I am not trying to have two different structs with the same name, but rather defining the same exact struct two different times (probably in different header files).
For example:
struct foo { int bar; }; struct foo { int bar; }; gives an error.
4 Answers
The way to do this is to surround your struct with preprocessor instructions
#ifndef STRUCT_FOO #define STRUCT_FOO struct foo { int bar; }; #endif /* STRUCT_FOO */ #ifndef STRUCT_FOO #define STRUCT_FOO struct foo { int bar; }; #endif /* STRUCT_FOO */ this has the effect of only defining struct foo once. In combination with the commonly accepted practice of putting such an item in a file called foo.h, like so
#ifndef INCLUDE_FOO_H #define INCLUDE_FOO_H struct foo { int bar; }; #endif /* INCLUDE_FOO_H */ it also protects against a person doing
#include "foo.h" #include "foo.h" (rest of code) As far as redefining the struct, that is not permitted in the C language; however, you can do some things that approximate a non-full redefine. I recommend avoiding them, as they tend to only make the code more obscure and difficult to maintain.
1 Comment
milanHrabos
I do not see the point. When you are just "worried" to not include the same struct in multiple places, then just one guard is sufficient. What is the point to have 2 guards for the same macro? The second one is redundant and can be omitted here. There is no way the second guard will process, so your example lack the intend of OP. Unless the OP asked about "how to protect against redefining" then "how to actually redefine struct".
No.
There is absolutely no point in doing this. If you have a structure that is to be used by multiple compilation units, put it in a .h header file, and #include it from those .c files. Or, if you need it in multiple header files, just include the common header file from those.
Now if you don't need the actual structure definition, but rather just need to declare that it exists (so you can create pointers to said struture), you can use a forward declaration:
struct foo; // defined elsewhere void somefunc(struct foo *ptr); answered Jun 15, 2015 at 0:44
Jonathon Reinhart
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1 Comment
Jonathon Reinhart
@whoKnows Well you never indicated what your use case was, so there's no way we could know.
Short answer: No
The compiler isn't that smart - you already have struct foo, so you can't have another struct foo even if you think it is the same as the first one.
Comments
No.
You should seperate the struct in another header, it sounds like you may be organizing your code poorly and should rethink the design.
You could use a ifndef:
#ifndef NAMEDEF struct name { int val; }; #define NAMEDEF #endif Although, I must reiterate that you need to rethink how your header files are designed and put this struct in a common header.
Its also possible to use a foreward declaration:
struct name; void function() { } 
3 Comments
too honest for this site
That would 1) result in problems if
name was already defined before as a macro or even another object. 2) it would defy any usage of the struct, as every occurence of name afterwards would be replaced by <nothing>. But your point about the headers might be worth to elaborate.
David Whiteside
I intended the define to use all caps to prevent a name collision with the struct. I just corrected this.
too honest for this site
It is not just caps, but, yes, that would work. It is called guard, btw. and the same as used for a header file. Still more of a hack, than a clean solution; that should simply go into a single header (which would then have its guard).
Cfrom your title since you already have a tag for that :)