mptole.c

这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

#include "os.h"
#include <mp.h>
#include "dat.h"

// convert an mpint into a little endian byte array (least significant byte first)

//   return number of bytes converted
//   if p == nil, allocate and result array
int
mptole(mpint *b, uchar *p, uint n, uchar **pp)
{
	int i, j;
	mpdigit x;
	uchar *e, *s;

	if(p == nil){
		n = (b->top+1)*Dbytes;
		p = malloc(n);
	}
	if(pp != nil)
		*pp = p;
	if(p == nil)
		return -1;
	memset(p, 0, n);

	// special case 0
	if(b->top == 0){
		if(n < 1)
			return -1;
		else
			return 0;
	}
		
	s = p;
	e = s+n;
	for(i = 0; i < b->top-1; i++){
		x = b->p[i];
		for(j = 0; j < Dbytes; j++){
			if(p >= e)
				return -1;
			*p++ = x;
			x >>= 8;
		}
	}
	x = b->p[i];
	while(x > 0){
		if(p >= e)
			return -1;
		*p++ = x;
		x >>= 8;
	}

	return p - s;
}