Chapter 18. Conclusions for File I/O.

General rules.

Always close your and only your files before leaving your procedures.
Never read records one by one.
Use Data In Memory (DIM) where applicable (e.g. when you have multiple actions on records).
Handle the CMS stack properly, or avoid it if possible.
Don't replace records in Variable format files.

Method	Main advantage	Other advantages	Drawbacks
CMS Pipelines	fast Possible to process the data 'while reading or writing'	many built-in stages for common data handling	VM/ESA only
EXECIO	fast Can keep file open while processing	+- compatible with OS/390	difficult syntax
XEDIT	fast File in storage while processing	many XEDIT commands to process the data	Only VM/ESA Programming not natural
LINEIN, LINEOUT	Portability with other REXX SAA platforms		slow on OS/2 and VM
CHARIN CHAROUT	portability with other REXX SAA platforms	fastest on OS/2
CSL calls	general file I/O in VM/ESA, a must if you want the full function use for SFS	same coding in all HLL, can use REXX for prototyping	only VM/ESA complex REXX programming
NAMEFIND	access to tagged data	NAMES panel for update and query	VM/ESA only

From this table, and from the results discussed in Appendix F. "Host versus Personal Systems", we can conclude this:

In general, CMS Pipelines is the winner. In special situations, EXECIO or XEDIT are a better choice and have approximately same I/O performance as CMS Pipelines.
Forget about LINEIN() when performance is important (and for large files).
Stream I/O is in general a good start if you want portability.
Stream I/O is useful to process SFS files residing on directories that are not accessed, and is easier to code than CSL calls.
CSL is good (or only) solution for other languages than REXX, and may even have better performance there as the interface between REXX and CSL is cumbersome.
On Personal Systems, use CHARIN() for input. Use LINEOUT() for text file output.

This was the last chapter for lesson 4. You should now be in a good position to work on the exercises for this lesson.