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GuruMeditation
Docs » Autodocs » dos.library » LockDosList

NAME

	LockDosList -- Locks the specified Dos Lists for use (V36)

SYNOPSIS

	dlist = LockDosList(flags)
	D0		     D1
	struct DosList *LockDosList(ULONG)

FUNCTION

	Locks the dos device list in preparation to walk the list.
	If the list is 'busy' then this routine will not return until it is 
	available.  This routine "nests": you can call it multiple times, and
	then must unlock it the same number of times.  The dlist
	returned is NOT a valid entry: it is a special value.  Note that
	for 1.3 compatibility, it also does a Forbid() - this will probably
	be removed at some future time.  The 1.3 Forbid() locking of this
	list had some race conditions.  The pointer returned by this is NOT
	an actual DosList pointer - you should use on of the other DosEntry
	calls to get actual pointers to DosList structures (such as
	NextDosEntry()), passing the value returned by LockDosList()
	as the dlist value.
	Note for handler writers: you should never call this function with
	LDF_WRITE, since it can deadlock you (if someone has it read-locked
	and they're trying to send you a packet).  Use AttemptLockDosList()
	instead, and effectively busy-wait with delays for the list to be
	available.  The other option is that you can spawn a process to
	add the entry safely.
	As an example, here's how you can search for all volumes of a specific
	name and do something with them:
	2.0 way:
		dl = LockDosList(LDF_VOLUMES|LDF_READ);
		while (dl = FindDosEntry(dl,name,LDF_VOLUMES))
		{
			Add to list of volumes to process or break out of
			the while loop.
			(You could try using it here, but I advise
			against it for compatability reasons if you
			are planning on continuing to examine the list.)
		}
		process list of volumes saved above, or current entry if
		you're only interested in the first one of that name.
		UnLockDosList(LDF_VOLUMES|LDF_READ);
				  \* must not use dl after this! *\
	1.3/2.0 way:
		if (version >= 36)
			dl = LockDosList(LDF_VOLUMES|LDF_READ);
		else {
			Forbid();
			/* tricky! note dol_Next is at offset 0! */
			dl = &(...->di_DeviceList);
		}
		while (version >= 36 ?
				dl = FindDosEntry(dl,name,LDF_VOLUMES) :
			        dl = yourfindentry(dl,name,DLT_VOLUME))
		{
			Add to list of volumes to process, or break out of
			the while loop.
			Do NOT lock foo1/foo2 here if you will continue
			to examine the list - it breaks the forbid
			and the list may change on you.
		}
		process list of volumes saved above, or current entry if
		you're only interested in the first one of that name.
		if (version >= 36)
			UnLockDosList(LDF_VOLUMES|LDF_READ);
		else
			Permit();
		\* must not use dl after this! *\
		...
		struct DosList *
		yourfindentry (struct DosList *dl, STRPTRname, type)
		{
		\* tricky - depends on dol_Next being at offset 0,
		   and the initial ptr being a ptr to di_DeviceList! *\
			while (dl = dl->dol_Next)
			{
			    if (dl->dol_Type == type &&
				stricmp(name,BADDR(dl->dol_Name)+1) == 0)
			    {
				break;
			    }
			}
			return dl;
		}

INPUTS

	flags - Flags stating which types of nodes you want to lock.

RESULT

	dlist - Pointer to the head of the list.  NOT a valid node!

SEE ALSO

AttemptLockDosList(), UnLockDosList(), exec.library/Forbid(), NextDosEntry()

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