Notes for variable parameter lists

Using the macros va_arg, va_end, and va_start, You can input variable parameters in the function. Such as printf and sprintf.

When I wrote a variable parameter list myself today, I found several notes.

 

The excerpt is as follows:

Here: http://www.cppblog.com/ownwaterloo/archive/2009/04/21/unacceptable_type_in_va_arg.html

Simply put, we use va_arg
(AP
, Type
) When a parameter is retrieved,
Type
Absolutely
The pair cannot be
The following types:
--Char

, Signed
Char

, Unsigned
Char


--Short

, Unsigned
Short


-- Signed
Short

,Short

Int
, Signed
Short

Int
, Unsigned
Short

Int

--Float

A simple reason is:
--No caller
Yes
To my_printf
Transfer
The precedingActual Parameters
.

When calling a function without a prototype declaration in C language:
The caller willEach
Run the "Default actual parameters" command.Upgrade
(Default argumentPromotions
)".

In addition, the variable length parameter listExceeds the last
YesType Declaration Form Parameter
AfterEach actual Parameter
.
The improvement is as follows:
-- The actual parameter of float type will be upgraded to double
-- The actual parameters of char, short, and corresponding signed and unsigned types are upgraded to int
-- If int cannot store the original value, it is upgraded to unsigned Int.

Then, the callerAfter Upgrade
ParametersTransfer
To the caller.
Therefore, my_printf isCannot receive
The actual parameters of the preceding type.