I think it's best to regard intmax_t as a failed experiment. E.g. GCC has long supported __int128, clang has _ExtInt, other compilers have similar extensions, but they refuse to increase intmax_t due to ABI concerns.
Well, that code is not, and was not ever, standards compliant.
After all, if predictable behavior in the future across all possible configurations were a high priority requirement for the tools people use to make computers do things, neither C nor C++ would have gotten off the ground. Instead, people don't memorize the entire standard. They write code that works on their machine, publish it, fix the bugs when people say it doesn't work on their machine, and maybe converge towards something that is standards compliant but, more likely, they converge towards something that works on the standard implementation on 90% of the operating systems available.
Would you be able to name three examples?
My intention was to make a good case for old code being compiled today by third parties.
But to my understanding intmax_t was introduced in C99, before widespread dominance of 64 bits platforms. So it would not be surprising that there exists code with lower assumptions about its size.
But actually I made also another mistake, the major problem had little to do with faulty logic and more to do with dynamic linking [0]
I will see myself out today.
If you want to know "what is the widest signed integer type available", you will get some kind of answer one way or another.
Obviously, when someone says “type X is size Y on architecture Z”, they’re not talking about the C standard, they’re talking about some particular ABI.
There are multiple ABIs for x86_64. I think the major compilers use 32 bits for int and 64 for long, but there could very well be a compiler that used different sizes, with a different ABI.
> A compiler vendor can set the sizes to whatever they want regardless of the target architecture and claim conformance if they meet the minimum.
Compilers claim conformance with other standards besides just the C standard.
Anyone who uses intmax_t in a durable API (public function argument, member of public structure) is just a goof.
How would you compile that to machine code which would, when run, still give you the maximum integer on next year’s CPU with all-new super-duper wide integers?
It is more reasonable to think of a binary to be compiled to a specific architecture triplet, and if the CPU changes, the architecture must change, and therefore the ABI, and if the OS wants to run a binary from an older architecture, some emulation layer is needed.
Of course, this would be a lot of work for the operating system people if they want to run binaries from lots of what are now different architectures, and it was apparently easier to just force the C standard to abandon the entire concept of intmax_t being the largest.
intmax_t by intent is the maximum integer available on a given platform. Abandoning that idea is a disservice to compiler writers, programmers -- especially scientific programmers -- everywhere.
That would add a lot of friction to changing CPUs, and making distributing software more complicated.
- POSIX defines that in_port_t is equal to uint16_t, so for that type there's no problem at all ;)
- Don't use printf/scanf for this, roll your own
- Refuse to print/read uid_t that are outside intmax_t range
- Use an autoconf test
Until then I guess I’ll have to do something like
#if sizeof(pid_t) > sizeof(uintmax_t)
#error Nope
#endif
Also this doesn't just apply "in the future" -- integer types larger than intmax_t already exist, C23 just updates the standard to match reality.
#if SIZEOF_PID_T > SIZEOF_UINTMAX_T
...
#endif
There are ways to detect sizes by compiling a source file to an object file, and then analyzing the object file (no execution), so things work under cross-compiling.
I've used a number of tricks over the years, and settled on this one:
https://www.kylheku.com/cgit/txr/tree/configure?h=txr-278#n1...
The basic idea is that we can take a value like sizeof(long), which is a constant, and using nothing but constant arithmetic, we can convert it to the characters " 8". These characters can be placed into a character array where they are delimited by some prefix that we can look for in the compiled object file with some common utilities.
This is quite durable in the sense that it can be reasonably expected to work through a wide variety of object formats.
In that test program, I have a structure with some character arrays. The larger character arrays hold the informative prefixes. The two-byte arrays hold decimal digits for sizes. Two digits go up to 99 bytes so we are safe for a number of years.
The DEC macro calculates the ASCII decimal value of its argument, expanding to a two-byte initializer for a character array:
#define D(N, Z) ((N) ? (N) + '0' : Z)
#define UD(S) D((S) / 10, ' ')
#define LD(S) D((S) % 10, '0')
#define DEC(S) { UD(S), LD(S) }
DEC(42) -> /* the equivalent of */ { '4', '2' }
DEC(42) -> { UD(42), LD(42) }
-> { D((42) / 10, ' '), D((42) % 10, '0') }
-> { ((42) / 10) ? ((42 / 10)) + '0' : ' ',
((42) % 10) ? ((42 % 10)) + '0' : '0' }