Extern

Extern is a programming term used to describe external linkage or declarations that reference symbols defined outside the current scope or translation unit. It signals that a symbol, such as a variable or function, is provided elsewhere, typically in another file or a library.

In C and C++, extern is a storage-class specifier that declares a variable or function without defining it, indicating external linkage. For example, extern int count; declares that count is defined in another translation unit; a separate definition such as int count = 0; in another file provides the symbol. Functions are extern by default; extern is often used in headers to declare functions or global variables. Static, by contrast, gives internal linkage. C++ adds extern "C" to specify C linkage, which disables name mangling to improve compatibility with C code.

Other languages use extern to reference foreign or native symbols as well. In Rust, extern is used in foreign-function interface (FFI) blocks to declare external functions, often with a specific ABI, for example extern "C" { fn printf(...); }. In C# and the .NET environment, extern appears in declarations for methods implemented in unmanaged code, typically together with DllImport attributes to import functions from native libraries. These uses share the common goal of exposing symbols outside the current language or module boundary.

Etymologically, extern derives from Latin exter, external. It is not itself a storage directive; rather, it modifies declarations to indicate external linkage or an external implementation. Correct use helps manage cross-module relationships and library interfaces, while misuse can lead to unresolved symbols or linkage errors.