Microsoft KB Archive/60199

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.