C-Kermit 7.0 Case Study #04

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Article: 10892 of comp.protocols.kermit.misc
From: fdc@watsun.cc.columbia.edu (Frank da Cruz)
Newsgroups: comp.protocols.kermit.misc
Subject: Case Study #4: Automatic text/binary mode switching
Date: 11 Jan 2000 02:03:46 GMT
Organization: Columbia University

Today's topic is automatic text/binary file-transfer mode selection.

The manual goes into some detail about just what "text" and "binary" mean, and what the consequences might be for choosing the wrong mode. This discussion assumes you know all that.

One of the hallmarks of C-Kermit 7.0 is that it tries to do the right thing for most people automatically whenever possible, by default, rather than making everybody cope with myriad settings to accomplish what they want. It's not exactly artificial intelligence, more like layer upon layer of tricks and ruses based on years of experience.

A common source of aggravation to users of any file-transfer software (FTP, Zmodem, or Kermit) is when find you have transferred a file in the wrong mode -- text when it should have been binary, or vice-versa. The fact is that on most platforms, application software has no good way of telling the difference between a "text file" and a "binary file" on its own. That's why file-transfer programs have commands that make you do it.

Back in the old days, Kermit's rules were simple, modeled on those of FTP: all files were transferred in text mode by default. If you wanted want to transfer files in binary mode, you had to give a command, SET FILE TYPE BINARY. From this point, all files would be transferred in binary mode until you gave another SET FILE TYPE command to change the mode back to TEXT.

In C-Kermit 7.0, the rules have changed, and in fact have become quite complicated. But since now it is all supposed to "just work", most people will never need to know what they are. A couple of new advances make this possible:

  1. Kermit protocol now includes a provision for "automatic peer recognition". If Kermit A is on Linux and Kermit B is on Solaris, they say "we are both on UNIX so we are alike" (modern versions of FTP do this too). In this case, they automatically transfer everything in binary mode except when character-set translation has been requested. When the two Kermits are not alike (e.g. some pairing of Unix, VMS, Windows, OS/2, OS-9, VM/CMS, etc), or if one of the Kermit partners fails to identify itself, then the text/binary file distinction is important.

  2. When the two Kermits do not have an "all-binary" connection (e.g. Unix-to-Unix without character-set translation), the file sender can switch automatically between text and binary mode on a per-file basis based on each filename. The sender informs the receiver of transfer mode of each file (this works with all but the most primitive Kermit implementations).

The upshot is, if you send files from C-Kermit 7.0 to C-Kermit 5A or later (on Unix, VMS, or other platform), to Kermit 95 (any version) or MS-DOS Kermit 3.00 or later, the right stuff should happen.

When the two platforms are alike, all files are transferred in binary mode unless you go out of your way to force text. When the text/binary distinction is significant, C-Kermit picks the mode for each file by comparing its name with lists of text and binary filename patterns. If a filename doesn't match any of the patterns, then the file is sent in the prevailing (SET FILE TYPE) mode, which, by new default, is BINARY.

Default lists are built in that catch the well-known types; you can see them by typing SHOW PATTERNS. Here's an example for UNIX:

 File binary-patterns:
  *.gz *.Z *.tgz *.gif *.tar *.zip *.o *.so *.a *.out *.exe *.jpg
  *.jpeg *.tif *.tiff *.pdf *.so.* *.class *.rpm *.bmp *.bz2 *.BMP
  *.dll *.doc *.vxd *.pdf *.xl* *.lzh *.lhz [wk]ermit

 File text-patterns:
  *.txt *.c *.h *.r *.w *.cpp *.ksc *.bwr *.upd *.html *.htm *.mss
  *.tex *.nr [Mm]akefile *.hex *.hqx *.for *.f77 *.f *.F *.s *.pas
  *.java *.el *.lisp *.sh *.perl *.awk *.sno *.spt *.sed *.TXT
  *read.me *READ.ME .* */.*

Of course you can change the lists by adding, removing, or replacing patterns.

The utility of this feature becomes apparent when you want to transfer a mixed group of files. For example, suppose you want to propogate a directory that contains a mixture of C source files, binaries, and ZIP or compressed tar archives to a variety of platforms. Now, for the first time, "send *" does it right.

This is just an introduction. For complete details on this topic, read ckermit2.txt Section 4.3, plus Section 4.9 about pattern syntax.

- Frank

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C-Kermit 7.0 / Columbia University / kermit@columbia.edu / 10 Jan 2000