According to the C Standard, subsection subclause 6.7.6.3, paragraph 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.
Subsection Subclause 6.11.6 states that
The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature.
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