DCL33-C. Do not define variadic functions

A variadic function – a function declared with a parameter list ending with ellipsis (...) – can accept a varying number of arguments of varying types. Variadic functions are flexible, but they are also hazardous. The compiler can't verify that a given call to a variadic function passes an appropriate number of arguments or that those arguments have appropriate types. Consequently, a runtime call to a variadic function that passes inappropriate arguments yields undefined behavior. Such undefined behavior could be exploited to run arbitrary code.

The best way to avoid calling variadic functions is the avoid defining them. However, declaring them does no harm.

When a function call expression appears in some contexts, notably as the argument in a sizeof expression, the compiler performs overload resolution to determine the result type of the call, but the object code doesn't execute the call at runtime. In such cases, the compiler uses only the function's declaration, not its definition.

Some template metaprogramming techniques that exploit SFINAE ("substitution failure is not an error") use variadic functions to implement compile-time type queries, as in:

typedef char True;
typedef struct { char a[2]; } False;

template <typename T>
True isPtr(T *);

False isPtr(...);

#define is_ptr(e) (sizeof(isPtr(e)) == sizeof(True))

In this example, is_ptr(e) returns true if expression e has a pointer type. is_ptr(e) calls variadic function isPtr(...), but the call takes place in a sizeof expression. Thus, isPtr(...) must be declared, but it need not, and should not, be defined.

Non-compliant Code Example

Compliant Solution

Exceptions

Risk Assessment

Incorrectly using a variadic function can result in abnormal program termination, unintended information disclosure, or execution of arbitrary code.

References

[[Dewhurst 03]] Gotcha 55: Runtime Static Initialization