MS-DOS Limit on Open Concurrent File Handles

ID Number: Q60199

3.3x 4.00 4.01

MS-DOS

Summary:

The maximum number of unique concurrent file handles that can be open

at any one time is 255. Although a single process can have up to 64K

of file handles open, the system only tracks up to 255 unique file

handles. The open file handles over 255 can only be duplicates of

existing file handles.

To use these file handles, the number of desired file handles must be

specified in the "FILES=" command in your CONFIG.SYS file.

If the desired number of file handles is greater than 20, INT 21H

Function 67H must be issued in the program using these file handles

in order to increase the file handle count to the desired number. The

program must also be resized with INT 21H, Function 4AH to allocate

memory for the additional duplicate file handles. This is necessary

since .COM files allocate all of existing memory when loaded. About

16 bytes per file handle will be needed to allow a sufficient amount

of memory to be reallocated.