path: root/HACKING

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 hwaddr 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.

For CPU virtual addresses there are several possible types.
vaddr is the best type to use to hold a CPU virtual address in
target-independent code. It is guaranteed to be large enough to hold a
virtual address for any target, and it does not change size from target
to target. It is always unsigned.
target_ulong is a type the size of a virtual address on the CPU; this means
it may be 32 or 64 bits depending on which target is being built. It should
therefore be used only in target-specific code, and in some
performance-critical built-per-target core code such as the TLB code.
There is also a signed version, target_long.
abi_ulong is for the *-user targets, and represents a type the size of
'void *' in that target's ABI. (This may not be the same as the size of a
full CPU virtual address in the case of target ABIs which use 32 bit pointers
on 64 bit CPUs, like sparc32plus.) Definitions of structures that must match
the target's ABI must use this type for anything that on the target is defined
to be an 'unsigned long' or a pointer type.
There is also a signed version, abi_long.

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.

3. Low level memory management

Use of the malloc/free/realloc/calloc/valloc/memalign/posix_memalign
APIs is not allowed in the QEMU codebase. Instead of these routines,
use the GLib memory allocation routines g_malloc/g_malloc0/g_new/
g_new0/g_realloc/g_free or QEMU's qemu_memalign/qemu_blockalign/qemu_vfree
APIs.

Please note that g_malloc will exit on allocation failure, so there
is no need to test for failure (as you would have to with malloc).
Calling g_malloc with a zero size is valid and will return NULL.

Memory allocated by qemu_memalign or qemu_blockalign must be freed with
qemu_vfree, since breaking this will cause problems on Win32.

4. String manipulation

Do not use the strncpy function.  As mentioned in the man page, it does *not*
guarantee a NULL-terminated buffer, which makes it extremely dangerous to use.
It also zeros trailing destination bytes out to the specified length.  Instead,
use this similar function when possible, but note its different signature:
void pstrcpy(char *dest, int dest_buf_size, const char *src)

Don't use strcat because it can't check for buffer overflows, but:
char *pstrcat(char *buf, int buf_size, const char *s)

The same limitation exists with sprintf and vsprintf, so use snprintf and
vsnprintf.

QEMU provides other useful string functions:
int strstart(const char *str, const char *val, const char **ptr)
int stristart(const char *str, const char *val, const char **ptr)
int qemu_strnlen(const char *s, int max_len)

There are also replacement character processing macros for isxyz and toxyz,
so instead of e.g. isalnum you should use qemu_isalnum.

Because of the memory management rules, you must use g_strdup/g_strndup
instead of plain strdup/strndup.

5. Printf-style functions

Whenever you add a new printf-style function, i.e., one with a format
string argument and following "..." in its prototype, be sure to use
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)

7. Error handling and reporting

7.1 Reporting errors to the human user

Do not use printf(), fprintf() or monitor_printf().  Instead, use
error_report() or error_vreport() from error-report.h.  This ensures the
error is reported in the right place (current monitor or stderr), and in
a uniform format.

Use error_printf() & friends to print additional information.

error_report() prints the current location.  In certain common cases
like command line parsing, the current location is tracked
automatically.  To manipulate it manually, use the loc_*() from
error-report.h.

7.2 Propagating errors

An error can't always be reported to the user right where it's detected,
but often needs to be propagated up the call chain to a place that can
handle it.  This can be done in various ways.

The most flexible one is Error objects.  See error.h for usage
information.

Use the simplest suitable method to communicate success / failure to
callers.  Stick to common methods: non-negative on success / -1 on
error, non-negative / -errno, non-null / null, or Error objects.

Example: when a function returns a non-null pointer on success, and it
can fail only in one way (as far as the caller is concerned), returning
null on failure is just fine, and certainly simpler and a lot easier on
the eyes than propagating an Error object through an Error ** parameter.

Example: when a function's callers need to report details on failure
only the function really knows, use Error **, and set suitable errors.

Do not report an error to the user when you're also returning an error
for somebody else to handle.  Leave the reporting to the place that
consumes the error returned.

7.3 Handling errors

Calling exit() is fine when handling configuration errors during
startup.  It's problematic during normal operation.  In particular,
monitor commands should never exit().

Do not call exit() or abort() to handle an error that can be triggered
by the guest (e.g., some unimplemented corner case in guest code
translation or device emulation).  Guests should not be able to
terminate QEMU.

Note that &error_fatal is just another way to exit(1), and &error_abort
is just another way to abort().