QuickBasic Uses File Handles, Not File Control Blocks (FCBs) (36030)

SUMMARY

MS-DOS supports two distinct but overlapping sets of file and record management services:

File control blocks (FCBs) (introduced in MS-DOS Versions 1.x)
File handles (introduced in MS-DOS Version 2.00)

The OPEN statement in the following products opens a file handle, not an FCB: Microsoft QuickBasic Versions 1.00, 1.01, 1.02, 2.00, 2.10, 3.00, 4.00, 4.00b, and 4.50 for MS-DOS, Microsoft Basic Compiler Versions 6.00 and 6.00b for MS-DOS, and Microsoft Basic PDS Version 7.00 for MS-DOS.

A QuickBasic or Basic compiler program can indirectly use FCBs by calling an assembly language routine or DOS interrupts to perform the FCB manipulations.

MORE INFORMATION

The Microsoft Basic Compiler Versions 5.35 and 5.36 both use File Control Blocks. These versions of the compiler were designed to operate under DOS 1.x and, therefore, always use FCBs to access files.

The handle-oriented MS-DOS functions use null-terminated (ASCII) filenames and 16-bit file identifiers, called handles, which are returned by MS-DOS after a file is opened or created. File handles allow names that may include paths that show the location of the file within the hierarchical directory structure. The following file-handle information is maintained in a table internal to MS-DOS:

The current read/write pointer for the file

The date and time of the last write to the file

The file's read/write permissions, sharing mode, and attributes

In contrast, the FCB-oriented MS-DOS functions use a 37-byte structure called a file control block, located in the application program's memory space, to specify the name and location of the file. After a file is opened or created, the FCB is used to hold other information about the file, such as the current read/write file pointer, while that file is in use.

Because FCBs predate the hierarchical directory structure introduced in MS-DOS Version 2.00, the FCB functions cannot be used to access files that are not in the current directory. FCBs also do not support the file and record locking functions that are essential to network applications.

This information about FCB's and file handles is taken from "The MS-DOS Encyclopedia" (Published by Microsoft Press, Copyright 1988).