[TUHS] x/y/zmodem on Unix

Paul Ruizendaal pnr at planet.nl
Tue Sep 19 08:31:30 AEST 2023

Last december Matt brought up xmodem and recently I needed it for an almost identical use case. Studied it a bit and in the context of Unix, it is an interesting piece of software history.

Although xmodem originated on CP/M in 1977, it seems to have arrived on Unix soon thereafter. The first Unix implementation seems to have gone by the name of “rbsb” and must have originated when V7 was prevalent: it uses alarm() to simulate non-blocking I/O. Over the course of the 80’s ymodem and zmodem were added and the package became lrzsz; the source continued to have a very V7-ish feel to it at least to the early 90’s. In the mid-90’s it was converted to ansi-C and had some other modernization, but it looks like it would still have run on SysIII (apart from the ansi).The last update to the upstream source package appears to be from 1998, i.e. 25 years ago.

It is still packaged today for FreeBSD and major Linux distros. On Unix it must be one the oldest code bases still in regular use, with the 1980 source still recognizable in its current incarnation. Any other contenders come to mind?

I had always associated x/y/zmodem with CP/M and MSDOS, not so much with Unix. Last December Clem already pointed out that it was popular for file exchange in the Unix scene as well, along with several other similar tools. Also, the ymodem approach to file metadata is very unix oriented, suggesting it originated on Unix or at least that Unix users were an important user demographic. Yet, I could find little trace of x/y/zmodem in the TUHS Unix Tree. The search tool finds it in 2.11BSD, in Minix 1.5 and 2.0 and in V10. Kermit is in those as well, and in 4.3BSD and 4.4BSD on top.

Maybe these programs were commonly pulled from a bulletin board and hence not on distribution media. Not sure how that would be bootstrapped, although the 2.11BSD files for x/y/zmodem include “minirb.c”, a 175 line implementation to receive files using the ymodem protocol. It is possible that this was transferred as plain text as a first step, or even retyped.

Any recollections?

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