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| -rw-r--r-- | HACKING | 20 |
1 files changed, 20 insertions, 0 deletions
@@ -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) |