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Q155703: PRB: MCI Fails to Read a File Using a Custom mmio Procedure

Article: Q155703
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:4.0
Operating System(s): 
Keyword(s): kbmm
Last Modified: 04-FEB-2000

4.00
WINDOWS
kbprg kbprb

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) 4.0 
- Microsoft Visual C++, version 4.1 
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SYMPTOMS
========

On the Windows 95 platform, the Media Control Interface (MCI) returns
MMSYSTEM348 error when using a custom mmio procedure to read a wave or an avi
file (device element). The text description for MMSYSTEM348 is "Cannot read this
file. Make sure the file hasn't been deleted, or check your disk or network
connection."

CAUSE
=====

The value in the adwInfo[2] member of the MMIOINFO structure is used by MCIWAVE
and MCIAVI drivers. Therefore, its content cannot be trusted when the custom
mmio procedure is invoked to read the custom device element. The stated problem
occurs when a custom mmio procedure uses adwInfo[2] to store the pointer to the
data to be read.

RESOLUTION
==========

A simple workaround is to use a static local variable to retain information
between invocations of the custom procedure. For instance:

  static DWORD dwMyLocalVar;

MORE INFORMATION
================

MCI allows the use of a custom mmio procedure to open a wave or an avi device.
When the custom procedure is invoked, the first argument is a pointer to
MMIOINFO structure. One field in MMIOINFO is adwInfo. In general, a custom mmio
procedure can use adwInfo for state information. However, if the custom
procedure is to be used by MCI drivers, it should use other local variables to
store information. Please note that this problem only occurs for 32-bit
applications running on Windows 95 operating system.

Additional query words: 4.00 kbdsd wave audio avi video mmioInstallIOProc

======================================================================
Keywords          : kbmm 
Technology        : kbVCsearch kbWinNT400xsearch kbAudDeveloper kbSDKSearch kbVC410 kbWinSDKSearch kbWinSDK400
Version           : WINDOWS:4.0

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