic. See, for example, https://arm.jonpalmisc.com/latest_aarch64/add_addsub_ext .

    typedef unsigned __attribute__((mode(HI))) uint16_t;
    typedef signed __attribute__((mode(SI))) int32_t;

It should indeed, and in hindsight it would have been better to move int to 64 bits (especially for C's integer promotion, which only really makes sense when the promotion happens to the register width.

But porting 32-bit code to 64-bit was a big deal back then, and C99 with its new fixed-width integer types overlapped with the first AMD64 CPUs (and Microsoft's MSVC didn't start to support C99 until around 2015 anyway), I guess keeping int on 32-bits in the popular compilers was deemed 'safer' for porting existing code. I guess we can already be lucky that all the big compilers agreed on the same int width.

What is the size of a pointer though?

On Intel 286 we had a 16-bit machine word and 24-bit addresses. A pointer wasn't just two machine words concatenated - the upper 8 bits were stored somewhere else - a segment register.

On modern machines we don't (usually) need to consider this because we have a single linear virtual address space, though the size is architecture dependant - usually above 40 bits and below 64. Most common size is 48-bits, but also up to 57-bits with 5 level paging enabled.

Either way we round up to 64-bits to store the pointer as one integer. C optionally provides types `intptr_t` and `uintptr_t`, which are integers large enough to hold the value of a pointer. Converting a pointer to `intptr_t` and back to the pointer type results in a pointer that compares equal to the original.

However, there is no guarantee that a pointer converted to `intptr_t` and back to a pointer can be dereferenced! It works most of the time because of our linear address space and non-use of segmentation, but segmentation can still be used - the FS and GS segment registers are still available on x86_64 and are commonly used for thread local storage. If you take a `thread_local T*`, convert it to `intptr_t`, and then convert it back to a `thread_local T*` on another thread and attempt to dereference it, then despite the pointers comparing equal, they dereference to different virtual addresses.

Integers tied to the size of a pointer would have been misguided. Pointers are not integers! (They just happen to use an integer in their representation).

Another one, `size_t` is supposed to represent the maximum size of any object. However, that's also not well-defined. The maximum object size on the Intel 256 would have been 16-bits, because that is all you can fit in a single segment.

On a modern machine, a `size_t` should really be 48-bits (4LP) or 57-bits (5LP), because we can't have an object larger than our maximum virtual address size - but `size_t` is typically 64-bits.

It turns out that you don't want integers the same size as a pointer because somebody might squirrel away capability bits in your pointer type (see CHERI) and you definitely do not want integers with capability bits.

Rust originally says that its types usize and isize are the same size as pointers, but this was ret-conned in later Rust to say actually they're the same size as addresses for this reason.

At the time its was probably very hard to know what the fixed sizes should be.

I came here to say exactly that. There’s int_leastN_t for storage and int_fastN_t for computation. Stdint.h really gets a bad rap here.

> int is basically the signed version of size_t aka a word sized data type. It's not meant to have a fixed size.

It isn't. `int` is at least as large as `short` and at least 16-bits. On modern systems `int` is still typically 32-bits whereas `size_t` is typically 64-bits. There's a `ssize_t` in POSIX for signed sizes.

> When people want the classic 4 byte data type they should choose long instead.

`long` is only at least 4 bytes, and at least as large as `int`. On MSVC (LLP64 data model) it's 4 bytes, but on SYSV (LP64 data model) it's 8 bytes. `long` should almost never be used if you actually want portable code today.

`int` is 32-bits and `long long` is 64-bits on both LP64 and LLP64. If you want portable code using the native integer types, these are the ones you should use, definitely not `long`.