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authorPeter Maydell <peter.maydell@linaro.org>2012-10-31 09:30:47 +0000
committerBlue Swirl <blauwirbel@gmail.com>2012-12-08 14:27:40 +0000
commit475363176c80feedb8feb5e335ba64de68c7b055 (patch)
tree41ac70d06db692789870336fb90873b77ab4c19a /HACKING
parent288fa40736e6eb63132d01aa6dc21ee831b796ae (diff)
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 <peter.maydell@linaro.org> Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
Diffstat (limited to 'HACKING')
-rw-r--r--HACKING20
1 files changed, 20 insertions, 0 deletions
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)