Comments for Hard to C

Comment on INT_MIN by David

David — Thu, 08 Sep 2011 08:10:55 +0000

Oops. I need to read harder.

Comment on INT_MIN by David

David — Thu, 08 Sep 2011 08:09:50 +0000

Typo: (-32766 – 1)

Comment on INT_MIN by Robin

Robin — Thu, 08 Sep 2011 07:57:41 +0000

@ Peter Hamilton

Because you’re compiling it as a 64-bit application on your 64-bit OS. The article is pointing out a 32-bit problem. If you compile the source code with “-arch i386″, forcing it as a 32-bit executable, the test will work.

Comment on INT_MIN by Jibz

Jibz — Thu, 08 Sep 2011 05:35:00 +0000

Thank you for the comments (even if the post is a few years old, I believe the issue remains).

Peter, normally I would expect GCC to default to C89 (with extensions) which should show ‘positive’. I wonder if it being a LLVM version makes a difference?

Josh, that is an interesting observation, thanks for the link!

lowell, it looks like clang follows the C99 standard mostly, so the output is expected, but it is a bit surprising it does not produce any warnings at all.

Comment on INT_MIN by lowell

lowell — Thu, 08 Sep 2011 01:34:47 +0000

WTF; NM. This was on HN so I thought it was new; didn’t notice the 2009 dates on the comments or the post itself.

Comment on INT_MIN by lowell

lowell — Thu, 08 Sep 2011 01:32:56 +0000

Clang in OS X Lion 10.7.2:

$ clang signed.c -o signed -Wall
$ ./signed
negative
equal
floating

Comment on INT_MIN by foolhunger

foolhunger — Thu, 08 Sep 2011 00:55:10 +0000

Great! I have saw this on CSAPP before, and the C99 output surprised me:)

Comment on INT_MIN by sdfghj

sdfghj — Wed, 07 Sep 2011 23:52:42 +0000

print(“%d\n”, -2147483648 < 0);
print("%d\n", (unsigned int)-2147483648 < 0);
print("%d\n", (int)-2147483648 < 0);
print("%d\n", (long)-2147483648 < 0);
print("%d\n", (unsigned long)-2147483648 < 0);
print("%d\n", -2147483647 – 1 < 0);

When compiled with Ken Thompson's C compiler for the 386, the above program prints the following output:

0
0
1
1
0
1

Comment on INT_MIN by Josh Haberman

Josh Haberman — Wed, 07 Sep 2011 23:49:38 +0000

This bit of asymmetry also leads to this interesting security implication: http://my.opera.com/taviso/blog/show.dml/639454

(In short, (INT_MIN / -1) can crash your program with an exception, since the result can’t be represented in an integer. It’s an easy way to crash a program that does integer math on untrusted input.

Comment on INT_MIN by Peter Hamilton

Peter Hamilton — Wed, 07 Sep 2011 23:19:19 +0000

Running on a Mac only prints negative, equal and floating.

melker:Programming peter$ gcc -v
Using built-in specs.
Target: i686-apple-darwin11
Configured with: /private/var/tmp/llvmgcc42/llvmgcc42-2335.15~25/src/configure –disable-checking –enable-werror –prefix=/Developer/usr/llvm-gcc-4.2 –mandir=/share/man –enable-languages=c,objc,c++,obj-c++ –program-prefix=llvm- –program-transform-name=/^[cg][^.-]*$/s/$/-4.2/ –with-slibdir=/usr/lib –build=i686-apple-darwin11 –enable-llvm=/private/var/tmp/llvmgcc42/llvmgcc42-2335.15~25/dst-llvmCore/Developer/usr/local –program-prefix=i686-apple-darwin11- –host=x86_64-apple-darwin11 –target=i686-apple-darwin11 –with-gxx-include-dir=/usr/include/c++/4.2.1
Thread model: posix
gcc version 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)
melker:Programming peter$ ./a.out
negative
equal
floating
melker:Programming peter$