HACKING

   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 hwaddr 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.  As mentioned in the man page, it does *not*
  95 guarantee a NULL-terminated buffer, which makes it extremely dangerous to use.
  96 It also zeros trailing destination bytes out to the specified length.  Instead,
  97 use this similar function when possible, but note its different signature:
  98 void pstrcpy(char *dest, int dest_buf_size, const char *src)
  99
 100 Don't use strcat because it can't check for buffer overflows, but:
 101 char *pstrcat(char *buf, int buf_size, const char *s)
 102
 103 The same limitation exists with sprintf and vsprintf, so use snprintf and
 104 vsnprintf.
 105
 106 QEMU provides other useful string functions:
 107 int strstart(const char *str, const char *val, const char **ptr)
 108 int stristart(const char *str, const char *val, const char **ptr)
 109 int qemu_strnlen(const char *s, int max_len)
 110
 111 There are also replacement character processing macros for isxyz and toxyz,
 112 so instead of e.g. isalnum you should use qemu_isalnum.
 113
 114 Because of the memory management rules, you must use g_strdup/g_strndup
 115 instead of plain strdup/strndup.
 116
 117 5. Printf-style functions
 118
 119 Whenever you add a new printf-style function, i.e., one with a format
 120 string argument and following "..." in its prototype, be sure to use
 121 gcc's printf attribute directive in the prototype.
 122
 123 This makes it so gcc's -Wformat and -Wformat-security options can do
 124 their jobs and cross-check format strings with the number and types
 125 of arguments.
 126
 127 6. C standard, implementation defined and undefined behaviors
 128
 129 C code in QEMU should be written to the C99 language specification. A copy
 130 of the final version of the C99 standard with corrigenda TC1, TC2, and TC3
 131 included, formatted as a draft, can be downloaded from:
 132  http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf
 133
 134 The C language specification defines regions of undefined behavior and
 135 implementation defined behavior (to give compiler authors enough leeway to
 136 produce better code).  In general, code in QEMU should follow the language
 137 specification and avoid both undefined and implementation defined
 138 constructs. ("It works fine on the gcc I tested it with" is not a valid
 139 argument...) However there are a few areas where we allow ourselves to
 140 assume certain behaviors because in practice all the platforms we care about
 141 behave in the same way and writing strictly conformant code would be
 142 painful. These are:
 143  * you may assume that integers are 2s complement representation
 144  * you may assume that right shift of a signed integer duplicates
 145    the sign bit (ie it is an arithmetic shift, not a logical shift)