What is the accepted standard way to define an exposed variable in C? Suppose the setup is the following:
In .h
typedef struct my_struct{
...
} my_struct;
extern my_struct var1;
In .c
my_struct var1;
Is this proper usage or is the compiler doing unnecessary work here? What is the extern actually doing here? My understanding has always been that everything already has an implicit extern by default.
There is a small difference between the implicit extern part when it comes to variables and functions.
If you put
void foo(void);
in a .h file and include the .h file in multiple .cc files, there is no harm since the function is not defined in the .h file. It's only declared.
If you put,
int x;
in the same .h file, then x is defined in every .c file that includes the .h file. You would get similar error for the function if the .h file had:
void foo(void){}
since that is a declaration as well as a definition.
To make a variable such as x to be only a declaration, you need to add the extern keyword.
My understanding has always been that everything already has an implicit extern by default.
This is true, however...
The purpose of explicitly externing var1 in the header is to:
Document your intentions, that var1 is not private.
Notify the compiler that source files that include the header are using a variable declared elsewhere.
Recommendation
Create getter/setter functions and make var1 private (static).
That's exactly right. You need to have the extern in the header in order to tell the compiler that var1 exists and what its type is when it's compiling code that uses it.
(I'm assuming that you are using var1 in more than one .c file, and one of them is defining it. If you're only using it in one .c file then there's no need for the declaration in the header if you define the variable before using it.)
Related
This question already has answers here:
How do I use extern to share variables between source files?
(19 answers)
Closed 6 years ago.
Hi I am a C++ developer now I am doing C programming.
My question is which place is better to declare global variable in c program.
Header or source file (provided my global variable is not used in other files)?
I want that variable like private variable in C++.
Assuming your variable is global and non static.
You need to declare it in a header file. We use extern keyword for this. As pointed out in comments, this keywords is not necessary, but most C users prefer to use it in headers, this is a convention.
stackoverflow.h:
#ifndef STACHOVERFLOW_H
#define STACHOVERFLOW_H
extern int my_var;
#ifndef
And you initialize it in source file. (Use of keyword extern is prohibited if you want to provide an initialization value).
stackoverflow.c
#include "stackoverflow.h"
int my_var = 50;
Do not put initialization value in a header, or you will get a linker error if the header is used at least twice.
Now you can use your variable in any other module by including the header.
main.c
#include <stdio.h>
#include "stackoverflow.h"
int main()
{
printf("my_var = %d\n", my_var);
return 0;
}
Including header "stackoverflow.h" in "stackoverflow.c" is a way to get sure definitions in source file match declarations in header file. This permit to have errors as soon as compilation instead of sometimes cryptic linker errors.
Edit: This is not at all the way to make a variable "private". You have to use a static variable to make it "private". See R Sahu's answer
which place is better to declare a global variable in c program
Answer: In source(*.c) file.
Assume the scenario like, I have declared a variable in a header file. I included this header in two different .c files. After the macro expansion step of compilation, these two files will have the global variable with the same name. So it will throw an error like multiple declarations of the variable during the linking time.
Conclusion:-
Keep all global variable declaration on .c file and put it as static if it is doesn't need in other files.
Add extern declaration of the variable in the corresponding header file if it's needed to access from other files
You should not place global non-constant variables anywhere. Global as in declared with extern and available to your whole project. The need to do this always originates from bad program design, period. This is true for C and C++ both.
The exception is const variables, which are perfectly fine to share across multiple files.
In the case you need file scope variables, they should be declared in the .c file and always as static. Don't confuse these for "globals" because they are local to the translation unit where they are declared. More info about how static file scope variables can make sense.
Also note the C standard future language directions:
Declaring an identifier with internal linkage at file scope without
the static storage class specifier is an obsolescent feature.
So if you don't use static your code might not compile in the next version of the C standard.
If you intend to use the global variables in multiple .c files, it is better to declare them in .h files. However, if you want to keep the variables like private member data of classes in C++, it will be better to provide access to the global data through functions.
Instead of
extern int foo;
Use
int getFoo();
void setFoo(int);
That sort of mimics the private access specifiers for member variables of classes.
Generally what you can do is define the variable in a source file, like int g_foo;, then reference this global in other files with extern, like extern int g_foo; do_sth(g_foo);. You could put the extern int g_foo; declaration in a header file, and include that in other source files. It's not recommend to have definitions of data in header files.
If you want it to be global (external linkage), you should put it in .h file. And this is one of the best practise, I think:
public_header.h
#ifdef YOUR_SOURCE
#define EXTERN
#else
#define EXTERN extern
#endif
EXTERN int global_var;
your_source.c
//your source makes definition for global_var
#define YOUR_SOURCE
#include <public_header.h>
other_source.c
#include <public_header.h> //other sources make declaration for global_var
If you want it to be private (internal linkage), the best solution, I think, is just make definition of it right in your source file instead of header file to prevent the header file is included by another source and then make confuse.
your_souce.c
static int private_var;
say I have 4 files.
file1.h
file1.c
file2.h
file2.c
in file1.h I have a variable :
static short mode=0;
in file1.c:
#include "file1.h"
...............
if ( mode ==0)
do stuff
else {
do something else
}
the problem I have is that I can only change mode value in file2.c
in file2.h I include file1.h and in the source file2.c, I change the value of to 1 but this has no influence on it in file1.c
When you write a line like
static short mode=0;
in a .h file and the .h file is included in multiple .c files, you effectively create multiple short mode variables, one in each .c file that ends up including the .h file.
If you want to have one variable, make it extern, make sure to define it in only one .c file.
in file1.h:
// Just the declaration in the .h file.
extern short mode;
in file1.c:
// The definition in only one .c file.
short mode = 0;
The whole point of having a variable declared as static is to reduce its scope and make it inaccessible to other files. This is known as private encapsulation and is good programming practice.
The opposite of private encapsulation is known as spaghetti coding, where you declare a variable as global, with the extern keyword. This is very bad programming practice (unless in some cases where the variable is declared const, which isn't the case here).
Under no circumstances should you attempt to rewrite good code based on private encapsulation into bad code based on spaghetti.
Also you should never define variables in header files, because that never makes any sense. A header file is just a description of the interface which is implemented in its corresponding c file, it should not implement anything (even though C allows one to do all kinds of stupid and crazy things).
What you should do is:
file1.h
short get_mode (void);
void set_mode (short m);
file1.c
#include "file1.h"
static short mode = 0;
short get_mode (void)
{
return mode;
}
void set_mode (short m)
{
mode = m;
}
some_other_file.c
#include "file1.h"
short mode = get_mode();
do_stuff_with(mode);
This is how you design programs properly, period. No matter which programming language that is used. Do not listen to anyone recommending extern or other such nonsense!
If you have static in .h file defined and including that .h file in .c files you are basically defining static variable in each .c file and they are all different objects.
You have to do
extern short mode;
in some .h file and include this .h in whichever .c file you need.
Once the variable is declared in some .h file you need to define it somewhere.. I mean in some .c file.
short mode;
Move the variable declaration to file1.c and get rid of static:
short mode=0;
Add an extern definition to file1.h:
extern short mode;
That is the semantics of static in this context: It creates a separate variable in each source file.
For a common variable which spans several source files you need extern declarations in each source file which wants to use it -- typically through a header --, and one definition in one of the source files.
On a general note, I can recommend the C FAQ (http://c-faq.com/). There is a section about declarations and definitions, http://c-faq.com/decl/decldef.html.
I'm working on an embedded c project and am having issues with global variable redefinition.
I've split up the function declarations and definitions into a handful of .h and .c files. Many of these functions need access to global variables, which are defined in global_vars.h. Therefore, at the beginning of the header files, inside the #ifndef, #define section, "global_vars.h". If I don't, as you might imagine I get a bunch of undefined variable errors.
However, even though global_vars.h has the #ifndef _GLOBAL_VARS_H_ #define... #endif, I get redefinition errors for all the global variables. My guess is that when the linker tries link the various object files, it sees the redefinition due to the #include "global_vars.h" in all the "blah_blah.h" files. It was my understanding, though, that the #ifndef... takes care of this issue.
Is there something I'm overlooking?
Thanks in advance
The linker never sees anything in the global_vars.h file, ever, unless -- bad news! -- some of the globals are actually defined in that file. global_vars.h should hold only declarations of those global variables, never (well, almost never) their definitions.
In global_vars.h, you should have declarations like:
extern int some_global;
You are not allowed to have:
int some_global;
If you have definitions in global_vars.h then, yes, they'll be multiply defined at link time because each of the .c files that #includes global_vars.h will have its own definition of each defined variable.
All of the definitions of the extern globals must be in some .c file, for sure. Usually it doesn't matter which .c file. Often all of the global-variable definitions are in a file called (surprise!) global_vars.c.
So make sure there aren't any global-variable definitions in global_vars.h and you'll be in good shape.
Is not a good idea to define globals in an H file. Better if you do that in a C or C++ file and you include and H file in other modules with those globals as externals.
Like this>>>
My module c file
unsigned short int AGLOBAL = 10; // definer and initializer
void MyFunc(void)
{
AGLOBAL+=1; // no need to include anything here cause is defined above
// more .....
}
My H file globals.h
// this is to include only once
#ifndef MYH
#define MYH
extern unsigned short int AGLOBAL; // no value in here!
#endif
Other module c file
#include globals.h
char SomeOtherFunc(void)
{
AGLOBAL+=10; // ok cause its defined by globals.h
// do more....
}
So let me start with saying that extern keyword applies to C variables (data objects) and C functions. Basically extern keyword extends the visibility of the C variables and C functions. Probably that’s is the reason why it was named as extern.
Use of extern with C functions. By default, the declaration and definition of a C function have “extern” prepended with them. It means even though we don’t use extern with the declaration/definition of C functions, it is present there.
For example, when we write.
int foo(int arg1, char arg2);
There’s an extern present in the beginning which is hidden and the compiler treats it as below.
extern int foo(int arg1, char arg2);
Same is the case with the definition of a C function (Definition of a C function means writing the body of the function). Therefore whenever we define a C function, an extern is present there in the beginning of the function definition. Since the declaration can be done any number of times and definition can be done only once, we can notice that declaration of a function can be added in several C/H files or in a single C/H file several times. But we notice the actual definition of the function only once (i.e. in one file only). And as the extern extends the visibility to the whole program, the functions can be used (called) anywhere in any of the files of the whole program provided the declaration of the function is known. (By knowing the declaration of the function, C compiler knows that the definition of the function exists and it goes ahead to compile the program).
So that’s all about extern with C functions.
Declaration can be done any number of times but definition only once.
“extern” keyword is used to extend the visibility of variables/functions().
Since functions are visible through out the program by default. The use of extern is not needed in function declaration/definition. Its use is redundant.
When extern is used with a variable, it’s only declared not defined.
As an exception, when an extern variable is declared with initialization, it is taken as definition of the variable as well.
This question already has answers here:
How do I use extern to share variables between source files?
(19 answers)
Closed 6 years ago.
beginner question about C declaration:
In a .c file, how to use variables defined in another .c file?
In fileA.c:
int myGlobal = 0;
In fileA.h
extern int myGlobal;
In fileB.c:
#include "fileA.h"
myGlobal = 1;
So this is how it works:
the variable lives in fileA.c
fileA.h tells the world that it exists, and what its type is (int)
fileB.c includes fileA.h so that the compiler knows about myGlobal before fileB.c tries to use it.
if the variable is :
int foo;
in the 2nd C file you declare:
extern int foo;
In 99.9% of all cases it is bad program design to share non-constant, global variables between files. There are very few cases when you actually need to do this: they are so rare that I cannot come up with any valid cases. Declarations of hardware registers perhaps.
In most of the cases, you should either use (possibly inlined) setter/getter functions ("public"), static variables at file scope ("private"), or incomplete type implementations ("private") instead.
In those few rare cases when you need to share a variable between files, do like this:
// file.h
extern int my_var;
// file.c
#include "file.h"
int my_var = something;
// main.c
#include "file.h"
use(my_var);
Never put any form of variable definition in a h-file.
Try to avoid globals. If you must use a global, see the other answers.
Pass it as an argument to a function.
Those other variables would have to be declared public (use extern, public is for C++), and you would have to include that .c file. However, I recommend creating appropriate .h files to define all of your variables.
For example, for hello.c, you would have a hello.h, and hello.h would store your variable definitions. Then another .c file, such as world.c would have this piece of code at the top:
#include "hello.h"
That will allow world.c to use variables that are defined in hello.h
It's slightly more complicated than that though. You may use < > to include library files found on your OS's path. As a beginner I would stick all of your files in the same folder and use the " " syntax.
The 2nd file needs to know about the existance of your variable. To do this you declare the variable again but use the keyword extern in front of it. This tells the compiler that the variable is available but declared somewhere else, thus prevent instanciating it (again, which would cause clashes when linking). While you can put the extern declaration in the C file itself it's common style to have an accompanying header (i.e. .h) file for each .c file that provides functions or variables to others which hold the extern declaration. This way you avoid copying the extern declaration, especially if it's used in multiple other files. The same applies for functions, though you don't need the keyword extern for them.
That way you would have at least three files: the source file that declares the variable, it's acompanying header that does the extern declaration and the second source file that #includes the header to gain access to the exported variable (or any other symbol exported in the header). Of course you need all source files (or the appropriate object files) when trying to link something like that, as the linker needs to resolve the symbol which is only possible if it actually exists in the files linked.
My question is about when a function should be referenced with the extern keyword in C.
I am failing to see when this should be used in practice. As I am writing a program all of the functions that I use are made available through the header files I have included. So why would it be useful to extern to get access to something that was not exposed in the header file?
I could be thinking about how extern works incorrectly, and if so please correct me.
Also.. Should you extern something when it is the default declaration without the keyword in a header file?
extern changes the linkage. With the keyword, the function / variable is assumed to be available somewhere else and the resolving is deferred to the linker.
There's a difference between extern on functions and on variables.
For variables it doesn't instantiate the variable itself, i.e. doesn't allocate any memory. This needs to be done somewhere else. Thus it's important if you want to import the variable from somewhere else.
For functions, this only tells the compiler that linkage is extern. As this is the default (you use the keyword static to indicate that a function is not bound using extern linkage) you don't need to use it explicitly.
extern tells the compiler that this data is defined somewhere and will be connected with the linker.
With the help of the responses here and talking to a few friends here is the practical example of a use of extern.
Example 1 - to show a pitfall:
stdio.h:
int errno;
myCFile1.c:
#include <stdio.h>
// Code using errno...
myCFile2.c:
#include <stdio.h>
// Code using errno...
If myCFile1.o and myCFile2.o are linked, each of the c files have separate copies of errno. This is a problem as the same errno is supposed to be available in all linked files.
Example 2 - The fix.
stdio.h:
extern int errno;
stdio.c:
int errno;
myCFile1.c:
#include <stdio.h>
// Code using errno...
myCFile2.c:
#include <stdio.h>
// Code using errno...
Now if both myCFile1.o and MyCFile2.o are linked by the linker they will both point to the same errno. Thus, solving the implementation with extern.
It has already been stated that the extern keyword is redundant for functions.
As for variables shared across compilation units, you should declare them in a header file with the extern keyword, then define them in a single source file, without the extern keyword. The single source file should be the one sharing the header file's name, for best practice.
Many years later, I discover this question. After reading every answer and comment, I thought I could clarify a few details... This could be useful for people who get here through Google search.
The question is specifically about using extern functions, so I will ignore the use of extern with global variables.
Let's define 3 function prototypes:
// --------------------------------------
// Filename: "my_project.H"
extern int function_1(void);
static int function_2(void);
int function_3(void);
The header file can be used by the main source code as follows:
// --------------------------------------
// Filename: "my_project.C"
#include "my_project.H"
void main(void) {
int v1 = function_1();
int v2 = function_2();
int v3 = function_3();
}
int function_2(void) return 1234;
In order to compile and link, we must define function_2 in the same source code file where we call that function. The two other functions could be defined in different source code *.C or they may be located in any binary file (*.OBJ, *.LIB, *.DLL), for which we may not have the source code.
Let's include again the header my_project.H in a different *.C file to understand better the difference. In the same project, we add the following file:
// --------------------------------------
// Filename: "my_big_project_splitted.C"
#include "my_project.H"
void old_main_test(void){
int v1 = function_1();
int v2 = function_2();
int v3 = function_3();
}
int function_2(void) return 5678;
int function_1(void) return 12;
int function_3(void) return 34;
Important features to notice:
When a function is defined as static in a header file, the compiler/linker must find an instance of a function with that name in each module which uses that include file.
A function which is part of the C library can be replaced in only one module by redefining a prototype with static only in that module. For example, replace any call to malloc and free to add memory leak detection feature.
The specifier extern is not really needed for functions. When static is not found, a function is always assumed to be extern.
However, extern is not the default for variables. Normally, any header file that defines variables to be visible across many modules needs to use extern. The only exception would be if a header file is guaranteed to be included from one and only one module.
Many project managers would then require that such variable be placed at the beginning of the module, not inside any header file. Some large projects, such as the video game emulator "Mame" even require that such variables appears only above the first function using them.
In C, extern is implied for function prototypes, as a prototype declares a function which is defined somewhere else. In other words, a function prototype has external linkage by default; using extern is fine, but is redundant.
(If static linkage is required, the function must be declared as static both in its prototype and function header, and these should normally both be in the same .c file).
A very good article that I came about the extern keyword, along with the examples: http://www.geeksforgeeks.org/understanding-extern-keyword-in-c/
Though I do not agree that using extern in function declarations is redundant. This is supposed to be a compiler setting. So I recommend using the extern in the function declarations when it is needed.
If each file in your program is first compiled to an object file, then the object files are linked together, you need extern. It tells the compiler "This function exists, but the code for it is somewhere else. Don't panic."
All declarations of functions and variables in header files should be extern.
Exceptions to this rule are inline functions defined in the header and variables which - although defined in the header - will have to be local to the translation unit (the source file the header gets included into): these should be static.
In source files, extern shouldn't be used for functions and variables defined in the file. Just prefix local definitions with static and do nothing for shared definitions - they'll be external symbols by default.
The only reason to use extern at all in a source file is to declare functions and variables which are defined in other source files and for which no header file is provided.
Declaring function prototypes extern is actually unnecessary. Some people dislike it because it will just waste space and function declarations already have a tendency to overflow line limits. Others like it because this way, functions and variables can be treated the same way.
Functions actually defined in other source files should only be declared in headers. In this case, you should use extern when declaring the prototype in a header.
Most of the time, your functions will be one of the following (more like a best practice):
static (normal functions that aren't
visible outside that .c file)
static inline (inlines from .c or .h
files)
extern (declaration in headers of the
next kind (see below))
[no keyword whatsoever] (normal
functions meant to be accessed using
extern declarations)
When you have that function defined on a different dll or lib, so that the compiler defers to the linker to find it. Typical case is when you are calling functions from the OS API.