Archipelago » qemu

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)