October 16, 2013

Bugs in ancient software: as88

In my Computer Organizations class, I use the venerable Tanenbaum textbook, now in its 6th edition, and this year I decided to go with the flow and use the assembler/simulator that he documents extensively (and tutorials at length) in an appendix to the book. The problem, of course, is that the code base is really old; it's written in K&R C, and copyrights and headers indicate that some of the files were last edited more than twenty years ago. But hey, it works! And it's not like the basics of assembly have changed all that much since then.

Except it doesn't quite work. The simulator and tracer run fine on the pre-assembled example files, but when I go to run the assembler, as88, even on an unmodified source file, it mostly runs (generating the appropriate extra files) and then segfaults right at the end. A little poking around shows that the crash occurs on line 36 of comm.c, where a call to fclose is failing. Some rummaging around the internet turns up a site in Italian that has discovered what appears to be the same problem, although their suggested solution doesn't work---the problem is not that input itself is NULL, but that something it references has already been closed.

Observing that this is cleanup at the end of the program, of resources that will automatically be cleaned up by the OS anyway, I thought about just commenting the line out. Buffer issues shouldn't be a problem, because this is an input buffer, so there shouldn't be anything buffered that needs to be written out (and we're done with the input, so we must be at EOF).

So, just commenting that line out seems to work. At some point I may find time to hack around further and see if I can figure out the real source of the problem, but if this duct tape I just applied continues to work....

"To put it bluntly, I believe the world is patriarchal because men are bigger and stronger than women, and can beat them up." --Roger Ebert

Posted by blahedo at 1:35pm on 16 Oct 2013
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