File:  [Qemu by Fabrice Bellard] / qemu / HACKING
Revision (vendor branch): download - view: text, annotated - select for diffs
Tue Apr 24 18:34:19 2018 UTC (3 years, 1 month ago) by root
Branches: qemu, MAIN
CVS tags: qemu0150, qemu0141, qemu0140, HEAD
qemu 0.14.0

    1: 1. Preprocessor
    3: For variadic macros, stick with this C99-like syntax:
    5: #define DPRINTF(fmt, ...)                                       \
    6:     do { printf("IRQ: " fmt, ## __VA_ARGS__); } while (0)
    8: 2. C types
   10: It should be common sense to use the right type, but we have collected
   11: a few useful guidelines here.
   13: 2.1. Scalars
   15: If you're using "int" or "long", odds are good that there's a better type.
   16: If a variable is counting something, it should be declared with an
   17: unsigned type.
   19: If it's host memory-size related, size_t should be a good choice (use
   20: ssize_t only if required). Guest RAM memory offsets must use ram_addr_t,
   21: but only for RAM, it may not cover whole guest address space.
   23: If it's file-size related, use off_t.
   24: If it's file-offset related (i.e., signed), use off_t.
   25: If it's just counting small numbers use "unsigned int";
   26: (on all but oddball embedded systems, you can assume that that
   27: type is at least four bytes wide).
   29: In the event that you require a specific width, use a standard type
   30: like int32_t, uint32_t, uint64_t, etc.  The specific types are
   31: mandatory for VMState fields.
   33: Don't use Linux kernel internal types like u32, __u32 or __le32.
   35: Use target_phys_addr_t for guest physical addresses except pcibus_t
   36: for PCI addresses.  In addition, ram_addr_t is a QEMU internal address
   37: space that maps guest RAM physical addresses into an intermediate
   38: address space that can map to host virtual address spaces.  Generally
   39: speaking, the size of guest memory can always fit into ram_addr_t but
   40: it would not be correct to store an actual guest physical address in a
   41: ram_addr_t.
   43: Use target_ulong (or abi_ulong) for CPU virtual addresses, however
   44: devices should not need to use target_ulong.
   46: Of course, take all of the above with a grain of salt.  If you're about
   47: to use some system interface that requires a type like size_t, pid_t or
   48: off_t, use matching types for any corresponding variables.
   50: Also, if you try to use e.g., "unsigned int" as a type, and that
   51: conflicts with the signedness of a related variable, sometimes
   52: it's best just to use the *wrong* type, if "pulling the thread"
   53: and fixing all related variables would be too invasive.
   55: Finally, while using descriptive types is important, be careful not to
   56: go overboard.  If whatever you're doing causes warnings, or requires
   57: casts, then reconsider or ask for help.
   59: 2.2. Pointers
   61: Ensure that all of your pointers are "const-correct".
   62: Unless a pointer is used to modify the pointed-to storage,
   63: give it the "const" attribute.  That way, the reader knows
   64: up-front that this is a read-only pointer.  Perhaps more
   65: importantly, if we're diligent about this, when you see a non-const
   66: pointer, you're guaranteed that it is used to modify the storage
   67: it points to, or it is aliased to another pointer that is.
   69: 2.3. Typedefs
   70: Typedefs are used to eliminate the redundant 'struct' keyword.
   72: 2.4. Reserved namespaces in C and POSIX
   73: Underscore capital, double underscore, and underscore 't' suffixes should be
   74: avoided.
   76: 3. Low level memory management
   78: Use of the malloc/free/realloc/calloc/valloc/memalign/posix_memalign
   79: APIs is not allowed in the QEMU codebase. Instead of these routines,
   80: use the replacement qemu_malloc/qemu_mallocz/qemu_realloc/qemu_free or
   81: qemu_vmalloc/qemu_memalign/qemu_vfree APIs.
   83: Please note that NULL check for the qemu_malloc result is redundant and
   84: that qemu_malloc() call with zero size is not allowed.
   86: Memory allocated by qemu_vmalloc or qemu_memalign must be freed with
   87: qemu_vfree, since breaking this will cause problems on Win32 and user
   88: emulators.
   90: 4. String manipulation
   92: Do not use the strncpy function.  According to the man page, it does
   93: *not* guarantee a NULL-terminated buffer, which makes it extremely dangerous
   94: to use.  Instead, use functionally equivalent function:
   95: void pstrcpy(char *buf, int buf_size, const char *str)
   97: Don't use strcat because it can't check for buffer overflows, but:
   98: char *pstrcat(char *buf, int buf_size, const char *s)
  100: The same limitation exists with sprintf and vsprintf, so use snprintf and
  101: vsnprintf.
  103: QEMU provides other useful string functions:
  104: int strstart(const char *str, const char *val, const char **ptr)
  105: int stristart(const char *str, const char *val, const char **ptr)
  106: int qemu_strnlen(const char *s, int max_len)
  108: There are also replacement character processing macros for isxyz and toxyz,
  109: so instead of e.g. isalnum you should use qemu_isalnum.
  111: Because of the memory management rules, you must use qemu_strdup/qemu_strndup
  112: instead of plain strdup/strndup.
  114: 5. Printf-style functions
  116: Whenever you add a new printf-style function, i.e., one with a format
  117: string argument and following "..." in its prototype, be sure to use
  118: gcc's printf attribute directive in the prototype.
  120: This makes it so gcc's -Wformat and -Wformat-security options can do
  121: their jobs and cross-check format strings with the number and types
  122: of arguments.
  124: Currently many functions in QEMU are not following this rule but
  125: patches to add the attribute would be very much appreciated.