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According to the C Standard, 7.1.3 [ISO/IEC 9899:2011],

All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use.

All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary and tag name spaces.

Each macro name in any of the following subclauses (including the future library directions) is reserved for use as specified if any of its associated headers is included, unless explicitly stated otherwise.

All identifiers with external linkage (including future library directions) and errno are always reserved for use as identifiers with external linkage.

Each identifier with file scope listed in any of the following subclauses (including the future library directions) is reserved for use as a macro name and as an identifier with file scope in the same name space if any of its associated headers is included.

Additionally, subclause 7.31 defines many other reserved identifiers for future library directions.

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