In C (stdarg.h), what does the va_arg macro do in a variadic function? Select the most accurate statement about va_arg's role when extracting values from a variable argument list (after va_start has been called).

Question

In C (stdarg.h), what does the va_arg macro do in a variadic function?  Select the most accurate statement about va_arg's role when extracting values from a variable argument list (after va_start has been called).

Accepted Answer

Introduction / Context: When writing variadic functions in C (functions with an ellipsis ... such as printf), the header stdarg.h provides macros to traverse the unnamed arguments. Understanding va_list, va_start, va_arg, and va_end is fundamental for safe and portable code. Given Data / Assumptions: We are discussing the macro va_arg from stdarg.h. A valid va_list has already been initialized with va_start. The function has at least one additional argument beyond the named parameters. Concept / Approach: The canonical pattern is: declare va_list ap; call va_start(ap, last_named_param); repeatedly fetch each argument by calling va_arg(ap, type); finally call va_end(ap). Each call to va_arg both returns the current argument (converted as if by default argument promotions) and advances the internal pointer so the next call fetches the following argument. Step-by-Step Solution: Declare va_list ap.Initialize with va_start(ap, last_named_param).Obtain first extra value with va_arg(ap, expected_type).Each additional va_arg moves the internal cursor forward to the next unnamed argument.Finish with va_end(ap) to clean up. Verification / Alternative check: You can verify by instrumenting a variadic function that prints two integers and a double. Each va_arg retrieves one value in sequence and subsequent calls advance to the next argument. Why Other Options Are Wrong: No: Incorrect; va_arg is precisely for extracting and advancing.Only available from C99: Not true; stdarg.h and va_arg predate C99.It retrieves only size: Wrong; it returns the actual value as the specified type.None of the above: Not applicable since “Yes” is correct. Common Pitfalls: Using the wrong type in va_arg, omitting va_end, or attempting to re-read the same argument after moving past it. Final Answer: Yes

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