HACKING - qemu - Git at Google

 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.

 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 GLib memory allocation routines g_malloc/g_malloc0/g_new/
 g_new0/g_realloc/g_free or QEMU's qemu_vmalloc/qemu_memalign/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_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.  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)
	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.

	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 GLib memory allocation routines g_malloc/g_malloc0/g_new/
	g_new0/g_realloc/g_free or QEMU's qemu_vmalloc/qemu_memalign/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_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. 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)