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extern.md

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In C and C++, the extern keyword is used to declare a global variable or function in another file. Here's a comprehensive overview:

Definition and Usage

  1. Global Variable Declaration:

    • extern is used to declare a global variable or function in another file.
    • It's a way of informing the compiler that the variable or function is defined elsewhere.

  2. Function Declaration:

    • When used with functions, extern specifies that the function is defined in another file.

Syntax

  1. Variable Declaration:

    • extern int var;
    • This line tells the compiler that the variable var is an integer and is defined in some other file.

  2. Function Declaration:

    • extern int func(int, int);
    • Declares a function func which is defined in another file.

Default Behavior

  1. In C:

    • Functions are extern by default.
    • When you declare a function without any storage class, it is assumed to be extern.

  2. In C++:

    • Same as C, functions are extern by default.

Usage in Multi-file Projects

  1. Declaring Global Variables:

    • Used to declare variables in header files which are defined in source files.

  2. Sharing Functions Across Files:

    • Functions defined in one file can be used in other files via extern declarations.

Linkage

  1. External Linkage:

    • Variables and functions with extern have external linkage, meaning they can be used in other files.

  2. Opposite Keyword - static:

    • While extern extends the visibility to other files, static restricts the visibility within the file.

Examples

  1. Variable Example:

    • File1.c: int var = 10;
    • File2.c: extern int var; (Access var from File1.c)

  2. Function Example:

    • File1.c: void foo() { /* definition */ }
    • File2.c: extern void foo(); (Access foo from File1.c)

Common Practices

  1. Header Files:

    • Declaring extern variables and functions in header files is common to ensure that all source files which include the header are aware of these declarations.

  2. Preventing Multiple Definitions:

    • Helps in managing large code bases where functions and variables are spread across multiple files.

Limitations and Considerations

  1. Definition Required:

    • extern declares but does not define. So, there must be a definition in some other file.

  2. Linker Errors:

    • If the extern variable or function is not defined in any of the linked files, it will lead to linker errors.

  3. Initialization:

    • extern can't be used to initialize variables. It's only for declaration.

Best Practices

  1. Avoid Global Variables:

    • Use extern sparingly for variables. Prefer function parameters or other methods to share data.

  2. Clear Module Interfaces:

    • Use extern to clearly define module interfaces in header files, making the code easier to read and maintain.

C vs. C++

  • Usage Similarity:

    • The usage in C and C++ is mostly similar, with differences primarily in language features and not in the concept of extern.

  • Name Mangling in C++:

    • In C++, extern "C" is used to prevent C++ name mangling when calling C functions.

extern "C" in C++

  • Purpose:
    • Used to tell C++ compiler to use C-style naming and linkage for the specified code.
  • Common Use:
    • When including C headers in C++ programs or calling C functions from C++.

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