That sounds great for testing, and thank you. Here you go:
rdmod.tar.gz

Extract using the command:
I say cd to root because it's tar'ed with a path to /sbin/rdmod already. Obviously if you want it anywhere else you can move it afterwords. Or you could usesome archive and file managers, of course. Up to you.

Anyway, much appreciated that you are willing to test.

i prefer R&D mode, less crashes in intensive file copying and i only have to press the button once to turn it on rather than hold it till the white light comes on

I prefer it for the same reasons, though I don't get the point of stating that. Anyone using my command line tool for turning it on/off would obviously be already in favor of using R&D mode for one reason/situation or another.

Okay, I spoke to dr_frost_dk and it sounds very likely that my current N900 (that I bought from him) never had R&D Mode turned on ever.

reinob: Did you get around to testing it? Any problems or need for more detailed instructions?

@ Everyone else: Tests on FRESHLY-reflashed N900s that HAVE been in R&D Mode before the reflash but haven't been put into R&D mode since, are also appreciated.

Hi all

I will be able to test it tomorrow (monday).

Cool. No rush by the way, and I'm sorry if I sounded like I was impatient, I was just curious given the two-day silence if you had seen my post or something had come up or wasn't working out.

No prob. Just busy on weekend (kids, guests + building a house

Anyway:

~# ./rdmod -q
Failed to read R&D Mode area from CAL.
Segmentation fault
~#

If you want me to strace/ltrace/syslog I might give it a go.. but tomorrow

Alright, so that's the same error I was getting. Great. No need for more detailed debugging currently... well, I suppose if you didn't I'd end up asking for it later anyway, but it's not urgent, as I currently lack the knowledge to figure out what the contents of strace/ltrace outputs mean anyway.

Well, I tried with ltrace and strace but got no useful info. Program is being killed by SISSEGV (Segmentation fault).

I don't know if the source code for the libcal are available. I might have a look at it some day..

Cheers.

Nope, libcal is closed. There was someone on this board who claimed they managed to reverse engineer all of the read functionality of it in an open source variant a long while back, though I don't know where the source for that is.

I suspect the segmentation fault is my own fault, perhaps the program isn't handling it correctly. But the error message you get before the segfault is indicative of what's happening (as I understand it it means the R&D Mode area for CAL isn't set up correctly at all yet on the devices where this happens, though I am not sure what that entails from a technical perspective). If you look at my source code posted in the first post, that error shows up at this moment in the code:
My C knowledge was and is just barely enough to write my program based on qwerty12's, so I'm not 100% sure what could cause the segmentation fault, unless there's something wrong with my "return 2" statement.