I found kind of a brute force way of doing this by opening the REL file in a loop and reading the error channel to determine if the record length was accepted. See code below. This method is obviously slow. Maybe there is an easier way?
I brought this up because of the current limitation of the uIEC 0.8.2 firmware and it's inability to read a directory's on-disk format with a non-zero secondary address. The on-disk format directory read returns a REL file's record length in byte 21 of the file entry as opposed to a directory read using a secondary address of zero which doesn't. I also want a solution that will work on the uIEC as well as other C= compatible drives.
10 d=8:l=0:input"enter filename: ";f$:print
80 close15:print"record length= "l