5 ms·

> #define sizeof(x) (size)sizeof(x) Technically, it's illegal to #define over a language keyword.

• chii 3y ago
  really? then how do you #define things like types like `int` and `char`?
  • antiquark 3y ago
    For example, it would be illegal to do the following: > #define int long Because you're replacing the int keyword with something else. The standard says: > 17.6.4.3.1 [macro.names] paragraph 2: A translation unit shall not #define or #undef names lexically identical to keywords, to the identifiers listed in Table 3, or to the attribute-tokens described in 7.6.
    • jenadine 3y ago
      That's C++. But I couldn't find the same restriction in C. In fact it seems that C allows it as long as you don't include any of the standard C header. > 7.1.2 "Standard headers" §5 [...] The program shall not have any macros with names lexically identical to keywords currently defined prior to the inclusion of the header or when any macro defined in the header is expanded.
      • Joker_vD 3y ago
        And so you still can re-define keywords, but only after you've included all the standard headers you want. Which makes sense: the meaning of including a standard header is entirely standard-mandated (they are not even required to be actual files) so making anything that could potentially mess with the implementation's implementation of standard headers UB is reasonable.
      • antiquark 3y ago
        Searching around gave me this from stackoverflow: In C standard, it is not allowed. > C11, § 6.4.1 Keywords > [...] > The above tokens (case sensitive) are reserved (in translation phases 7 and 8) for use as keywords, and shall not be used otherwise. https://stackoverflow.com/questions/12286691/keywords-redefinition-in-c-c https://stackoverflow.com/questions/12286691/keywords-redefi...
  • tom_ 3y ago
    You don't.
• oh_sigh 3y ago
  Also, `#define countof(a) (sizeof(a) / sizeof(*(a)))` is unsafe since the arg is evaluated twice.
  • david2ndaccount 3y ago
    Arguments to sizeof aren’t actually evaluated (except for variably-modified-types, AKA VLAs, but don’t use those).
    • uecker 3y ago
      Please use those. They are useful, make code clearer, improve bounds checking, ... Don't let attackers influence the size of a buffer (neither for VLAs nor for heap allocations).
  • badsectoracula 3y ago
    This is a very common macro to get static array lengths and i'm not sure there is any other way to do the same thing (i.e. give a static array, get back the number of items in it) in any other way.
  • f33d5173 3y ago
    The arg is expanded twice but evaluated zero times, since sizeof gives the size of its argument type without executing anything.
    • Izkata 3y ago
      countof(foo()) looks like foo() is only called once, but would actually be called twice. That's what GP is talking about, it's evaluated twice after the expansion when the code is actually running, not during the expansion.
      • deleted 3y ago
        [deleted]
      • uecker 3y ago
• wannacboatmovie 3y ago
  If you're dead set on doing this, the correct way would be to name the macro in all caps e.g. #define SIZEOF(x) as C is case sensitive. It is somewhat self-documenting to the next guy that SIZEOF() != sizeof().
  • loeg 3y ago
    Any name is fine, as long as it isn't literally "sizeof".
• mianos 3y ago
  Considering '#defines' are done in a textual pre-precessing by the C pre-processor, they don't know much at all about the C language. You can define out int, long, struct or anything. I have seen many people redefine 'for' and 'while'. These people often argue that it is an improvement. cpp #define sizeof(x) (size)sizeof(x) sizeof(UU) ^D # 1 "<stdin>" # 1 "<built-in>" # 1 "<command-line>" # 31 "<command-line>" # 1 "/usr/include/stdc-predef.h" 1 3 4 # 32 "<command-line>" 2 # 1 "<stdin>" (size)sizeof(UU)
  • orthoxerox 3y ago
    > I have seen many people redefine 'for' and 'while'. #define while if