Performing low-level file operations on streams

Because the file streams in VA Smalltalk are implemented using the CfsFileDescriptor class, it is possible to obtain the CfsFileDescriptor instance that a file stream is streaming over. To do this, send the fileDescriptor message to the file stream instance. The CfsFileDescriptor instance that is answered can then be used for low-level file descriptor operations. An example of cooperation between streams and file descriptors is shown below:

"An example of combining file descriptor locking with file streams"

| fileStream |

(fileStream := CfsReadWriteFileStream openEmpty: 'lockable.fil') isCfsError

ifTrue: [^self error: fileStream message].

fileStream nextPutAll: 'This is a LOCKED area'.

"Lock the word LOCKED"

fileStream fileDescriptor lock: FMDLOCK start: 10 len: 6.

"Unlock it"

fileStream fileDescriptor unlock: FMDLOCK start: 10 len: 6.

"Close the file stream"

fileStream close.