According to C11, Section 6.7.6.3, paragraph para. 14 [ISO/IEC 9899:2011],
An identifier list declares only the identifiers of the parameters of the function. An empty list in a function declarator that is part of a definition of that function specifies that the function has no parameters. The empty list in a function declarator that is not part of a definition of that function specifies that no information about the number or types of the parameters is supplied.
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