C - extern, static, include - c

Question

Scoping in C is confusing as hell. I have a variable: "int qwe". This var should be visible in one or more files - f1.c in this case, but not the another f2.c .
Say i have: main.c, f1.c, f2.c, header.h
main:
call f1();
call f2();
header:
#ifndef HEADER_INCLUDED
#define HEADER_INCLUDED
int qwe = 1;
void f1();
void f2();
#endif // HEADER_INCLUDED
f1.c:
#include <stdio.h>
extern int qwe;
void f1(){
printf("In f1: %d\n", qwe);
}
f2.c:
#include <stdio.h>
static int qwe = 2;
void f2(){
printf("In f2: %d\n", qwe);
}
Now this gets confusing. There is definition and declaration. I have defined qwe in the header, declared it in f1.c. Is that correct? Should definition happen in header and declaration in f1.c instead? I tried the latter case, but got an error - "multiple definition of qwe". When i removed the #include directive from f1.c, it worked... It also works when i remove the extern keyword. Is extern redundant?
f2.c i guess it's ok and behaves as expected, or is it? But if i put an #include with header, it breaks. Why is that?
When should i use #include in source files? If i don't include it f1.c or f2.c it works...
Also, if i define a variable as static inside a function, like static int i = 0; This variable will not be destroyed when function exist, it will keep it's memory and data. And next time that same function gets called, will have access to it, right? But the var won't be reinitialized to 0, i.e. the line where is defines won't execute. Correct?
Life stomps me :(

Answer 1

In the header, declare the variable. In e.g f1.c, define the variable, e.g. int qwe = 1; // at global scope.
In all files that want to see/change qwe, include the header that declares the variable. Easy-peasy.

Answer 2

You need to declare the variable in the header, and define it in one and only one C file.
In C, you cannot have a variable that doesn't "belong" to a given translation unit (source file). So it must be defined by exactly one translation unit in the whole program.
When you declare a variable as extern, you're telling the compiler the symbol is (possibly) external to your translation unit (c file).
It's probably also worth noting that when you try to declare a variable without extern, the variable is also defined, e.g.:
/* Declares, but does not defines external symbol 'foo' */
extern int foo;
/* Both declares AND defines bar */
int bar;
This is also different from how functions work to where the "default" syntax for declaration does NOT define a function:
/* Declare, but don't define spam */
void spam(void);
/* Declare, but don't define eggs */
extern void eggs(void);
/* Declare & define 'cheese' */
void cheese(void){ return; }
So your example should look more like this:
qwe.h:
#ifndef QWE_H
#define QWE_H
/* Declare qwe here */
extern int qwe;
#endif
f1.c:
/* DEFINE qwe here */
int qwe = 1;
f2.c:
#include "qwe.h" /*header includes the `extern int qwe` declaration */
void my_function(void)
{
/* use external symbol here! */
qwe = 10;
}

Answer 3

The scoping system isn't that confusing. The rule is this:
if you define something in a .c file EVERY other .c file in your program can access it (it is put in the global namespace).
if you specify static in front of its definition then only things in the SAME file can see it. This should be your default position for all functions and variables you don't want other .c files to be able to access.
It is very important to remember that extern only tells the compiler that the variable/function in question is not defined in the current file. This prevents the compiler for otherwise issuing an error because it can't find the symbol, but it has nothing to do with scoping - your .c file sees everything in the global namespace and if you have not followed the second part of the rule you will quickly find out about this at link time.
Header files similarly have nothing to do with scoping. They are just convenient places to put a bunch of extern statements and macros.

Answer 4

In C, you should use the header file in general to declare the data but not define the data. You don't want to define global data in a header because it will then be redundantly defined in multiple modules. The header indicates to multiple modules the existence of some data or function somewhere and what its type is, as well as common constants and macros (#defines). Outside of that, things in C are very simple. Just about anything is, technically, global unless you declare it static, keeping it scoped to the module it's defined in. The extern declarations for data in the headers and the function prototypes help the compiler know that these items are being accessed by a particular module and what the data types are for access so that the correct code can be generated.
You have:
Two functions f1 and f2 that are defined in separate modules but used in main. So these need to be declared in a header file.
A global data item qwe being used in more than one module.
A static qwe used in one module.
Assuming you want this done with one header file (you might need separate ones for f1, f2, and global data - see below), you could set up
your header as:
#ifndef MY_HEADER_H
#define MY_HEADER_H
extern int qwe;
void f1(void);
void f2(void);
#endif // MY_HEADER_H
Then in your main.c:
...
#include "my_header.h"
int qwe; // This is global and can be accessed from other modules
void main(...)
{
// call f1
// call f2
...
I just defined the global variable, qwe, in main.c arbitrarily. If you have a few globals, you can define them in their own glob_data.c module, for example, and have it's own header, glob_data.h, to declare them. Any other module that needs to access them would include the glob_data.h header so that compilation can properly be done on that module to access that data. Keeping the global data in separate headers helps with cases like you have where you have a static instance of the data versus the global, which are in conflict. You can avoid including the data header file for that global item when you want to compile with the static item.
Then in your C file, f1.c:
...system headers included...
#include "my_header.h"
void f1() {
printf("In f1: %d\n", qwe);
}
And in f2.c:
...system headers included...
#include "my_header.h" // Only if it doesn't contain `extern int qwe;`
static int qwe = 2; // This hides the global qwe and is known only
// to f2.c
void f2(){
printf("In f2: %d\n", qwe);
}
As I mentioned above, you might want to separate your function prototypes and your global data declaration in separate headers. That way, you can include only what's needed in each module and avoid conflicts, such as when you have a static int qwe; versus the global int qwe;.

Answer 5

I have defined qwe in the header, declared it in f1.c. Is that correct?
No, it should be the other way around. You are supposed to have a definition of a global variable in a single translation unit (that's geek speak for a .C file) but you may declare it in as many translation units as you wish. Since headers potentially get included from many translation units, declarations go in the headers.
When should i use #include in source files?
You do it when the header has anything that is needed for your translation unit to compile, with very few exceptions. Note that in some situations it may be necessary or desirable to make a forward declaration manually without including the header.
Also, if I define a variable as static inside a function, like static int i = 0; This variable will not be destroyed when function exist, it will keep it's memory and data.
That is correct, the static variable inside the function will be assigned the initial value only once, and retain the value that you assign to it for as long as your program continues to run.