File:  [Qemu by Fabrice Bellard] / qemu / HACKING
Revision 1.1.1.4 (vendor branch): download - view: text, annotated - select for diffs
Tue Apr 24 19:35:01 2018 UTC (23 months, 2 weeks ago) by root
Branches: qemu, MAIN
CVS tags: qemu1101, HEAD
qemu 1.1.1

    1: 1. Preprocessor
    2: 
    3: For variadic macros, stick with this C99-like syntax:
    4: 
    5: #define DPRINTF(fmt, ...)                                       \
    6:     do { printf("IRQ: " fmt, ## __VA_ARGS__); } while (0)
    7: 
    8: 2. C types
    9: 
   10: It should be common sense to use the right type, but we have collected
   11: a few useful guidelines here.
   12: 
   13: 2.1. Scalars
   14: 
   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.
   18: 
   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.
   22: 
   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).
   28: 
   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.
   32: 
   33: Don't use Linux kernel internal types like u32, __u32 or __le32.
   34: 
   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.
   42: 
   43: Use target_ulong (or abi_ulong) for CPU virtual addresses, however
   44: devices should not need to use target_ulong.
   45: 
   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.
   49: 
   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.
   54: 
   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.
   58: 
   59: 2.2. Pointers
   60: 
   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.
   68: 
   69: 2.3. Typedefs
   70: Typedefs are used to eliminate the redundant 'struct' keyword.
   71: 
   72: 2.4. Reserved namespaces in C and POSIX
   73: Underscore capital, double underscore, and underscore 't' suffixes should be
   74: avoided.
   75: 
   76: 3. Low level memory management
   77: 
   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 GLib memory allocation routines g_malloc/g_malloc0/g_new/
   81: g_new0/g_realloc/g_free or QEMU's qemu_vmalloc/qemu_memalign/qemu_vfree
   82: APIs.
   83: 
   84: Please note that g_malloc will exit on allocation failure, so there
   85: is no need to test for failure (as you would have to with malloc).
   86: Calling g_malloc with a zero size is valid and will return NULL.
   87: 
   88: Memory allocated by qemu_vmalloc or qemu_memalign must be freed with
   89: qemu_vfree, since breaking this will cause problems on Win32 and user
   90: emulators.
   91: 
   92: 4. String manipulation
   93: 
   94: Do not use the strncpy function.  According to the man page, it does
   95: *not* guarantee a NULL-terminated buffer, which makes it extremely dangerous
   96: to use.  Instead, use functionally equivalent function:
   97: void pstrcpy(char *buf, int buf_size, const char *str)
   98: 
   99: Don't use strcat because it can't check for buffer overflows, but:
  100: char *pstrcat(char *buf, int buf_size, const char *s)
  101: 
  102: The same limitation exists with sprintf and vsprintf, so use snprintf and
  103: vsnprintf.
  104: 
  105: QEMU provides other useful string functions:
  106: int strstart(const char *str, const char *val, const char **ptr)
  107: int stristart(const char *str, const char *val, const char **ptr)
  108: int qemu_strnlen(const char *s, int max_len)
  109: 
  110: There are also replacement character processing macros for isxyz and toxyz,
  111: so instead of e.g. isalnum you should use qemu_isalnum.
  112: 
  113: Because of the memory management rules, you must use g_strdup/g_strndup
  114: instead of plain strdup/strndup.
  115: 
  116: 5. Printf-style functions
  117: 
  118: Whenever you add a new printf-style function, i.e., one with a format
  119: string argument and following "..." in its prototype, be sure to use
  120: gcc's printf attribute directive in the prototype.
  121: 
  122: This makes it so gcc's -Wformat and -Wformat-security options can do
  123: their jobs and cross-check format strings with the number and types
  124: of arguments.

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