According to C11the C Standard, Section section 6.7.6.3, para. 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.
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When the compliant solution is used and foo(3) is called, the GCC compiler will issue issues the following diagnostic, which alerts the programmer about the misuse of the function interface.
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In C++, foo() and foo(void) have exactly the same meaning and effect, so this rule doesn't apply to C++. However, foo(void) should be declared explicitly instead of foo() to distinguish it from foo(...), which accepts an arbitrary number and type of arguments.
ISO/IEC 9899:2011 Forward Foreword and Section section 6.9.1, "Function definitions"
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