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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 Portable Operating System Interface ([POSIX®)], Base Specifications, Issue 7, section 2.2, "The Compilation Environment" [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 (see undefined behavior 106).
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In addition to symbols defined as functions in each C standard library header, identifiers with external linkage include , among others, errno and math_errhandling, among others, regardless of whether any of them are masked by a macro of the same name.
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DCL37-EX1: Provided that a library function can be declared without reference to any type defined in a header, it is permissible to declare that function without including its header as long as provided that declaration is compatible with the standard declaration.
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