Usage and example of variable parameters in C Language

Usage and example of variable parameters in C language-Linux general technology-Linux programming and kernel information. The following is a detailed description. Syntax:
# Include
Type va_arg (va_list argptr, type );
Void va_end (va_list argptr );
Void va_start (va_list argptr, last_parm );

The va_arg () macros are used to pass a variable number of arguments to a function.

First, you must have a call to va_start () passing a valid va_list and the mandatory first argument of the function. this first argument can be anything; one way to use it is to have it be an integer describing the number of parameters being passed.
Next, you call va_arg () passing the va_list and the type of the argument to be returned. The return value of va_arg () is the current parameter.
Repeat callto va_arg () for however extends arguments you have.
Finally, a call to va_end () passing the va_list is necessary for proper cleanup.
For example:

Int sum (int num ,...){
Int answer = 0;
Va_list argptr;

Va_start (argptr, num );

For (; num> 0; num --){
Answer + = va_arg (argptr, int );
}

Va_end (argptr );

Return (answer );
}


Int main (void ){

Int answer = sum (4, 4, 3, 2, 1 );
Printf ("The answer is % d \ n", answer );

Return (0 );
}

This code displays 10, which is 4 + 3 + 2 + 1.

Here is another example of variable argument function, which is a simple printing function:

Void my_printf (char * format ,...){
Va_list argptr;

Va_start (argptr, format );

While (* format! = '\ 0 '){
// String
If (* format ='s '){
Char * s = va_arg (argptr, char *);
Printf ("Printing a string: % s \ n", s );
}
// Character
Else if (* format = 'C '){
Char c = (char) va_arg (argptr, int );
Printf ("Printing a character: % c \ n", c );
Break;
}
// Integer
Else if (* format = 'D '){
Int d = va_arg (argptr, int );
Printf ("Printing an integer: % d \ n", d );
}

* Format ++;
}

Va_end (argptr );
}


Int main (void ){

My_printf ("sdc", "This is a string", 29, 'x ');

Return (0 );
}

This code displays the following output when run:

Printing a string: This is a string
Printing an integer: 29
Printing a character: X



// One instance
Program requirements:
Create a LOG function

Log_printfbuf (char * fmt ,...)

Print the same content of printf, and then print the buffer content of the remaining two parameters.
The first parameter is buffer, and the second parameter is the number of bytes to be printed.


For example

Char buf [256];

Buf [0] = 0;
Buf [1] = 0;
Buf [2] = 1;
Strcpy (buf + 3, "abcdskkkklkkkaskk ");
Log_printfbuf ("buffer at % p size % d:", buf, sizeof (buf), buf, 56 );

Will print out
Buffer at 08001000 size 256: {00} {00} {01} ABCASKKKLKKLWIKKKK ....

This is based on actual needs. It may be useful.
Program Implementation:
# Include
# Include
# Include

Void foo (char * fmt ,...);
Void show_hex (char * s, int d );
Void main ()
{
Char buf [256] = {0 };
Buf [0] = 0;
Buf [1] = 0;
Buf [2] = 1;
Strcpy (buf + 3, "ABCDSKKKKKLLLLKKKASKK ");

Foo ("buffer at % p size % d:", buf, sizeof (buf), buf, 56 );

}
Void foo (char * fmt ,...){
Va_list ap;
Int d;
Char c, * s;
Va_start (ap, fmt );
While (* fmt)
{
If (* fmt = '% ')
{

Switch (* (++ fmt )){
Case's ':/* string */
S = va_arg (ap, char *);
Printf ("% s", s );


Break;
Case 'p ':
S = va_arg (ap, char *);
Printf ("% 04X", (int) s );
Break;
Case 'D':/* int */
D = va_arg (ap, int );
Printf ("% d", d );

Break;


Case 'C':/* char */
/* Need a cast here since va_arg only
Takes fully promoted types */
C = (char) va_arg (ap, int );
Printf ("% c", c );

Break;
}
}
Else
Printf ("% c", * fmt );

Fmt ++;

}
S = va_arg (ap, char *);
D = va_arg (ap, int );
Show_hex (s, d );

Va_end (ap );
}
Void show_hex (char * s, int d)
{
For (int I = 0; I <d; I ++)
{
Printf ("{% 02X}", (unsigned char) * (s + I ));

}
Printf ("\ n ");
}