In the function prototype, the number of parameters is fixed in general, but what to do if you want to receive an indefinite number of parameters at different times? c language Variable parameter list is provided for implementation.

The variable parameter list is implemented by macros, which are defined in the header file of stdarg.h. A va_list type and three macros, va_start, va_arg and va_end, are declared in the header file. When we use the variable parameter list, we need to declare a variable of VA ﹣ list type to be used with these three macros.

VA start (VA list variable name, the last named parameter before the ellipsis): this macro must be called for initialization before variable parameters can be extracted.

Va'arg (va'list variable name, type'of'var): used to extract variables. Type'of'var is the type of extracted variables. Returns the parameter of the corresponding type.

VA end (VA end variable name): after the parameter processing, you must call VA end to do some cleaning.

 

The following example is taken from c and pointer

Example code:

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<stdlib.h>
#include<stdarg.h>
#include<string.h>
float average(int n_value,...)  //Average the specified quantity value
{
 va_list var_arg;      //Declare the VA list variable
 int count = 0;
 float sum = 0;
 va_start(var_arg, n_value);  //Prepare to access variable parameters
 {
  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;
}

#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 variables in parameter list by va_arg(pi,char *)
 {
  strcpy(dest, p);
  dest += strlen(p);        //Copy one variable to the next
 }
 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", a);
 system("pause");
 return 0;
}

Reference link: https://www.2cto.com/kf/201511/449297.html