On Sat, 8 Mar 2008 12:21:09 -0600 (CST), ericshufro@[EMAIL PROTECTED]
wrote in
comp.lang.c.moderated:
> Although we have scanned the ANSI-C do***ent, we cannot determine
> whether "prototyping" structures, including a struct's name beyond its
> tag name, is ANSI-C & most likely sup****ted by ANSI-C compliant
> compilers.
The rule is really fairly simple. A structure type can be declared
without defining it. Without the definition, it is an incomplete
type. You can define pointers to incomplete types.
> Similar to prototyping functions, the reason we ask this question is
> to determine whether we can resolve circular definition situations
> that occur when ordering header files which have data type definitions
> each dependent on the other.
>
>
> Example Scenario :
>
>
> #include <types.h>
> #include <header_a.h>
> #include <header_b.h>
>
>
> In 'types.h' :
>
> typedef struct some_tag_name SOME_STRUCT_NAME;
Note that the typedef above is not actually necessary to do what you
want. But it does serve to tell the compiler that the program might
use a type named "struct some_tag_name", which has an alias of
"SOME_STRUCT_NAME", and if it does, a definition of the type will be
available when needed.
> In 'header_a.h' :
>
> typedef struct some_other_struct {
> ...
> /* Allowed because of prior prototype? : */
> SOME_STRUCT_NAME *psome_struct;
Again, you could do this without the typedef, and indeed even without
a forward declaration at all. You could just code:
struct some_tag_name *psome_struct;
That would also tell the compiler that there will be a structure type
named "some_tag_name", and that is all the compiler needs to know to
define an object that can hold a pointer to such a struct.
> ...
> } SOME_OTHER_STRUCT;
>
>
> In 'header_b.h' :
>
> typedef struct some_tag_name {
> ...
> } SOME_STRUCT_NAME;
>
> If SOME_STRUCT_NAME can be "prototyped" in a header PRIOR to
> 'header_a.h', then that structure name can be referenced in other
> header files PRIOR to its actual data type definition.
The C standard requires that all pointers to structure types have the
same size, alignment, and representation. That means that the
compiler does not need to know any details about a structure type to
know that create a pointer to it.
This is valid C code:
struct x; /* forward declaration */
struct y; /* forward declaration */
struct x { int a; struct y *yp; };
struct y { int c; struct x *xp; };
It is also valid without the first two lines, the forward
declarations. You can define a pointer to a previously unseen
function type. The appearance of a previously undefined structure
tag, with the struct keyword, serves as a forward declaration of the
type all by itself.
> We have tried this method on one compiler so
> far & it compiles & builds without additional
> warnings or errors. However, we'd like some
> input before we consider using it in released
> software.
>
> If this was a fully ANSI-C compliant method
> to resolve circular data type definitions, it
> would be an immense improvement to our code
> base since it would allow us to remove many
> instances of 'void *' definitions that we
> believe had to be used in order to avoid the
> circular definition problem.
>
>
>
> Thank you for your time & expertise,
--
Jack Klein
Home: http://JK-Technology.Com
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