Re: Newbie question

After further review:

On 2005-12-17, Flash Gordon <spam@xxxxxxxxxxxxxxxxxx> wrote:
> Longfellow wrote:

<snip>
>> What I've done is declare a struct in a header file, which is included
>> in the files for both ends. For example:
>
> Declaring structs, typedefs, function prototypes etc yes, defining
> objects and function no.
>
>> -------
>> /* --- data.h --- */
>> struct data {
>> char question[80];
>> char answer[80];
>> } calc[80];
>> -------
>
> Here you are not merely defining the struct, you are also defining an
> object called calc which is an array of structs.

Not defining, just initializing. This declaration is made only once in
the header file and not again referenced in the code. It is assumed
that every file that has that header included should know what it needs
to know about that array of structs.

>> This works fine. My question is this: Is this legal and conforming?
>
> No, absolutely not. Any object that you use must be defined EXACTLY
> once, and because data.h is included in more than one source file any
> objects it defines are defined multiple times.

Prohibition is against declaring an object more than once, I think. H&S
says on page 79 that one of the two exceptions to this rule is when an
object is declared as the same type every time, as in a header file that
can be shared with all code files. You are correct that an inclusion of
the header file in each code file is a duplicate declaration, but this
exception exists because otherwise the standard C library would be
illegal.

> You want something like:
> /* --- data.h --- */
> #define CALC_SIZE 80
>
> struct data {
> char question[80];
> char answer[80];
> };
> extern struct data calc[CALC_SIZE];

This does not compile.

> Then, in EXACTLY one C source file (not header) which includes data.h
> you have
> struct data calc[CALC_SIZE];

See above.

>> I get from K&R2, page 82, that functions can indeed be declared in
>> header files. Do I understand this correctly?
>
> Yes, but only if you understand the difference between a declaration
> (which says basically this thing exists somewhere, but I'm not defining
> it here) and a definition (which says OK, this is the actual
> object/function right here so allocate the space/put the code here).

Apparently an initialized declaration at the top level is considered a
legal defining declaration [H&S pp113-116].

For my purposes, having this array of structs globally available via a
header file serves my purpose. Where I don't need it, the header file
will not be included, and I'm going to make the variable names unique
enough so that they will not be duplicated elsewhere.

In any case, it compiles cleanly with -Wall -ansi -pedantic. I've no
idea how this setup will fare as I flesh it all out, but I think I'll
drag out lclint and use it continuously. That way, I'll know that I'm
staying out of trouble.

Thanks anyway,

Longfellow

.

Relevant Pages

  • Re: Newbie question
    ... >>> What I've done is declare a struct in a header file, ... >> Declaring structs, typedefs, function prototypes etc yes, defining ... >the header file and not again referenced in the code. ... >the header file in each code file is a duplicate declaration, ...
    (comp.lang.c)
  • Re: Error C2259 while building application on 64-bit
    ... build to the header file you use in the 64-bit build. ... 'HRESULT IPersistFolder::Initialize': is ... declaration of 'IShellFolder::ParseDisplayName' ... while compiling class template member function 'HRESULT ...
    (microsoft.public.vc.mfc)
  • Re: A question about One Definition Rule
    ... >> I just realized I might have broken the rule by defining the exception ... >> classes in the header file. ... Or do you mean a class declaration, ... I'm aware that a declaration can also be a definiton (as in int x; ...
    (comp.lang.cpp)
  • Re: External structs - newbie question
    ... The header file for csd.c contains the typedef for a ... There is only one struct in use ... Declaring it in a header file is the right way to go. ... If you put the keyword extern before the declaration, it will not be a problem declaring it twice. ...
    (comp.lang.c)
  • Re: Register my own messages
    ... include the header file except in the modules that actually use it. ... I defined int CODERSOURCEMSG in a header file appart. ... the declaration merely declares its type and name so that it can be used. ... extern const int UWM_MYMSG; ...
    (microsoft.public.vc.mfc)