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 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.

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 replacement qemu_malloc/qemu_mallocz/qemu_realloc/qemu_free or

qemu_vmalloc/qemu_memalign/qemu_vfree APIs.

Please note that NULL check for the qemu_malloc result is redundant and

that qemu_malloc() call with zero size is not allowed.

Memory allocated by qemu_vmalloc or qemu_memalign must be freed with

qemu_vfree, since breaking this will cause problems on Win32 and user

emulators.

4. String manipulation

Do not use the strncpy function.  According to the man page, it does

*not* guarantee a NULL-terminated buffer, which makes it extremely dangerous

to use.  Instead, use functionally equivalent function:

void pstrcpy(char *buf, int buf_size, const char *str)

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 qemu_strdup/qemu_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.