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From: "luigighiron at gmail dot com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug libc/31470] New: inttypes.h provides incorrect macro
 definitions
Date: Mon, 11 Mar 2024 01:05:29 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D31470

            Bug ID: 31470
           Summary: inttypes.h provides incorrect macro definitions
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: luigighiron at gmail dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

The inttypes.h header provides macros for printing out the types defined in
inttypes.h and stdint.h. In C23, the following text was added to the C stan=
dard
in section 7.8.1 "Macros for format specifiers":

> The functions in the fprintf and fwprintf families shall behave as if the=
y use
> va_arg with a type argument naming the type resulting from applying the
> default argument promotions to the type corresponding to the macro and th=
en
> convert the result of the va_arg expansion to the type corresponding to t=
he
> macro.

For example:

#include<stdio.h>
#include<inttypes.h>
int main(){
    uint8_t u=3D-1;
    printf("%"PRIu8"\n",u+1);
}

This program should output 0, at least in C23. u contains the value 255, th=
en
one is added which results in the value 256 (after integer promotions). 256=
 is
provided to printf which will do va_arg with int (the promoted type of uint=
8_t)
and that value is then converted back to uint8_t which will result in the v=
alue
0.

However, libc (I tested it on godbolt) seems to print 256 here because it d=
oes
not do the conversion back to uint8_t. This seems to be because libc defines
PRIu8 here as "u". I tested this on MSVC and Cygwin and it printed 0 as
expected, with those compilers it is defined as "hhu". The text which requi=
res
that the conversion back after va_arg was added by GB-108 (N3148). See N3167
for discussion about accepting GB-108.

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