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From: "subc2 at wp dot pl" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/63417] New: scanf: problem handling %hhd format
Date: Tue, 30 Sep 2014 15:48:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63417

            Bug ID: 63417
           Summary: scanf: problem handling %hhd format
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: subc2 at wp dot pl

The following code runs properly on any UNIX-like operating system, but under
Windows with GCC up to the version 4.8.1 (ports: MinGW, MinGW-w64, TDM-GCC)
gives wrong results.

file.c:
#include <stdio.h>
int main()
{
  int logical = 1;
  signed char s_char;
  scanf("%hhd", &s_char);
  if (s_char == 0)
    logical = s_char;
  printf("%d", logical);
  return 0;
}

C:\> gcc -std=gnu99 file.c
C:\> a.exe
0
0
C:\> a.exe
1
0
C:\> a.exe
2
0

What's even more interesting, after compiling this code

magic.c:
#include <stdio.h>
int main()
{
  int logical = 1;
  signed char s_char;
  scanf("%hhd", &s_char);
  if (s_char == 0) {
    int nothing = 0;
    logical = s_char;
  }
  printf("%d", logical);
  return 0;
}

C:\> gcc -std=gnu99 magic.c
C:\> a.exe
0
0
C:\> a.exe
1
1
C:\> a.exe
2
1

everything is OK.