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_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)