QEMU is not a pure C99 code-base as anyone attempting to build with
-std=c99 will find out.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
---
 HACKING | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/HACKING b/HACKING
index 0fc3e0fc04..e042dd1f4f 100644
--- a/HACKING
+++ b/HACKING
@@ -189,6 +189,11 @@ In addition, QEMU assumes that the compiler does not use the latitude
 given in C99 and C11 to treat aspects of signed '<<' as undefined, as
 documented in the GNU Compiler Collection manual starting at version 4.0.
 
+Finally QEMU also uses some GNU extensions to the C language including
+making extensive use of the typeof keyword for type inference. While
+typeof() is a GCC extension it is fairly widely understood by other
+compilers.
+
 7. Error handling and reporting
 
 7.1 Reporting errors to the human user
-- 
2.17.1