Translation Units
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The term translation unit refers to a source code file together with any included files, but less any source lines omitted by conditional preprocessor directives. Syntactically, a translation unit is defined as a sequence of external declarations:
translation-unit: external-declaration translation-unit external-declaration external-declaration function-definition declaration
word external has several connotations in C; here it refers to declarations made outside of any function, and which therefore have file scope. (External linkage is a distinct property; see the section Linkage..) Any declaration that also reserves storage for an object or function is called a definition (or defining declaration). For more details, see External declarations and definitions.