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Additionally, subclause 7.31 defines many other reserved identifiers for future library directions.
No other identifiers are reserved. (Note that the POSIX standard extends the set of identifiers reserved by the C Standard to include an open-ended set of its own. See section 2.2, "The Compilation Environment," in IEEE Std 1003.1-2013.) The behavior of a program that declares or defines an identifier in a context in which it is reserved or that defines a reserved identifier as a macro name is undefined
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.
Noncompliant Code Example (Header Guard)
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a macro definition of
errnois suppressed in order to access an actual object, or the program defines an identifier with the nameerrno. [ISO/IEC 9899:2011]
(See subclause 7.5, paragraph 2, and undefined behavior 114 in Annex J of the C Standard.)
The errno identifier expands to a modifiable lvalue that has type int but is not necessarily the identifier of an object. It might expand to a modifiable lvalue resulting from a function call, such as *errno(). It is unspecified whether errno is a macro or an identifier declared with external linkage. If a macro definition is suppressed to access an actual object, or if a program defines an identifier with the name errno, the behavior is undefined.
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