Variable Parameter Function Variable Arguments in C

Today, we can see in csdn that this example is used to assign values to an array and va_list is used. I have never been very concerned about the use and details of this variable parameter function. In the past, I used int argc and char * argv [] to do this.

I posted the code of Mr. Xing and made a few changes:

# Include <stdio. h>
# Include "stdarg. H"

Void array_set (char * parray ,...)
{
Va_list Va;
Int n = 0;
Char C = 0;
Va_start (va, parray );
While (1)
{
C = va_arg (va, char );
If (C! = 0)
{
Parray [n ++] = C;
}
Else
{
Break;
}
}

Va_end (VA );

}

// This is the instance code for testing. Note that the final parameter 0 of array_set is required.

// At least the terminator should not be manually added.

Int main ()
{
Char arr [10];
Array_set (ARR, 48, 49, 49,49, 49,49, 49,0); // a 0 string is required to terminate the string.
Printf ("% s/n", arr );
Getchar ();
}

 

Below are some references:

The information in the <stdarg. h> header file stdarg:
--------------------------------------------------------------------------------

ANSI routines for creating functions with variable numbers of arguments

Functions
Va_arg
Returns the current argument in the list.
Va_end
Va_end helps the called function perform a normal return.
Va_start
Initializes a pointer to a variable argument list.
Predefined types
Va_list
A void pointer which can be interpreted as an argument list.

--------------------------------------------------------------------------------

Va_arg
Type va_arg (va_list & AP, type );

Returns the current argument in the list.

This function (also implemented as a macro) expands to an expression that has the same type and value as the next argument being passed (one of the Variable Arguments ). the variable AP to va_arg shoshould be the same AP that va_start initialized. because of default promotions, you can't use Char or unsigned char types with va_arg.

The first time va_arg is used, it returns the first argument in the list. each successive time va_arg is used, it returns the next argument in the list. it does this by first dereferencing AP, and then incrementing AP to point to the following item.

Va_arg uses the parameter type (which must be an expected type name) to both perform the dereference and to locate the following item. each successive time va_arg is invoked, it modifies AP to point to the next argument in the list.

--------------------------------------------------------------------------------

Va_end
Void va_end (va_list & AP );

Va_end helps the called function perform a normal return.

Va_end might modify AP in such a way that it cannot be used unless va_start is recalled. va_end shocould be called after va_arg has read all the arguments.

Note: va_end is introduced here only to increase compatibility with ansi c. In this implementation, va_end in fact does nothing.

--------------------------------------------------------------------------------

Va_start
Void va_start (va_list & AP, & lastfix );

Initializes a pointer to a variable argument list.

Some C functions, such as sprintf have a variable number of arguments. this function, together with va_arg and va_end, provides a portable way to access these argument lists. they are used for stepping through a list of arguments when the called function does not know the number and types of the arguments being passed. function va_start (implemented as a macro) sets AP to point to the first of the variable arguments being passed to the function. it must be used before the first call to va_arg or va_end.

Va_start takes two parameters: AP and lastfix. AP is explained above, and lastfix is the name of the last fixed parameter being passed to the called function.

Note: I used notation "& AP" in the prototype description, although passing by reference does not exist in ordinary C (only in C ++ ). however, this macro is implemented in such a way that it simulates "passing by reference ".

--------------------------------------------------------------------------------

Va_list
Typedef void * va_list;

A void pointer which can be interpreted as an argument list.

Va_list is the type of the void pointer passed to one of the funents that accepts a pointer to a list of arguments. This array holds information needed by va_arg and va_end.

Topics in GCC Library:

Http://www.delorie.com/gnu/docs/glibc/libc_670.html

Other links:

Http://www.yourblog.org/Data/20041/2523.html

Http://blog.chinaunix.net/index.php? Blogid = 97