Volume-Locking Guidelines

Applications that lock and modify volumes should follow these guidelines to avoid degrading system performance and to prevent data loss:

• If there are no open files on the volume, applications should perform direct disk writes in a level 0 lock. Otherwise, they should use the locking hierarchy and perform disk write operations in a level 3 lock.
• Applications should utilize the locking hierarchy to minimize the time spent in a level 3 lock. They should only call disk I/O functions inside a level 3 lock and drop down to a level 1 or 2 lock whenever possible.
• Applications should neither terminate or relinquish control nor leave a level 0 or 3 lock if the volume information is incomplete or invalid. When applications leave one of these locks, the file system must be consistent with what it was when they entered the lock because other applications will regain access to the drive.
• Because the Interrupt 21h file handle I/O functions rely on accurate information about the volume, applications should not use these functions when the volume information is incomplete or invalid.
• Applications should not move the swap file.
• Applications should not move memory-mapped files opened for write access. Read-only memory-mapped files may be moved cluster by cluster.
• Applications may only move 32-bit Windows-based DLLs and executables cluster by cluster.
• Applications may move directory entries for the swap file and open memory-mapped files, but the path to them must always be consistent, even in a level 3 lock.

Because read operations are blocked in the level 3 lock, all applications written for 16-bit Windows, 32-bit Windows, or MS-DOS should follow these guidelines to avoid deadlock while in a level 3 lock:

• Applications should only access the disk by using the low-level disk functions (Interrupt 13h, Interrupt 25h, and Interrupt 26h) or the Interrupt 21h file handle read, write, seek, and IOCTL functions. Other MS-DOS functions are not guaranteed to work. Windows or C run-time library file I/O functions should not be used, because these functions may contain code or call code that is not safe to execute inside the level 3 lock.
• Applications should not yield control, update the screen, execute any user-interface code, or do anything else that could cause Windows 95 to load a new or previously discarded segment, such as by spawning an application or loading a DLL.
• Windows-based applications must have all the code for a level 3 lock contained within the processing for a single message. The application should not process other messages or call any Windows functions.