External linkage is an important concept within programming languages that allows multiple files to be linked together to form a complete program. In C++, the "extern" keyword is used to specify external linkage for a variable or function.

When a variable or function is declared as "extern," it means that it is being explicitly defined in another file and can be accessed by other files within the same program. This is particularly useful when multiple files need to share the same data or functionality.

For example, consider a program that includes a header file containing function declarations. To ensure that these functions can be accessed by other files in the program, they would need to be defined as "extern" in the header file. When these functions are later implemented in a separate source file, they can be linked together using the "extern" keyword, allowing them to be shared across the entire program.

It is important to note that external linkage should be used sparingly and only when necessary. Excessive use of external linkage can make a program more difficult to understand, maintain, and debug. Additionally, it can introduce potential issues with naming conflicts or unexpected behavior.

In summary, the "extern" keyword is an important concept in C++ that enables external linkage, allowing multiple files to be linked together to form a complete program. Proper use of external linkage can help make a program more efficient and easier to work with.

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