multiple inclusions - Linking .h files with .c with #ifdef header guards

Question

im having trouble linking .h and .c files, i've also read some threads regarding this problem and all of them is a bit vague and still i can't fully grasp the concept of it, and im having a lot of linking problems, Say i have b.c and b.h which i will use in a.c, and im confused whether to include b.h both a.c and b.c cuz b.c itself needs to know the structure defined in b.h, i have some function which has its prototype in b.h and is defined in b.c which also use the structure in b.h, im am not including b.h in b.c cuz as what i know b.h is more like an interface to a.c which will use the functions in b.c... Here a more clear example

b.h file

typedef struct{
int x, y;
}myStruct;

void funct1(myStruct);
void funct2(myStruct);

b.c file

void funct1(myStruct x)
{
    //do something
}

void funct2(myStruct y)
{
     //do something
} 

a.c file

#include "b.h"

int main()
{
myStruct x;
  funct1(x);
  funct2(y);
return 0;
}

Executed the command in cygwin: gcc b.c a.c -g

Now the confusing part, i have a linking error wherein when b.c is compiled it can't detect the structure and the prototypes in b.h. Cuz all i know is that b.h is used to link b.c from a.c but when both .c is compiled it seems that b.c can't find its strucutre and prototypes,

Why didn't i include b.h in b.c? Answer: Cuz as what i know, b.h is already included in a.c and when i include it again in b.c, i'll be doing double inclusions <--- thats what i learn so far and i know there is #ifdef but it seems it won't work, maybe i still don't know how to use it, if you know please feel free to discuss this.

If you have any idea as to how to go about this feel free to tell me some.

there is a #ifdef directive but i can't seem to have any idea how to do this.

NOTE: ASSUME THAT ALL ABOVE CODES IS SYNTACTICALLY CORRECT if there are any misspelled word please ignore, i'm only after the inclusions between .h and .c

Answer 1

You do indeed need to #include b.h in b.c. Each file is compiled separately before the linker takes over, so it doesn't matter that you have included b.h in a.c, because b.c is compiled by itself and has no idea about the contents of b.h unless you include it.

Here's an example of a #include guard

// some_header_file.h
#ifndef SOME_HEADER_FILE_H
#define SOME_HEADER_FILE_H
// your code
#endif

When some_header_file.h is included anywhere, everything in between the #ifndef and the #endif will be ignored if SOME_HEADER_FILE_H has been defined, which will happen on the first time it is included in the compilation unit.

It is common practice to name the #define after the name of the file, to ensure uniqueness within your project. I like to prefix it with the name of my project or namespace as well, to reduce the risk of clashes with other code.

NOTE: The same header file CAN be included multiple times within your project even with the above include guard, it just can't be included twice within the same compilation unit. This is demonstrated as follows:

// header1.h
#ifndef HEADER_H
#define HEADER_H
int test1 = 1;
#endif

// header2.h
#ifndef HEADER_H
#define HEADER_H
int test2 = 2;
#endif

Now let's see what happens when we try to include the above two files. In a single compilation unit:

// a.cpp
#include "header1.h"
#include "header2.h"
#include <iostream>
int main()
{
   std::cout << test1;
   std::cout << test2;
};

This generates a compiler error because test2 is not defined - it is ignored in header2.h because HEADER_H is already defined by the time that is included. Now if we include each header in separate compilation units:

// a.cpp
#include "header2.h"
int getTest2()
{
   return test2;
};

// b.cpp
#include "header1.h"
#include <iostream>
int getTest2(); // forward declaration
int main()
{
   std::cout << test1;
   std::cout << getTest2();
};

It compiles fine and produces the expected output (1 and 2), even though we are including two files which both define HEADER_H.