Re: [PATCH] C undefined behavior fix

From: Aaron Lehmann (
Date: Wed Jan 02 2002 - 06:28:21 EST

On Wed, Jan 02, 2002 at 01:03:25AM +0200, Momchil Velikov wrote:
> Thus
> strcpy (dst, "abcdef" + 2)
> gives
> memcpy (dst, "abcdef" + 2, 5)

IMHO gcc should not be touching these function calls, as they are not
made to a standard C library, and thus have different behaviors. I'm
suprised that gcc tries to optimize calls to these functions just
based on their names.

The gcc manpage mentions

           Assert that compilation takes place in a freestanding
           environment. This implies -fno-builtin. A freestand­
           ing environment is one in which the standard library
           may not exist, and program startup may not necessarily
           be at "main". The most obvious example is an OS ker­
           nel. This is equivalent to -fno-hosted.

Why is Linux not using this? It sounds very appropriate. The only
things the manpage mentions that -fno-builtin would inhibit from being
optimized are memcpy() and alloca(). memcpy() has its own assembly
optimization and inlining on some (most?) archs, and as for alloca(),
I only see it being used a bit in the S/390 code, where the gcc
optimizations could quite possibly break something. I think
-ffreestanding definately should be used by the kernel to prevent gcc
from messing with its code in broken ways.

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