Re: -std=c90 -pedantic-errors and <stdint.h>

[Xi Ruoyao <ryxi at stu dot xidian dot edu dot cn>]

On 2017-06-01 13:14 +0200, David Brown wrote:
> On 31/05/17 01:23, Vincent Lefevre wrote:
> > On 2017-05-30 23:04:02 +0200, Florian Weimer wrote:
> > > * Vincent Lefevre:
> > > 
> > > > Is it normal that -std=c90 -pedantic-errors allows to use
> > > > 
> > > > #include <stdint.h>
> > > > 
> > > > ?
> > > 
> > > Yes.  It's difficult where to draw the line.  <inttypes.h> was
> > > apparently available with some C90 compilers, so it would make sense
> > > to allow it in C90 mode.  But this means that it wouldn't be
> > > completely out of line to accept <stdint.h>, too.
> 
> I have used C90 compilers that provided a <stdint.h> too.
> 
> And since C90 does not define a <stdint.h>, an implementation or user is
> free to put a file called "stdint.h" in a directory that is searched for
> system includes - a compiler cannot arbitrarily blacklist a header name
> in a standard that does not mention that name!
> 
> > 
> > Now, what actually mattered in my case was the use of (u)intmax_t.
> > For instance, consider the following program:
> > 
> > #include <stdio.h>
> > #include <stdint.h>
> > #include <limits.h>
> > 
> > int main (void)
> > {
> >   uintmax_t i = -1;
> >   printf ("%d\n", i > ULONG_MAX);
> >   return 0;
> > }
> > 
> > In C90, "long" is the largest type, so that one should always expect
> > the output 0. But:
> 
> "long" is the largest standards-mandated type in C90, but it is not
> necessarily the largest type available.  An implementation is allowed to
> provide additional types, even in "pedantic" C90 mode.

For example we have __int128 and some targets has __int20.

> In this particular case, uintmax_t is defined using the "__extension__"
> keyword inside the <stdint.h> file.  (At least, that is the case in my
> sample <stdint.h> here.)  The implementation is allowed to do that - the
> identifier "__extension__" is reserved for the implementation, and the
> compiler can use it however it wants.
> 
> So in C90 mode, <stdint.h> is not standardised and can contain whatever
> the implementation (or user) wants.  The same applies to the type
> "uintmax_t" - C90 says nothing about it.  You have no guarantees, and
> the working of the code depends entirely on the contents of <stdint.h>.
> 
> If you want to stick to C90, and you want to have a uintmax_t that
> matches "long", then I suggest you don't include headers that are not
> relevant for C90, and define the types yourself.  Perhaps start with:
> 
> #ifdef __STRICT_ANSI__
> typedef long int intmax_t;
> typedef unsigned long int uintmax_t;
> #else
> #include <stdint.h>
> #endif

It's wrong.  __STRICT_ANSI__ is also defined in "-std=c99". We need

#if defined(__STRICT_ANSI__) && __STDC_VERSION__ < 199901L
#include <our/customized/stdint.h>
#else
#include <stdint.h>
#endif

> > 
> > cventin:~> gcc -std=c90 -pedantic-errors -o tst tst.c -m32
> > cventin:~> ./tst
> > 1
> > 
> > under GNU/Linux.
> > 
> > IMHO, at least some features of <stdint.h> should yield a warning
> > with -pedantic (an error with -pedantic-errors).
> > 

I'll send a RFC patch to gcc-patch, after finishing my baechlor
thesis...  But it's nearly impossible the patch would be approved by
GCC developers.
-- 
Xi Ruoyao <ryxi@stu.xidian.edu.cn>
School of Aerospace Science and Technology, Xidian University