convm2d.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 "lib.h"
#include <string.h>
#include "sys9.h"
#include "dir.h"
#define nil ((void*)0)

static char nullstring[] = "";

uint
_convM2D(uchar *buf, uint nbuf, Dir *d, char *strs)
{
	uchar *p, *ebuf;
	char *sv[4];
	int i, ns, nsv[4];

	p = buf;
	ebuf = buf + nbuf;

	p += BIT16SZ;	/* ignore size */
	d->type = GBIT16(p);
	p += BIT16SZ;
	d->dev = GBIT32(p);
	p += BIT32SZ;
	d->qid.type = GBIT8(p);
	p += BIT8SZ;
	d->qid.vers = GBIT32(p);
	p += BIT32SZ;
	d->qid.path = GBIT64(p);
	p += BIT64SZ;
	d->mode = GBIT32(p);
	p += BIT32SZ;
	d->atime = GBIT32(p);
	p += BIT32SZ;
	d->mtime = GBIT32(p);
	p += BIT32SZ;
	d->length = GBIT64(p);
	p += BIT64SZ;

	d->name = nil;
	d->uid = nil;
	d->gid = nil;
	d->muid = nil;

	for(i = 0; i < 4; i++){
		if(p + BIT16SZ > ebuf)
			return 0;
		ns = GBIT16(p);
		p += BIT16SZ;
		if(p + ns > ebuf)
			return 0;
		if(strs){
			nsv[i] = ns;
			sv[i] = strs;
			memmove(strs, p, ns);
			strs += ns;
			*strs++ = '\0';
		}
		p += ns;
	}

	if(strs){
		d->name = sv[0];
		d->uid = sv[1];
		d->gid = sv[2];
		d->muid = sv[3];
	}else{
		d->name = nullstring;
		d->uid = nullstring;
		d->gid = nullstring;
		d->muid = nullstring;
	}
	
	return p - buf;
}