A simple understanding of variable parameter lists in C Language

A simple understanding of variable parameter lists in C Language
In the function prototype, the number of parameters is usually fixed, but what if you want to receive parameters with an indefinite number at different times? The C language provides a variable parameter list. The variable parameter list is implemented through macros, which are defined in the header file of stdarg. h. The header file declares a va_list type and three macros, va_start, va_arg, and va_end. When using the variable parameter list, we need to declare a va_list type variable for use with these three macros. Va_start (va_list variable name, the last parameter with a name before the ellipsis): You must call this macro to initialize the variable parameters before extracting them. Va_arg (va_list variable name, type_of_var): used to extract the variable. type_of_var is the type of the extracted variable. Return parameters of the corresponding type. Va_end (va_list variable name): After processing the parameters, you must call va_end for cleanup. The following example is taken from c and pointer

# Define _ CRT_SECURE_NO_WARNINGS # include <stdio. h> # include <stdlib. h> # include <stdarg. h> # include <string. h> float average (int n_value ,...) // calculate the average value of a specified number of values {va_list var_arg; // declare the va_list variable int count = 0; float sum = 0; va_start (var_arg, n_value ); // prepare the variable access parameter {for (count = 0; count <n_value; count ++) {sum + = va_arg (var_arg, int);} va_end (var_arg ); // complete Variable Parameter processing return sum/n_value;} int main () {printf ("% lf \ n", average (6, 1, 2, 3, 4, 5, 6 )); system ("pause"); return 0 ;}

 

The preceding example shows how to implement a variable parameter list that calculates the average value of a specified number of values. Of course, there are many applications for variable parameters, such as copying strings.

# Define _ CRT_SECURE_NO_WARNINGS # include <stdio. h> # include <stdlib. h> # include <stdarg. h> # include <string. h> void nstrcpy (char * dest ,...) {va_list pi; // declare the va_list variable char * p; va_start (pi, dest); while (p = va_arg (pi, char *))! = NULL) // extract the variable {strcpy (dest, p); dest + = strlen (p); In the parameter list through va_arg (pi, char ); // copy a variable to make the next copy} va_end (pi);} int main () {char a [100]; char * B = "asdg "; char * c = "qwewq"; char * d = "aswq"; nstrcpy (a, B, c, d); printf ("% s \ n", ); system ("pause"); return 0 ;}

 

Copies multiple strings. A simple understanding of the Variable Parameter List is not very clear about the specific definitions of the three macros, and a complete blog on the variable parameter list will be completed.