From 475363176c80feedb8feb5e335ba64de68c7b055 Mon Sep 17 00:00:00 2001 From: Peter Maydell Date: Wed, 31 Oct 2012 09:30:47 +0000 Subject: HACKING: List areas where we may rely on impdef C behaviour Add a section to HACKING saying which version of the C spec we use and describing the bits of implementation defined C compiler behaviour which C code in QEMU is allowed to rely on. Signed-off-by: Peter Maydell Signed-off-by: Blue Swirl --- HACKING | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) (limited to 'HACKING') diff --git a/HACKING b/HACKING index 89a6b3ad44..6654d33249 100644 --- a/HACKING +++ b/HACKING @@ -123,3 +123,23 @@ gcc's printf attribute directive in the prototype. This makes it so gcc's -Wformat and -Wformat-security options can do their jobs and cross-check format strings with the number and types of arguments. + +6. C standard, implementation defined and undefined behaviors + +C code in QEMU should be written to the C99 language specification. A copy +of the final version of the C99 standard with corrigenda TC1, TC2, and TC3 +included, formatted as a draft, can be downloaded from: + http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf + +The C language specification defines regions of undefined behavior and +implementation defined behavior (to give compiler authors enough leeway to +produce better code). In general, code in QEMU should follow the language +specification and avoid both undefined and implementation defined +constructs. ("It works fine on the gcc I tested it with" is not a valid +argument...) However there are a few areas where we allow ourselves to +assume certain behaviors because in practice all the platforms we care about +behave in the same way and writing strictly conformant code would be +painful. These are: + * you may assume that integers are 2s complement representation + * you may assume that right shift of a signed integer duplicates + the sign bit (ie it is an arithmetic shift, not a logical shift) -- cgit v1.2.3