奇怪的 _printf_p 行为。 为什么会这样？

Question

_printf_p ("%1$c\n", 'a', 'b', 'c');    // OK
_printf_p ("%2$c\n", 'a', 'b', 'c');    // Debug Assertion Failed
_printf_p ("%3$c\n", 'a', 'b', 'c');    // Debug Assertion Failed
_printf_p ("%1$c %2$c\n", 'a', 'b', 'c');    // OK
_printf_p ("%2$c %1$c\n", 'a', 'b', 'c');    // OK
_printf_p ("%1$c %3$c\n", 'a', 'b', 'c');    // Debug Assertion Failed
_printf_p ("%3$c %1$c\n", 'a', 'b', 'c');    // Debug Assertion Failed
_printf_p ("%2$c %3$c\n", 'a', 'b', 'c');    // Debug Assertion Failed
_printf_p ("%3$c %2$c\n", 'a', 'b', 'c');    // Debug Assertion Failed
_printf_p ("%1$c %2$c %3$c\n", 'a', 'b', 'c');    // OK
_printf_p ("%3$c %2$c %1$c\n", 'a', 'b', 'c');    // OK
_printf_p ("%2$c %1$c %3$c\n", 'a', 'b', 'c');    // OK

If not all of the output arguments are used, function _printf_p will work only when continious positional arguments starting from 1$ are specified in the format string, otherwise it will emit error "Debug Assertion Failed". Why does this happen?

Answer 1

Why does this happen?

Navigating via a list of variable arguments of different types is hard. The requirement of printf is that you have to give a format specifier for all arguments. (In reality, you have to give format specifications for the initial part of all arguments up to the maximum <num>$ argument you requested).

When printf is navigating arguments, it has to know what type to use to "jump" over to the next argument - va_arg(va, int) ? va_arg(va, long) ? va_arg(va, char) ? For printf to know, you have to give all format specifiers for all arguments. For example, to get to the 3$ , printf walks thought format string, finds the first %1$ part in format string, extracts the type c , and does va_arg(char) , then it has to walk over format string again, find the %2$ part, extract its type c , and then it knows to do va_arg(char) , and repeats for all arguments until it finally can get to the 3$ type requested to print.

If one of the specifications is missing, printf doesn't know how to jump over elements in va_list to get to the element requested to print. So it fails.