path.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 <u.h>
#include <libc.h>
#include <bio.h>
#include <libsec.h>

#include "iso9660.h"

/*
 * Add the requisite path tables to the CD image.
 * They get put on the end once everything else is done.
 * We use the path table itself as a queue in the breadth-first
 * traversal of the tree.  
 *
 * The only problem with this is that the path table does not
 * store the lengths of the directories.  So we keep an explicit
 * map in an array in memory.
 */

enum {
	Big,
	Little
};

static void
Crdpath(Cdimg *cd, Cpath *p)
{
	p->namelen = Cgetc(cd);
	if(p->namelen == 0) {
		Crseek(cd, (Croffset(cd)+Blocksize-1)/Blocksize * Blocksize);
		p->namelen = Cgetc(cd);
		assert(p->namelen != 0);
	}

	p->xlen = Cgetc(cd);
	assert(p->xlen == 0);	/* sanity, might not be true if we start using the extended fields */

	Cread(cd, p->dloc, 4);
	Cread(cd, p->parent, 2);
	p->name[0] = '\0';
	Crseek(cd, Croffset(cd)+p->namelen+p->xlen+(p->namelen&1));	/* skip name, ext data */
}

static void
writepath(Cdimg *cd, Cdir *c, int parent, int size)
{
/*
	DO NOT UNCOMMENT THIS CODE.
	This commented-out code is here only so that no one comes
	along and adds it later.

	The ISO 9660 spec is silent about whether path table entries
	need to be padded so that they never cross block boundaries.
	It would be reasonable to assume that they are like every other
	data structure in the bloody spec; this code pads them out.

	Empirically, though, they're NOT padded.  Windows NT and
	derivatives are the only known current operating systems
	that actually read these things.

	int l;

	l = 1+1+4+2+c->namelen;
	if(Cwoffset(cd)/Blocksize != (Cwoffset(cd)+l)/Blocksize)
		Cpadblock(cd);
*/
	Cputc(cd, c->namelen);
	Cputc(cd, 0);
	Cwrite(cd, c->dloc + (size==Little ? 0 : 4), 4);
	(size==Little ? Cputnl : Cputnm)(cd, parent, 2);
	Cwrite(cd, c->name, c->namelen);
	if(c->namelen & 1)
		Cputc(cd, 0);
}

static ulong*
addlength(ulong *a, ulong x, int n)
{
	if(n%128==0)
		a = erealloc(a, (n+128)*sizeof a[0]);
	a[n] = x;
	return a;
}

static ulong
writepathtable(Cdimg *cd, ulong vdblock, int size)
{
	int rp, wp;
	uchar buf[Blocksize];
	ulong bk, end, i, *len, n, rdoff, start;
	Cdir *c;
	Cpath p;

	Creadblock(cd, buf, vdblock, Blocksize);
	c = (Cdir*)(buf+offsetof(Cvoldesc, rootdir[0]));

	rp = 0;
	wp = 0;
	len = nil;
	start = cd->nextblock*Blocksize;
	Cwseek(cd, start);
	Crseek(cd, start);
	writepath(cd, c, 1, size);
	len = addlength(len, little(c->dlen, 4), wp);
	wp++;

	while(rp < wp) {
		Crdpath(cd, &p);
		n = (len[rp]+Blocksize-1)/Blocksize;
		rp++;
		bk = (size==Big ? big : little)(p.dloc, 4);
		rdoff = Croffset(cd);
		for(i=0; i<n; i++) {
			Creadblock(cd, buf, bk+i, Blocksize);
			c = (Cdir*)buf;
			if(i != 0 && c->namelen == 1 && c->name[0] == '\0')	/* hit another directory; stop */
				break;
			while(c->len && c->namelen && (uchar*)c+c->len < buf+Blocksize) {
				if((c->flags & 0x02) && (c->namelen > 1 || c->name[0] > '\001')) {	/* directory */
					writepath(cd, c, rp, size);
					len = addlength(len, little(c->dlen, 4), wp);
					wp++;
				}
				c = (Cdir*)((uchar*)c+c->len);
			}
		}
		Crseek(cd, rdoff);
	}
	end = Cwoffset(cd);
	Cpadblock(cd);
	return end-start;
}


static void
writepathtablepair(Cdimg *cd, ulong vdblock)
{
	ulong bloc, lloc, sz, sz2;

	lloc = cd->nextblock;
	sz = writepathtable(cd, vdblock, Little);
	bloc = cd->nextblock;
	sz2 = writepathtable(cd, vdblock, Big);
	assert(sz == sz2);
	setpathtable(cd, vdblock, sz, lloc, bloc);
}

void
writepathtables(Cdimg *cd)
{
	cd->pathblock = cd->nextblock;

	writepathtablepair(cd, cd->iso9660pvd);
	if(cd->flags & CDjoliet)
		writepathtablepair(cd, cd->jolietsvd);
}