C program extern

For more information about this use of extern , see Explicit instantiation. When the linker sees extern before a global variable declaration, it looks for the definition in another translation unit. Declarations of non- const variables at global scope are external by default.

Only apply extern to the declarations that don't provide the definition. A const global variable has internal linkage by default. If you want the variable to have external linkage, apply the extern keyword to the definition, and to all other declarations in other files:. In Visual Studio version Eventually the option will become the default. C functions and data can be accessed only if they're previously declared as having C linkage. However, they must be defined in a separately compiled translation unit.

If a function has more than one linkage specification, they must agree. Furthermore, if two declarations for a function occur in a program, one with a linkage specification and one without, the declaration with the linkage specification must be first. Improve Article. Save Article. Like Article. Take a step-up from those "Hello World" programs.

Learn to implement data structures like Heap, Stacks, Linked List and many more! Check out our Data Structures in C course to start learning today. Previous Understanding "register" keyword in C. Next Storage Classes in C. Recommended Articles. Article Contributed By :. Easy Normal Medium Hard Expert. Writing code in comment? Please use ide. We will just include the header file and directly use the variables.

You should also try it and move things around to get the idea. The sources from the examples above are availble on GitHub. I also prepared a zip file you. It contains the source and header files of the examples. You can download it, unzip and create projects with the source files: c-extern-examples. In conclusion, the extern keyword in C forces a declaration of a variable, that was defined elsewhere.

In practice it should be used in our header files and then we will include the necessary headers.