Translation unit (programming)

In C programming language terminology, a translation unit is the ultimate input to a C compiler from which an object file is generated. In casual usage it is sometimes referred to as a compilation unit. A translation unit roughly consists of a source file after it has been processed by the C preprocessor, meaning that header files listed in #include directives are literally included, sections of code within #ifdef may be included, and macros have been expanded.

Context

A C program consists of units called source files (or preprocessing files), which, in addition to source code, includes directives for the C preprocessor. A translation unit is the output of the C preprocessor – a source file after it has been preprocessed.

Preprocessing notably consists of expanding a source file to recursively replace all #include directives with the literal file declared in the directive (usually header files, but possibly other source files); the result of this step is a preprocessing translation unit. Further steps include macro expansion of #define directives, and conditional compilation of #ifdef directives, among others; this translates the preprocessing translation unit into a translation unit. From a translation unit, the compiler generates an object file, which can be further processed and linked (possibly with other object files) to form an executable program.