ID Number: Q76682
3.00
WINDOWS
Summary:
Many application developers try to use a dynamic-link library (DLL) as
a second application. However, DLLs are designed to provide resources
to applications.
Two problems typically arise with the use of DLLs:
1. A DLL opens a file and leaves it open.
2. The DLL improperly allocates memory.
This article addresses these two issues.
More Information:
When a DLL opens a file, the file handle is stored in the task
database (TDB). The TDB is similar to, but not the same as the program
segment prefix (PSP) used by non-Windows applications. Because the DLL
does not have a TDB, the handle is stored in the TDB of the
application that called the DLL.
If the file is left open when the application terminates, the handle
is freed and becomes invalid. A future attempt to use the handle
results in an attempt to access an invalid file handle.
Files should be opened, read from or written to, and closed as quickly
as possible within the application.
When a DLL allocates memory, the handle is stored in the TDB. For
similar reasons to the file case, if a DLL allocates memory and the
application that caused the allocation terminates, the memory block is
freed. An attempt to use the memory block will result in a general
protection violation, usually reported as an unrecoverable application
error (UAE).
To avoid this problem, memory allocations from a DLL should specify
the GMEM_DDESHARE flag. When this flag is specified, the allocated
memory is bound to the DLL code segment, not an application. Because
the DLL owns the memory, when the application that caused the
allocation terminates, the memory remains valid.
When the DLL terminates, the memory block is freed.
Additional reference words: 3.00 3.0