a.h:
#ifndef A_H
#define A_H
struct a
{
int i;
struct b
{
int j;
}
};
#endif
there you go, now you just need to include a.h to the files where you want to use this structure.
a.h:
#ifndef A_H
#define A_H
struct a
{
int i;
struct b
{
int j;
}
};
#endif
there you go, now you just need to include a.h to the files where you want to use this structure.
For a structure definition that is to be used across more than one source file, you should definitely put it in a header file. Then include that header file in any source file that needs the structure.
The extern
declaration is not used for structure definitions, but is instead used for variable declarations (that is, some data value with a structure type that you have defined). If you want to use the same variable across more than one source file, declare it as extern
in a header file like:
extern struct a myAValue;
Then, in one source file, define the actual variable:
struct a myAValue;
If you forget to do this or accidentally define it in two source files, the linker will let you know about this.
When a type is used in a file (i.e. func.c file), it must be visible. The very worst way to do it is copy paste it in each source file needed it.
The right way is putting it in an header file, and include this header file whenever needed.
This is the solution I like more, because it makes the code highly modular. I would code your struct as:
#ifndef SOME_HEADER_GUARD_WITH_UNIQUE_NAME
#define SOME_HEADER_GUARD_WITH_UNIQUE_NAME
struct a
{
int i;
struct b
{
int j;
}
};
#endif
I would put functions using this structure in the same header (the function that are "semantically" part of its "interface").
And usually, I could name the file after the structure name, and use that name again to choose the header guards defines.
If you need to declare a function using a pointer to the struct, you won't need the full struct definition. A simple forward declaration like:
struct a ;
Will be enough, and it decreases coupling.
This is another way, easier somewhat, but less modular: Some code needing only your structure to work would still have to include all types.
In C++, this could lead to interesting complication, but this is out of topic (no C++ tag), so I won't elaborate.
I fail to see the point, perhaps, but Greg Hewgill has a very good answer in his post http://stackoverflow.com/questions/228684/how-to-declare-a-structure-in-a-header-that-is-to-be-used-by-multiple-files-in-c#228691.
The reason being that C struct managing can be a pain: You have to declare the struct keyword everywhere it is used:
struct MyStruct ; /* Forward declaration */
struct MyStruct
{
/* etc. */
} ;
void doSomething(struct MyStruct * p) /* parameter */
{
struct MyStruct a ; /* variable */
/* etc */
}
While a typedef will enable you to write it without the struct keyword.
struct MyStructTag ; /* Forward declaration */
typedef struct MyStructTag
{
/* etc. */
} MyStruct ;
void doSomething(MyStruct * p) /* parameter */
{
MyStruct a ; /* variable */
/* etc */
}
It is important you still keep a name for the struct. Writing:
typedef struct
{
/* etc. */
} MyStruct ;
will just create an anonymous struct with a typedef-ed name, and you won't be able to forward-declare it. So keep to the following format:
typedef struct MyStructTag
{
/* etc. */
} MyStruct ;
Thus, you'll be able to use MyStruct everywhere you want to avoid adding the struct keyword, and still use MyStructTag when a typedef won't work (i.e. forward declaration)
Corrected wrong assumption about C99 struct declaration, as rightfully remarked by Jonathan Leffler.