C varargs diagnostics: point out the specific error(s) in this program using <stdarg.h>. Rewrite focuses on why ellipsis functions need a named parameter and a correct va_start anchor. #include <stdio.h> #include <stdarg.h> fun(...); int main() { fun(3, 7, -11.2, 0.66); return 0; } fun(...) { va_list ptr; int num; va_start(ptr, n); /* 'n' is not a named parameter here */ num = va_arg(ptr, int); printf("%d", num); }

Introduction / Context:
This question examines correct use of C variable-argument functions (varargs) with . Specifically, it tests whether the function with ellipsis is declared and defined properly and whether va_start is anchored to a valid, named parameter that precedes the ellipsis. Understanding these rules prevents undefined behavior and compilation errors in real-world code such as logging or formatting routines.

Given Data / Assumptions:

• A function is declared as fun(...); with no named fixed parameter.
• The definition fun(...) also uses only an ellipsis.
• va_start(ptr, n) uses an identifier n that is not a parameter.
• Old-style (K&R) return type is omitted.

Concept / Approach:
In standard C, a variadic function must have at least one fixed, named parameter before .... The macro va_start requires that the second argument be that last named parameter. Using only ... makes it impossible to position the va_list at the first variable argument. Therefore, such a declaration/definition is invalid. Missing an explicit return type is also an error in modern C, but the most fundamental issue is the invalid variadic signature.

Step-by-Step Solution:
Identify function signature: fun(...) → no named parameter → invalid for varargs.Check va_start(ptr, n) → n is not a parameter → misuse of va_start.Conclusion → declaration/definition of fun(...) is invalid.

Verification / Alternative check:
Correct pattern: void fun(int count, ...)va_list ap; va_start(ap, count);This compiles and allows safe extraction of arguments with va_arg.

Why Other Options Are Wrong:
Error: fun() needs return type — true in modern C, but not the primary varargs error being tested. Error: ptr Lvalue required — ptr is a proper lvalue; the issue is not lvalueness. No error — incorrect; there is a clear signature misuse.

Common Pitfalls:
Forgetting the required named parameter, anchoring va_start to an identifier that is not a parameter, and assuming legacy implicit-int rules still apply.

Final Answer:
Error: Invalid declaration of fun(...)