Commit 47536317 authored by Peter Maydell's avatar Peter Maydell Committed by Blue Swirl

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: default avatarPeter Maydell <>
Signed-off-by: default avatarBlue Swirl <>
parent 288fa407
......@@ -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:
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)
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