1. Preprocessor

For variadic macros, stick with this C99-like syntax:

#define DPRINTF(fmt, ...)                                       \
    do { printf("IRQ: " fmt, ## __VA_ARGS__); } while (0)

2. C types

It should be common sense to use the right type, but we have collected
a few useful guidelines here.

2.1. Scalars

If you're using "int" or "long", odds are good that there's a better type.
If a variable is counting something, it should be declared with an
unsigned type.

If it's host memory-size related, size_t should be a good choice (use
ssize_t only if required). Guest RAM memory offsets must use ram_addr_t,
but only for RAM, it may not cover whole guest address space.

If it's file-size related, use off_t.
If it's file-offset related (i.e., signed), use off_t.
If it's just counting small numbers use "unsigned int";
(on all but oddball embedded systems, you can assume that that
type is at least four bytes wide).

In the event that you require a specific width, use a standard type
like int32_t, uint32_t, uint64_t, etc.  The specific types are
mandatory for VMState fields.

Don't use Linux kernel internal types like u32, __u32 or __le32.

Use target_phys_addr_t for guest physical addresses except pcibus_t
for PCI addresses.  In addition, ram_addr_t is a QEMU internal address
space that maps guest RAM physical addresses into an intermediate
address space that can map to host virtual address spaces.  Generally
speaking, the size of guest memory can always fit into ram_addr_t but
it would not be correct to store an actual guest physical address in a
ram_addr_t.

Use target_ulong (or abi_ulong) for CPU virtual addresses, however
devices should not need to use target_ulong.

Of course, take all of the above with a grain of salt.  If you're about
to use some system interface that requires a type like size_t, pid_t or
off_t, use matching types for any corresponding variables.

Also, if you try to use e.g., "unsigned int" as a type, and that
conflicts with the signedness of a related variable, sometimes
it's best just to use the *wrong* type, if "pulling the thread"
and fixing all related variables would be too invasive.

Finally, while using descriptive types is important, be careful not to
go overboard.  If whatever you're doing causes warnings, or requires
casts, then reconsider or ask for help.

2.2. Pointers

Ensure that all of your pointers are "const-correct".
Unless a pointer is used to modify the pointed-to storage,
give it the "const" attribute.  That way, the reader knows
up-front that this is a read-only pointer.  Perhaps more
importantly, if we're diligent about this, when you see a non-const
pointer, you're guaranteed that it is used to modify the storage
it points to, or it is aliased to another pointer that is.

2.3. Typedefs
Typedefs are used to eliminate the redundant 'struct' keyword.

2.4. Reserved namespaces in C and POSIX
Underscore capital, double underscore, and underscore 't' suffixes should be
avoided.