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.
Related
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.
I'm new to C and I came across this statement:
"Functions need to be declared in .h files and not defined with exception of inline functions".
My question is then, where are standard functions defined?
"Functions need to be declared in .h files and not defined with exception of inline functions".
You declare the existence of a function in the header. For all dependents to see.
You implement (define) the function in the source file. Dependents cannot see this.
The header file is the API or your C file for use elsewhere in your application, hiding the implementation.
The exception for inline functions has to do with the fact that when compiling the dependent, the compiler must know what to put instead of the function call.
Functions for (private) use only inside your source file do not need to be declared in the header file. Why tell other code about it?
My question is then where are standard functions defined?
In the compiled library. You only know that they exist, not where they exist. The implementation is hidden from you. The linker connects the dots.
Yes functions are declared in .h file and they are defined in .c file
"Functions need to be declared in .h files and not defined with exception of inline functions"
Unknown Source
Unfortunately, this quote is ambiguous and not entirely correct as well.
Functions do not necessary need to be declared in .h files. You can declare a function in .c files as well, or even omit the declaration entirely (dependent upon your coding style and the place of the function definition).
Note for the latter, that the compiler reads a file from top to bottom.
For example, This is the content of the source file foo.c:
void foo (void)
{
// absolute foo / translated: It really does not make anything useful.
// this function is only to illustrate the omission of the prototype.
}
int main (void)
{
foo();
}
To call foo() we need no declaration of foo() at all because the definition is before its use in the source code (compiler reads from top to bottom).
Would it be behind, we would need a declaration to tell the compiler what foo() is, f.e.:
void foo (void); // foo() needs to be declared. Else the compiler does not know
// what foo is when foo is called in main.
int main (void)
{
foo();
}
void foo (void)
{
// absolute foo.
}
It is just a common use to place the declaration/ prototype of a function in a separate header, but you actually do not need to do so. This is the header version:
foo.c:
#include "foo.h" // We need to include the header to gain the prototype.
// Note that `foo.h` needs to be in the same directory
// as `foo.c` in this case. Else you need to specify the
// full path to foo.h .
int main (void)
{
foo();
}
void foo (void)
{
// absolute foo.
}
foo.h:
void foo (void);
What the quote really means is:
A function definitely should not be defined in an .h file (as these are headers, not source files), with the only exception of inline functions, which indeed can be defined in .h files.
"My question is then, where are standard functions defined?"
That is a completely different question and implementation-specific. The functions are stored in an either fully or semi-compiled form. I can not answer it here without the focus to any specific implementation (OS, compiler).
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;
If a function declaration isn't in a header file (.h), but is instead only in a source file (.c), why would you need to use the static keyword? Surely, if you only declare it in a .c file, it isn't seen by other files, as you're not supposed to #include .c files, right?
I have already read quite a few questions and answers about this (eg. here and here), but can't quite get my head around it.
What static does is make it impossible to declare and call a function in other modules, whether through a header file or not.
Recall that header file inclusion in C is just textual substitution:
// bar.c
#include "header.h"
int bar()
{
return foo() + foo();
}
// header.h
int foo(void);
gets preprocessed to become
int foo(void);
int bar()
{
return foo() + foo();
}
In fact, you can do away with header.h and just write bar.c this way in the first place. Similarly, the definition for foo does not need to include the header in either case; including it just adds a check that the definition and declaration for foo are consistent.
But if you were to change the implementation of foo to
static int foo()
{
// whatever
return 42;
}
then the declaration of foo would cease to work, in modules and in header files (since header files just get substituted into modules). Or actually, the declaration still "works", but it stops referring to your foo function and the linker will complain about that when you try to call foo.
The main reason to use static is to prevent linker clashes: even if foo and bar were in the same module and nothing outside the module called foo, if it weren't static, it would still clash with any other non-static function called foo. The second reason is optimization: when a function is static, the compiler knows exactly which parts of the program call it and with what arguments, so it can perform constant-folding, dead code elimination and/or inlining.
The static keyword reduces the visibility of a function to the file scope. That means that you can't locally declare the function in other units and use it since the linker does not add it to the global symbol table. This also means that you can use the name in other units too (you may have a static void testOutput(); in every file, that is not possible if the static is omitted.)
As a rule of thumb you should keep the visibility of symbols as limited es possible. So if you do not need the routine outside (and it is not part of some interface) then keep it static.
It allows you to have functions with identical names in different source files, since the compiler adds an implicit prefix to the name of every static function (based on the name of the file in which the function is located), thus preventing multiple-definition linkage errors.
It helps whoever maintains the code to know that the function is not exposed as part of the interface, and is used only internally within the file (a non-static function can be used in other source files even if it's not declared in any header file, using the extern keyword).
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.