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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: Why does C allow structs to have a tag?
Date: Wed, 28 Jul 2021 16:07:28 -0700
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Bart <bc@freeuk.com> writes:
> On 28/07/2021 18:21, Scott Lurndal wrote:
>> Bart <bc@freeuk.com> writes:

[...]

> I want to call a C function via an FFI from a language that I've
> devised. The public API says one parameter type is off_t.
>
> Which type matches that in my language? I have to choose from i8-i64
> and u8-u64.
>
> The kind of answer I want is (a) 'It uses i64' etc; or (b) 'It uses
> i32 or u64 depending on platform'.

POSIX says that off_t is a signed integer type.

Is there some reason that compiling and running this program on the
target system doesn't solve your problem?

#include <stdio.h>
#include <limits.h>
int main(void) {
    printf("off_t is %c%zu\n",
           (off_t)-1 < (off_t)0 ? 'i' : 'u',
           CHAR_BIT * sizeof (off_t));
}

> Not keeping the answer buried for decades under a dozen typedefs,
> dependent on a dozen conditional macros, dependent on the C compilers 
> used [what C compiler? I might not have one] and hidden in include
> files 6 levels deep, scattered across multiple directories, to make it
> as hard as possible to find out.

Yeah, you don't need to do any of that.

[...]

>>> This is exactly my point. A compiler installation can exist for just one
>>> platform. This goes for Windows compilers such as Pelles C (2 targets),
>>> DMC, lccwin (2 targets), Tcc (2 targets).
>> So what?   Programmers generally[*] don't write code for a
>> particular compiler or platform,
>> they write code to the language specification to solve some problem, which is independent
>> of any particular compiler implementation.  Portability of C code is what
>> has built the internet as we know it.
>> And for other languages, the language provides the binding, for
>> example:
>> http://www.nongnu.org/posix90/#SEC7
>
> This doesn't do struct stat. But supposing it did, how did /they/ pick
> up the necessary information? Although they seem to have clocked that 
> 'clock_t' is an integer type, rather than float, something else that C
> prefers to keep under wraps.

POSIX says that clock_t can be either integer or floating-point.  You
can use a method similar to what I wrote above to find out what it is in
a particular implementation (without reading the header files).

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips
void Void(void) { Void(); } /* The recursive call of the void */