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From: hpoppe@ncar.ucar.edu (Herb Poppe)
Newsgroups: comp.sys.mac.programmer
Subject: Sound Manager
Message-ID: <9426@ncar.ucar.edu>
Date: 6 Dec 90 18:19:31 GMT
Sender: news@ncar.ucar.edu
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Organization: National Center for Atmospheric Research
Lines: 95

What with the availability of the latest and greatest Sound Manager in 
System 6.0.7 I though I'd rework a sound application I wrote last year so 
that it will support multi-channel sampled sound. (I don't have the new 
Sound Manager chapter from IM 6; I'm assuming that all the bugs, 
restrictions, and limitations that Jim Reekes discussed in his Oct. '88 
update to the Sound Manager chapter in IM 5 have been removed. Sure would 
be nice, though, if the new Sound Manager chapter could be put up on 
apple.com; or, better yet, if the dts directory on ftp.apple.com could be 
activated.)

This time I thought I would allocate my own SndChannel records and pass a 
pointer to them to SndNewChannel instead of passing nil and having 
SndNewChannel allocate the storage each time it was called. I planned to 
allocate the records statically, and since I never have more than 3 or 4 
SndCommands in the channel I'd take advantage of the innocent sounding 
sentence that accompanies the description of SndNewChannel (page 17):

"Each channel will hold 128 commands as a default size. The length of a 
channel can be expanded by the application creating its own channel in 
memory."

(I assumed that "expanded" really meant "changed" and that I could make 
the channel smaller than 128 commands.)

I then tried to figure out how to tell SndNewChannel what the maximum 
number of SndCommands were in my SndChannel. There is no parameter to 
SndNewChannel that one can use to provide this information, nor does there 
appear to be any field in the SndChannel record:

SndChannel = PACKED RECORD
  nextChan:      SndChannelPtr;
  FirstMod:      ModifierStubPtr;
  callBack:      ProcPtr;
  userInfo:      LONGINT;
  wait:          Time;
  cmdInProgress: SndCommand;
  flags:         INTEGER;
  qLength:       INTEGER;
  qHead:         INTEGER;
  qTail:         INTEGER;
  queue:         ARRAY [0..stdQLength-1] OF SndCommand;
END;

where this information can be provided (I assume the fields qLength, 
qHead, qTail refer to the dynamic length of the channel, and not to its 
maximum size).

It seems to me, then, that SndNewChannel has no way to determine what the 
maximum length of the channel is, unless it uses a call like GetPtrSize, 
which requires that all SndChannel records be allocated in the heap by the 
Memory Manager. This in turn means that it is bad news to statically 
allocate SndChannel records. IF THIS IS SO, THIS REQUIREMENT SHOULD BE 
POINTED OUT IN THE DOCUMENTATION! Is this discussed in the IM 6 
documentation? Can SndChannel records be (safely) statically allocated?

I was just about to post this when it occurred to me that it might be 
prudent to look back at the Sound Manager chapter in IM 5. Although Jim 
Reekes' update was intended to replace the chapter in IM 5, I have 
occasionally discovered little gems of information there that weren't 
carried over into the update document.  On page V-481, "The SndChannel 
Record", it says:

"QLength specifies the number of commands that the channel can hold; it 
must always be ORD(stdQLength)."

I was apparently wrong in assuming that qLength was the dynamic length. 
Presumably I could set qLength to the length of my SndCommand array and 
SndNewChannel (or whoever) could look there for the max channel length 
without resorting to GetPtrSize. Therefore, I could statically allocate my 
SndChannel records. On the other hand, it does say: "it MUST ALWAYS be 
ORD(stdQLength)" which says I can't have records that are different in 
size from the standard, thus contradicting Jim Reekes' update.

The last paragraph on the page goes on to say:

"When you've allocated the SndChannel record yourself, the Sound Manager 
saves the state information from the pointer you pass, (calling 
HGetState)."

This says, I guess, that SndChannel records must be allocated via the 
Memory Manager (no static allocation).

So I'm left asking:

1) Am I permitted to allocate a SndChannel record smaller than the 
standard length one.

2) Must I set qLength to the number of SndCommands I've allocated before I 
call SndNewChannel?
(SndChannel.queue: ARRAY[0..SndChannel.qLength-1] OF SndCommand; in effect)

Herb Poppe             hpoppe@ncar.ucar.edu
NCAR                      (303) 497-1296
1850 Table Mesa Dr.
Boulder, CO  80307-3000