touch-packages team mailing list archive

[Colin Watson <cjwatson@xxxxxxxxxxxxx>]

This is a standard gotcha in C operator precedence: == has higher
precedence than ^, and so your expression is parsed as 3 ^ (1 == 1),
which is indeed non-zero.  To fix it, use explicit parentheses as
follows: (3 ^ 1) == 1.

It would perhaps be nice if the C language had been defined differently
(Wikipedia mentions this, for instance:
https://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B#Criticism_of_bitwise_and_equality_operators_precedence),
but we're stuck with it now, and GCC certainly cannot arbitrarily decide
to change the language's operator precedence.

** Changed in: gcc-4.8 (Ubuntu)
       Status: New => Invalid

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https://bugs.launchpad.net/bugs/1533295

Title:
  Wrong result when comparing xor and int

Status in gcc-4.8 package in Ubuntu:
  Invalid

Bug description:
  Hello,
  I'm on Ubuntu 14.04 LTS (64bits)
  With gcc:
  gcc --version
  gcc (Ubuntu 4.8.4-2ubuntu1~14.04) 4.8.4
  Copyright (C) 2013 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.  There is NO
  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

  
  lsb_release -rd
  Description:	Ubuntu 14.04.3 LTS
  Release:	14.04

  
  apt-cache policy gcc
  gcc:
    Installé : 4:4.8.2-1ubuntu6
    Candidat : 4:4.8.2-1ubuntu6
   Table de version :
   *** 4:4.8.2-1ubuntu6 0
          500 http://fr.archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages
          100 /var/lib/dpkg/status

  So:
  3 ^ 1 = 2 ok?

  
  Try this code:
  #include <stdio.h>
  int main()
  {
  	printf("%d\n", 3 ^ 1);
  	if(3 ^ 1 == 1)//Bug in this test
  	{
  		printf("SURPRISE !\n");
  	}
  	else
  	{
  		printf("FAIL !\n");
  	}
  	return 0;
  }

  
  Just compile this with defaut options:
  gcc -o youprogram yourfile.c

  
  Then, run, and you will see:
  2
  SURPRISE !

  
  So, normally it will show "FAIL !", because 3 ^  1 = 2, and 3 ^ 1 == 1 must return 0, but this test return 1 !

  
  If you do this:

  int v = 3 ^ 1;
  if(v == 1)//This test work normally
  {
  }

  
  And:
  if((int) (3 ^ 1) == 1)//This test work normally
  {
  }

  
  Globally, if you have:
  int x, y, z;//Choose random values for this

  if(x ^ y == z)//Return always true
  {
  }

  The invert is bugged too:

  if(z == y ^ x)//Same thing
  {
  }

  I don't know if this bug is fixed on more recent releases, but a patch
  is welcome for this version :)

  Thank you :)

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