On the realization of variable parameter function from printf

I've always felt that printf seems to be one of the most powerful functions in the C language library, not only because it can format output, but because it has no limits on the number of parameters, and a few to give. printf's strong adaptability to parameter numbers and parameter types gives rise to a strong interest in exploring it.

1. Use case

int a =10;
double b = 20.0;
char *str = "Hello world";
printf("begin print\n");
printf("a=%d, b=%.3f, str=%s\n", a, b, str);
...

From the use of printf, it is not hard to find a rule that the first argument to printf is always a string, regardless of the number of variable arguments. And it is this first parameter that makes it possible to confirm how many trailing parameters are left behind. The amount of stack space used for each trailing parameter is determined by the first format string. However, how does printf get the value of the parameter after the first argument, see the following code

Implementation of 2.printf function

//acenv.h
typedef char *va_list;
#define　_AUPBND　　　　(sizeof (acpi_native_int) - 1)
#define　_ADNBND　　　　(sizeof (acpi_native_int) - 1)

#define _bnd(X, bnd)　　(((sizeof (X)) + (bnd)) & (~(bnd)))
#define va_arg(ap, T)　 (*(T *)(((ap) += (_bnd (T, _AUPBND))) - (_bnd (T,_ADNBND))))
#define va_end(ap)　　　(void) 0
#define va_start(ap, A) (void) ((ap) = (((char *) &(A)) + (_bnd (A,_AUPBND))))
//start.c
static char sprint_buf[1024];
int printf(char *fmt, ...)
{
　　va_list args;
　　int n;
　　va_start(args, fmt);
　　n = vsprintf(sprint_buf, fmt, args);
　　va_end(args);
　　write(stdout, sprint_buf, n);
　　return n;
}
//unistd.h
static inline long write(int fd, const char *buf, off_t count)
{
　　return sys_write(fd, buf, count);
}