extern function declaration in header file

Likewise, for C code to call a C++ function bar(), the C++ code for bar() must be declared with extern "C". How do I write a C header file that can be used in C++ programs? The question has been modified to reflect this. One With It extern . What if there were 20 of these variables and 50 more functions? Now, I'll modify the math.h header file created in the previous section to contain the declaration for the pi variable as follows: As you can see, the variable has been declared as an extern in the header file, which means this should be accessible anywhere in the program. Why would Henry want to close the breach? Can virent/viret mean "green" in an adjectival sense? Now this header file can be included to C source files that need to access this variable: #include "vars.h". The code for the math.cpp file is as follows: This file contains the definition for the previously declared sum function and it returns the sum of the given parameters as an integer. Stan program file header files that function. It reminds the readers that they are extern, and since humans are more fallible than computers, I find the reminder helps. Its use is implicit. /*Once the following function is specified as a low priority interrupt service routine, the system willWhen entering this function, it will automatically protect the scene, and automatically restore the scene before exiting. Hi, I have put my global variables in the main source file for the library. And how is it going to affect C++ programming? This function declarations do functions before using external with declaring it will be. Its use is implicit. Although there are other ways of doing it, the clean, reliable way to declare and define global variables is to use a header file file3.h to contain an extern declaration of the variable. Is this an at-all realistic configuration for a DHC-2 Beaver? Why would someone declare some variable or function as extern in a header file that can otherwise be accessed directly!! And we can do this declaration as many times as we want. In the case of functions, the extern keyword is used implicitly. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, @user827992: I'm pretty sure you are talking about. In general, if you have to write something twice, you are doing something wrong. Declare them in a header file and define them in the corresponding source file: C++ // Header file a.h // A global variable that can be accessed from other modules. There's such thing as extern "C" in C++, but that is a completely different matter. The keyword [ extern "C" ] is used to declare functions in C++ which is implemented and compiled in C language. Personally, I prefer to see the explicit keyword there - but the compiler doesn't need it. Is it correct to say "The glue on the back of the sticker is dying down so I can not stick the sticker to the wall"? Asking for help, clarification, or responding to other answers. Variable is declared of course in some *.c file in project. Thanks for the careful edit, @LokiAstari. toString() methods A Swift 4 function can be as simple as a simple C function to as complex as an If the value of the string can't be represented as an int, 0 is returned So, to convert a time by a given number, you need to divide the number of hours by 24 to get . Not sure if it was just me or something she sent to the whole team. To use the C++ named C library need explicitly make it visible with using directive using namespace std; If the source code is in C++ but not C, to prevent C/ C++ compiler compile it in . How do I use extern to share variables between source files? As an exception, when an extern variable is declared with initialization, it is taken as the definition of the variable as well. Answer (1 of 5): Yes it can. That's because (in the c89 standard) if your function was not declared before it's being used (i.e. storage class specifier (or both), If the header file defines a function using a macro, declare it as though it were an ordinary . Declaring a variable simply declares the existence of the variable to the program. Would it be possible, given current technology, ten years, and an infinite amount of money, to construct a 7,000 foot (2200 meter) aircraft carrier? What is the difference between #include and #include "filename"? You can compile this using: gcc first. Create your own Header File: Instead of writing a large and complex code, we can create your own header files and include them in our program to use it whenever we want. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Why did the Council of Elrond debate hiding or sending the Ring away, if Sauron wins eventually in that scenario? The extern keyword in C and C++ extends the visibility of variables and functions across multiple source files. Where does the idea of selling dragon parts come from? Counterexamples to differentiation under integral sign, revisited, Looking for a function that can squeeze matrices. How do I use extern to share variables between source files? It will be connected when program and initialized data definitions cannot have external functions inside a header file in function declaration. In header files of a C library, should one declare functions: In a C++ program, the functions are declared as functions returning no value and taking no arguments. Is there any reason on passenger airliners not to have a physical lock between throttles? Making static When we write extern some_data_type some_variable_name; no memory is allocated. In C++, C library header file is always the C name prefixed with the letter c in which the .h file suffix has been dropped, for example <cassert> and <cstdlib>. In *.h header files of a C library, should one declare functions. Should I give a brutally honest feedback on course evaluations? A plain extern is redundant on a function declaration. A common practice for header files to maintain both C and C++ compatibility is to make its declaration be extern "C" for the scope of the header: If you have GCC installed on your system you may follow along. How to pass a 2D array as a parameter in C? I think a lot probably depends on how the declarations in the .h file are created, and how they relate to the main .c file. In your example, you didn't need to write. exception from the above in C, which is probably not directly related to what you are asking about: in C language (C99) if in some translation unit a function is defined as inline and also declared as extern (an explicit extern is used) then the inline definition of that function also serves as an external definition. By using our site, you Thou shalt not functional behaviour of extern structs, or go in header file, and gives a compatibility statement is fairly readable. Ready to optimize your JavaScript with Rust? Not repeating yourself is not a small thing. Why is this usage of "I've to work" so awkward? It's a bit fiddly, but not too hard. Thanks for contributing an answer to Stack Overflow! We accomplish this by creating thousands of videos, articles, and interactive coding lessons - all freely available to the public. (I'd made a mild error in the last sentence, which is why it is now last edited by me once more. Maybe you should add that nifty define '#ifdef _CPLUSPLUS' (or whatever your compiler uses to differ between CPP and C-mode). However, I use it in headers to emphasize that it is a declaration of an externally defined function, and for symmetry with those (rare) occasions when there is a global variable declared in the header. Therefore, any C++ file that contains "test1.h" will have internal linkage of the variable one - Mutating Algorithm the same as that of the visible The "extern" attribute is the default linkage (i.e, in the absence of static or extern). How do I tell if this single climbing rope is still safe for use? Even though it's not used that often, the extern keyword in C/C++ is undoubtedly one of the most important concept to understand. What if you wanted to change int to uint32_t? To learn more, see our tips on writing great answers. In this context, functions differ clearly from variables, but that's a different matter. Are there good reasons to use the `extern` linkage specifier in new code? MOSFET is getting very hot at high frequency PWM. If the function is declared in a header, it does not need to be declared again. But this isnt the case with variables. Till the next one, stay safe and keep learning. // my_program.cpp #include "my_class.h" using namespace N; int main() { my_class mc; mc.do_something(); return 0; } Otherwise, Stack Overflow is always there to help. I usually start by typing in the .h file prototypes, and then copy/paste to the .c file and add the function body (striking the semicolon at the end of the prototype), so "extern" would require have to be added to the header file or struck from the main .c file after the copy/paste. Image Processing: Algorithm Improvement for 'Coca-Cola Can' Recognition. One of the biggest uses of a header file is to share function declarations across C modules, as well. The compiler throws a warning for every such function declared but not called in the source file. Why did the Council of Elrond debate hiding or sending the Ring away, if Sauron wins eventually in that scenario? You can then refer to the C functions by qualifying them with the name of the module. I'm trying to include some C code in my .cl code to call from the kernel. Why is apparent power not measured in Watts? Extern can be used access variables across C files. How to deallocate memory without using free() in C? Since the extern keyword extends the functions visibility to the whole program, the function can be used (called) anywhere in any of the files of the whole program, provided those files contain a declaration of the function. In file2 we declare the variable callCount. What declarations should be placed in a C header file? Better way to check if an element only exists in one array. Sorry for the confusion. Are there conservative socialists in the US? So I'll quickly setup a simple C++ program for demonstration. What is this fallacy: Perfection is impossible, therefore imperfection should be overlooked. Why is Singapore considered to be a dictatorial regime and a multi-party democracy at the same time? Not the answer you're looking for? Use static to declare a variable has two functions: (1) For local variables to be declared with static . Get started, freeCodeCamp is a donor-supported tax-exempt 501(c)(3) nonprofit organization (United States Federal Tax Identification Number: 82-0779546). It enhances code functionality and readability. union template for header file in function declaration declares that the other source file. The header is included by the one source file that defines the variable and by all the source files that reference the variable. Presumably it will be provided by the linker, program loader, or some part of the operating system. When extern is used with a variable, it's only declared, not defined. extern with Functions In the example, I have two C++ files named main.cpp and math.cpp and a header file named math.h. Different compilation results not using extern in C vs in C++. The strange habit of declaring functions in header files with extern probably has some historical roots, but it has been completely irrelevant for decades already. Personally, I prefer to see the explicit keyword there - but the compiler doesn't need it. Is it required to add 'extern C' in source file also? from the header file test1.h I get no warning or error during compilation and execution. As you continue to use the keyword in your programs, you'll definitely come across problems and situations that are outside the scope of this article. Below are the steps to create our own header file: Write your own C/C++ code and save that file with ".h" extension. Not the answer you're looking for? freeCodeCamp's open source curriculum has helped more than 40,000 people get jobs as developers. There are two types of header files: the files that the programmer writes and the files that comes with your compiler. You can #include the source file that implements your template class ( TestTemp.cpp) in your header file that defines the template class ( TestTemp.h ), and remove the source file from the project, not from the folder. Whenever a compiler sees a prototype, it assumes a function is defined somewhere else (in the current or another translation unit). Could you please help me a little bit more? It reminds the readers that they are extern, and since humans are more fallible than computers, I find the reminder helps. Guitar. So thats all about extern and functions.Now lets consider the use of extern with variables. You should always declare your C functions in header files. Not the answer you're looking for? What is the difference between const int*, const int * const, and int const *? Why can templates only be implemented in the header file? CGAC2022 Day 10: Help Santa sort presents! By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Pardon me if this sounds a question that has been asked many times but I assure you this is a little different. Can virent/viret mean "green" in an adjectival sense? Program (s) are built with this: Cl.BuildProgram (program, 1, new [] { device . Method 3. A GNU C model. file scope, the resulting linkage is Thats probably the reason why it was named extern.Though most people probably understand the difference between the declaration and the definition of a variable or function, for the sake of completeness, I would like to clarify them. Appealing a verdict due to the lawyers being incompetent and or failing to follow instructions? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. beginners) as it will be to those who are well-versed in C. So let me start by saying that the extern keyword applies to C variables (data objects) and C functions. Notice var is never used so no problems arise.Example 3: This program throws an error in the compilation(during the linking phase, more info here) because var is declared but not defined anywhere. Why are some functions declared extern and header file not included in source in Git source code? If you also affects shifts and leave it extern function declaration in header file, they are helpful. Is there a higher analog of "category with all same side inverses is a groupoid"? You can declare the function as extern int sum(int a, int b) instead but this will only cause redundancy. Difference Between malloc() and calloc() with Examples, Dynamic Memory Allocation in C using malloc(), calloc(), free() and realloc(). If you try to compile this program at this point, the compilation process will fail. This holds for both of the languages. The macros would normally be defined in one general-purpose header that's used everywhere, and then the particular header would ensure that the general purpose header is included and then use the appropriate form of the macro. Thanks much. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. "extern" is necessary when you need to directly share global variables among multiple source files (modules). If there is already a visible declaration of that identifier with Functions have external linkage by default But here is something i tried and now I am confused. The extern keyword means "declare without defining". ETC Blog Posts EDI Chapel and the C function it calls. extern tells the compiler to assume that there is a function or variable that it has not created. // In the header file, declare the variable // MyGlobal.h extern int a; // In exactly one source file, define the variable // MyGlobal.cpp int a; // or with an initializer int a = 42; // Other source files may include the header and use the variable // OtherSource.cpp #include "MyGlobal.h" void IncrementGlobal () { ++a; } Igor Tandetnik rev2022.12.9.43105. But there are global variables that are used in the functions I'm moving to the second source file. Declare variables on main. Find centralized, trusted content and collaborate around the technologies you use most. the extern keyword is used to extend the visibility of variables/functions. The strange habit of declaring functions in header files with extern probably has some historical roots, but it has been completely irrelevant for decades already. Finally, the code for the main.cpp file is as follows: This file includes the math.h header file containing the declaration for the sum function. If you put them in header files, you will get multiple different definitions of the same function, which can all be accessed from different translation units. Why do American universities have so many general education courses? Therefore, we need to include the extern keyword explicitly when we want to declare variables without defining them. What happens if you score more than 99 points in volleyball? What are the default values of static variables in C? An identifier with external linkage can be seen and used both from the file in which it is defined, and from other code files (via a forward declaration). Declaration in a header is better, because it ensures that all users are using the same prototype. Therefore, the declaration does not allocate storage space, only when it is defined. to force a definition. Should functions be made extern in header files? The latter is perfectly fine, since it's only a function definition, which tells those, who include this header: 'There's a function with this prototype somewhere around here'. Why is this usage of "I've to work" so awkward? Maybe you need to put some code in a function, write a macro or typedef something. Without headers, you would have to copy-paste the following code in every file that wanted to do I/O: Providing function declarations in a header will at least protect you from stupid errors like passing arguments of incompatible types. The following thread has some useful comments in general about extern. ( extern "C", on the other hand, is used to denote a function with no C++ name decoration.) Essentially, the var isnt allocated any memory. This is not the case with automatic variables. How is the merkle root verified if the mempools may be different? It didnt matter with my simple program but it surely would if we deal with like 100 files. Did the apostolic or early church fathers acknowledge Papal infallibility? Tricky, but the normal rule would that you should declare the functions to C++ code as extern "C". So far so good. Is there a higher analog of "category with all same side inverses is a groupoid"? The above code works with and without commenting out the declarations in the header file. This is an error. Users can also convert plain english data File to Hex by uploading the file. Is this an at-all realistic configuration for a DHC-2 Beaver. Why does my stock Samsung Galaxy phone/tablet lack some features compared to other Samsung Galaxy models? Specifying extern in function prototype has no effect, since it is assumed by default. When an extern variable is initialized, then memory for this is allocated and it will be considered defined. You can define the variable anywhere in the program but I chose the math.cpp file for definition to prove the point that this extern variable indeed is available to all the other source files as well. Well, here comes another surprise from C standards. When we write. Yes, which is why you shouldn't put them in header files. If you see the "cross", you're on the right track. Declare the variable extern in the header file: extern int global_int;, then define it and optionally initialize it in one and only one source file: int global_int = 17;. P.S. Note that the .c file should also use the header and so the standard pattern looks like: // file.h extern int x; // declaration // file.c #include "file.h" int x = 1; // definition and re-declaration Making statements based on opinion; back them up with references or personal experience. How to smoothen the round border of a created buffer to make it look more natural? (updated) I prefer to omit the extern in headers and use the rule: if it is static it is. Affidavit. The question actually concerns forward declarations of functions, not "extern functions". a : b; } But with variables, you have to use the keyword explicitly. Should teachers encourage good students to help weaker ones? You can also put function prototypes in header files, whose names end with .h, and then include them with stdio.h as shown in the listings here. Appropriate translation of "puer territus pedes nudos aspicit"? Connect and share knowledge within a single location that is structured and easy to search. You define a variable as follows: You can declare a variable as many times as you want, but you can define a variable only once. Received a 'behavior reminder' from manager. To create a function prototype: Create a new program named functionprototype.m. Does not in function from the compiler adds the declarations for the problem when building c language is encountered in the keyword at compile. Only property of variable is announced. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? Exactly. All the compiler needs to know is that my_class is a class that has a public member function called do_something(). Our mission: to help people learn to code for free. Making statements based on opinion; back them up with references or personal experience. Is there a verb meaning depthify (getting more depth)? What is this fallacy: Perfection is impossible, therefore imperfection should be overlooked. Finally, header can be considered as an external API for your module. But variables defined in a .h header and then #included in multiple .c files will need to be declared extern. C++ header file, Extern, declaration & definition-analysis. Consequently, for symmetry with the (very few) global variables declared in headers, I use extern with the function too - even though it is strictly not necessary. Lego Wii. You must differentiate between definitions and code (.c files), and declarations (in .h files). How to correctly use the extern keyword in C. Why are #ifndef and #define used in C++ header files? Why should I use a pointer rather than the object itself? I want to split the source file and put group of functions in another source file. You would do something like this: Here, an integer type variable called var has been declared (it hasnt been defined yet, so no memory allocation for var so far). called), the function is assumed to be declared as: extern int func(); where func is actually your function name. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. This variable is now a global that you can use in any source file by declaring it extern, for example, by including the header file. In the example, I have two C++ files named main.cpp and math.cpp and a header file named math.h. And the program is trying to change the value to 10 of a variable that doesnt exist at all.Example 4: Assuming that somefile.h contains the definition of var, this program will compile successfully.Example 5: Do you think this program will work? Its value is not retained between function calls. I have problem with compiling project when i declare this variable as an extern in header file, but if i declare this in each *.c file as an extern it compiles without errors! But what if the variables are initialized ? You can avoid this with having access functions but these come at a cost of course. Is there any reason on passenger airliners not to have a physical lock between throttles? Here is what I heard someone mention: The functions are declared static as an optimization. So it looks like you're going to have to go into the code generation templates. Before using, you have to include the header file into your program and you can do this with the help of a preprocessor directive called #include. In C and in C++ all functions have external linkage by default. C header files are a way to share global pointers, macros (#define ), common structure types declared as uninstatated structures or typedefs. What this means is that the function cannot be called from other .c files (Yes, best practice says the declaration should go in a header but for simplicity lets assume no header) Thirdly, by way of example, let's see the file called mixedFoo.c below: Code: rev2022.12.9.43105. I'm using C# with OpenCL.NetCore source code is located in a folder named cl. Functions declared in headers are normally (unless you work really hard) extern. No, functions declared in header files do not need to be declared extern. Surely, duplicating definitions is tedious and error-prone. And Hell; Schema Database; By Mail. To begin with, how would you declare a variable without defining it? Is it appropriate to ignore emails from a student asking obvious questions? From looking at the symbols in the linker map file found that the SIZE operator This article explains how to create a new memory section and place a function in a specific memory region using the ARM GCC linker script.The easiest way to fix this would be to modify F28M35x_generic_M3_FLASH.cmd to rename the C0 memory region to C03SRAM and all . I believe a simple code example can explain things better in some cases than a wall of text. Why is the federal judiciary of the United States divided into circuits? To get the 'no arguments' meaning in C, use one of: The same notation also means the same thing in C++, though for pure C++ code, using void in the argument list is not idiomatic (do not do it in pure C++ code). Do you want to write extern int one in all of them? Let's take a look at stdio.h for example. Find centralized, trusted content and collaborate around the technologies you use most. The extern keyword has four meanings depending on the context: First, static specifier when used on global variables limits the variable's scope to the source file in which it is defined. Use extern inline in a common header and provide a definition in a .c file somewhere, perhaps using macros to ensure that the same code is used in each case. Code for the math.h file is as follows: int sum (int a, int b); As you can see, the header file contains the declaration for a simple function called sum that takes two integers as parameters. . It is done by assigning an initialization value to a variable. How do I use extern to share variables between source files? This is because you can not allocate memory to the same variable multiple times. How to declare a structure in a header that is to be used by multiple files in c? We also have thousands of freeCodeCamp study groups around the world. Here is the sample code: Example 2:. I hope you've understood how the keyword works at a basic level from this short article. There's [almost] never any need to use the keyword extern when declaring a function, either in C or in C++. The extern keyword is used to share variables across translation units. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. There's one [obscure?] Improve INSERT-per-second performance of SQLite. 2. I used to declare global variable, which are used by many *.c files, in header file with 'extern'. extern void show(); Ready to optimize your JavaScript with Rust? extern inline functions can be access in more than one translation units. Basically, the extern keyword extends the visibility of the C variables and C functions. How can I use a VPN to access a Russian website that is banned in the EU? extern int one; Its like useless here. Functions declared in headers are normally (unless you work really hard) extern. declaration; otherwise the result is external linkage. How to dynamically allocate a 2D array in C? used when a particular files need to access a variable from another file. I use Codeblocks for C programming and lately I have started to wonder why would someone use a header file in C. I understand it is used for declaring and/or defining variables structures. Asking for help, clarification, or responding to other answers. Examples of frauds discovered because someone tried to mimic a random sequence. We #include the header file so that the compiler pulls in the declaration. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Then inside the main function, the std::cout << sum(10, 8) << std::endl; statement calls the sum functions by passing 10 and 8 as the two parameters and prints out whatever the returned value is. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, Advantage of using extern in a header file. Since functions are visible throughout the program by default, the use of extern is not needed in function declarations or definitions. Ready to optimize your JavaScript with Rust? Effects of the extern keyword on C functions. Thanks I am starting to get it. the extern keyword is used to extend the visibility of variables/functions. My question: Am i doing something wrong coz header doesnt seem to be of any use? rev2022.12.9.43105. In other words, it is a way to explicitly declare a variable, or to force a declaration without a definition. When extern is used with a variable, its only declared, not defined. Otherwise, I'll include outputs from each code snippet with them for you to read through. extern C tells the compiler that, in addition, it should use the C function call method, rather than the C++ function call method. Is there any reason for declaring functions as static in a header file if that header file is going to be included in several other files? Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. When would I give a checkpoint to my D&D party that they can return to if they die? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. When I can directly access int one and void show() from headertest.c because of them having external linkage implicitly then whats the use of Header file here? (Remember the basic principle that you cant have two locations of the same variable or function).Now back to the extern keyword. A header file is a file with extension .h which contains C function declarations and macro definitions to be shared between several source files. Penrose diagram of hypothetical astrophysical white hole. To my mind, having extern before variables but not functions makes it more visually obvious which things are functions and which things are variables (possibly including function pointers). To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Extern variable says to compiler go outside my scope and you will find the definition of the variable that I declared.. For example: extern int incr (int); extern int add (int a, int b) { return a+b; } Second, static and extern specifiers are mutually exclusive therefore decl. It is also possible to explicitly define a variable, i.e. Donations to freeCodeCamp go toward our education initiatives, and help pay for servers, services, and staff. A-143, 9th Floor, Sovereign Corporate Tower, We use cookies to ensure you have the best browsing experience on our website. Thanks for contributing an answer to Stack Overflow! You can make a tax-deductible donation here. We will just include the header file and directly use . What to do ? Personally, I prefer to see the explicit keyword there - but the compiler doesn't need it. because the purpose of a header is to document your API, your types, enums, and public functions, and make it readable to a human. Difference between intrinsic, inline, external in embedded system? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. It has no memory allocation at all. extern means that this variable is defined elsewhere and we will use that variable - we will not create a new variable here.. It specifies that the symbol has external linkage. Imagine you have a hundred files that use this global variable (one). C++11 introduced a standardized memory model. In C, we use header files for all declarations. Find centralized, trusted content and collaborate around the technologies you use most. These variables are defined outside the function and are available globally throughout the function execution. Understanding volatile qualifier in C | Set 2 (Examples). Make sure though, that you do not include the function body, unless you declare it 'inline' or as part of a class definition (C++) or as a 'template function' (also C++). now C++11 add new semantic meaning for extern keyword, like. Declaration must include any qualifiers used in the definition. BIG EDIT: Okay, I goofed. Now, create a header file (say vars.h) and declare the IOstate as: extern volatile int IOstate; // declaration of IOstate. Im sure this post will be as interesting and informative to C virgins (i.e. If any identifier (static or extern) is used before it is declared, that will generate an error, but that's a file scope compilation error, remedied by a forward declaration (with or without static or extern). Multiple declarations of extern variable is allowed within the file. Thank you /u/raevnos. Consider a simple program which calls a function foo(). The extern keyword may be applied to a global variable, function, or template declaration. Why is this usage of "I've to work" so awkward? I see how the question got to be as it was, and your edit to my answer made sense. Linker throws an error when it finds no such variable exists. Code for the math.h file is as follows: As you can see, the header file contains the declaration for a simple function called sum that takes two integers as parameters. One can briefly take a look at a header to understand how to use it without dealing with an implementation (which sometimes is not available at all). You should declare it as extern in a header file, and define it in exactly 1 .c file. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. In C and in C++ all functions have external linkage by default. As global . Should functions be made "extern" in header files? C compilers that use the so-called "def/ref" model get indigestion at link time when . There are some details implicit in that wording, reduce maintenance, so we can do the . Since this is a definition, the memory for var is also allocated. People can, and do, disagree over this; I go with the local rules but when I'm the rule-maker, the extern is included in a header. Moreover, headers are used not only for function declarations. Now if you try to compile this program you'll see it compiles without any problem and upon executing the resultant binary file, you'll see following output in the console: This works (even though the definition of the sum function is in a separate file than main.cpp) because all the functions in C/C++ are declared as extern. 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