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Pet Bugs II - Debugger War Stories
AlphaHelix queries: "A few weeks back there was an article on Pet Bugs, where people were asked about their favorite bugs. I have a different sort of question: what was your greatest debugging challenge? I've been debugging for a long time, from analog circuits all the way up to multi-kLOC multithreaded servers, and I have some pretty grisly war stories, like the time I debugged a problem in a third-party DLL in machine code because the client didn't have the source for it (yay open source.) What was your greatest debugging triumph?" The first time Slashdot did this it was more about bugs that you had encountered (and may not have solved), this one is about bugs in your own projects code and the trials and tribulations you had to go thru to get them fixed.
Once I had to debug a program written in MFC... Wait. Sorry. The memory is too painful to recall.
This one time, at computer camp, I found a bug and stuffed it into my pussy!
Without a doubt, the most difficult problems I've ever had to debug are the multi-threading/synchronisation issues. You can get good tools to deal with things like buffer overruns or off-by-one errors, but I've never seen anything that helps debug a serious multi-threaded app.
If you disagree, post your argument. (-1, Overrated) isn't your personal censorship tool for views you don't like.
Linus Torvalds, widely respected throughout the industry (yay open source!) as a programmer par excellence, has stated in public that debuggers are for wimps.
Thanks to Linus, we of the Free Software community can rest assured in the knowledge that we have the most stable, most secure operating system in the world. And to what is it that makes Linux so great? Why, the fact that it was debugged entirely with printf's.
Karma: Good (despite my invention of the Karma: sig)
My favorite bug was allocating memory inside of an assert() using VisualC++ (I hate MS tools; I had to use it for work).
...
...
So the gist of the code went something like this:
0. int array[];
1. assert(array = new int[SIZE]);
2. for (int i = 0; i < SIZE; i++) {
3. array[i] = i;
and the code would segfault on line 3. So I brought it into debug mode, and stepped through. But it worked fine. Back to release mode, and it segfaults.
To restate, here we have the classic example of something you don't want: it works fine in debug mode, but it bombs in release mode.
Of course, since I have simplified the code the answer should be obvious -- in release mode, VisualC++'s compiler was stripping out the assert(), and the allocation inside. In debug mode, it left the assert() in, so the allocation worked fine. I had never changed a flag that said I wanted it to strip them, so I assumed it wouldn't. Never trust M$...
That one is an oldie :) Back in the Amiga days I had made this game that worked fine on my A500 but that stopped working after a while on most other A500. That was strange as the machines were supposed to be identical and I couldn't make more tests at home.
So I used the Action Replay cartridge. For those who don't know about Action Replay, those were "hardware debuggers" that pluged on the bus and could stop and restore the execution of the running program. They were very powerful debugging tools.
After inspecting the content of the hardware registers thanks to the Action Replay, the result was that on some revision of the A500 motherboard the audio interupts had a slightly different timing that caused an improbable case were the audio samples always stopped playing on offset 0 retrigering an audio interupt as soon as one was handled.
The Amiga was so much fun...
True warriors use the Klingon Google
When we were working on a game title on the Nintendo 64, we were maintaining a parallel version for Windows, as it was substantially easier to develop for. Basically, we created some rendering, sound, controller and I/O code for Windows that duplicated what we'd created on the N64.
At one point, we were trying to find some problems with the camera behavior, so we created a flying camera object that coincided with the real camera. It looked like an old Hollywood camera, though the lens cap, reels, everything was just flat black. Then, we'd set up a fixed camera and watch what the game camera would be trying to do by observing where the flying camera went.
Time passed, and we'd forgotten about the added camera altogether. Then, as we were approaching a critical milestone, we went to bring the N64 build up to date... and the screen was black.
The game seemed to be playing, the menus were there, and the framerate counter was up, but - black. The Z-buffer had a constant value all the way across it, meaning there was some mysterious polygon that was exactly covering the screen all the time.
We were there until something like 2am, trying to figure out what the hell was going on. We were risking blowing this milestone, and with that, taking on a pretty hefty late delivery penalty.
We finally figured it out. Stepping back... well, the N64 engine didn't support back-face culling at that point, whereas the Windows engine did. So what's the upshot of the whole thing?
We'd left the lens cap on.
Says the RIAA: When you EQ, you're stealing bass!
It's really easy to set up a bad DMA chain on most architectures, and when that happens, it can do wonderful damage that's tough to reproduce.
One of the more fun things about it is that DMA generally ignores the MMU completely, so you can consistently trash whatever's at some physical address time and again, ignoring all protection.
Even better, DMA doesn't cause hardware breakpoints, so even if your debugger/system are capable of watching for all writes to a given address or page, it'll still merrily corrupt it.
Even more fun if data has been corrupted, but the correct data is still in the data cache from a previous access, making failures even more unpredictable, often relying on an interrupt or other random bit of interference clearing the cache.
On top of all this, a bad DMA chunk may not manifest itself in an obvious way. The program may crash in random sections well before you realize that the DMA you intended didn't happen, or the program may just keep on running with a single fram graphic glitch or a brief bit of static in the case of DMA meant to go to video or audio hardware. That's easy to miss when you're focussing on the debugger and not the running program.
I've seen products ship weeks late just because of a single hard-to-find DMA glitch.
Says the RIAA: When you EQ, you're stealing bass!
I got stung by that on a Friday before a long weekend in 1984 or 1985. A dirty INT21 hook I was applying to DOS worked on ATs but not on XTs (or was it the other way around?). I had set up a structure on the stack and needed to pass its address to a higher language (prolly K&R C) routine, so PUSH SP seamed like the right thing to do.
Hardly a complex bug, but one where it is non-obvious that a 286 is not a superset of an 86.
Then there was the time I had to download a patch to over a thousand embedded controllers spread over a whole country whose problem was that downloading didn't work.... a truck roll to wach one was not an option. But, that's another story (bootstrapping the fix was horrendously more complex that finding the bug).
You could've hired me.
I worked on code for a satellite ground-control system for a few years. It was all Fortran-5/77 and handled a couple-dozen satellites and ground stations in real-time. The problem was that it was written back in the 60's and the programmers who implemented it had really old slow MV-8000's. There weren't enough spare clock cycles to have decent synchronization between modules so they just depended on different subroutines taking an exact number of cycles to execute so they'd match up with whatever they were talking to. Change a single line in anything and you had to recompile it and time every possible way it could execute. Horrible stuff...
Once I was writing some C code to run on an old Motorolla DSP in an embedded system.
One particular function kept crashing. My debugging tools were very limited in this environment--basically, I had a total of 4 LEDs that I could blink on and off by insert function calls into my code. That and a logic analyzer for when things got really nasty.
Well, things did get really nasty. After reviewing and rewriting that function dozens of times, I finally decided the bug couldn't be in my C code. So I had the compiler spit out the assembly it was generating, brushed up on my DSP assembly, and read through its code on the hunch that there was a bug in the compiler (the compiler was very new and still pretty crappy).
But after spending a couple of days staring at the assembly, I concluded that it was perfectly fine. What else could be going wrong? I started thinking maybe something was going wrong in the link step or in the process of getting the file transferred down onto the embedded controller.
I went and learned more than I wanted to know about XCOFF format and used a little binary file editor to see what the linker output was. Again, everything just as it should be.
I just knew that somehow, what was getting executed was different from what was in the file. So we fired up the logic analyzer, and attached it to the DSP, and set it up to watch the contents of the address bus and data bus at each clock cycle.
This is incredibly painstaking--you have to look at 32 lines of step-functions to read off the address, and 48 lines of step-functions to read off the data (yes, it was a 48-bit data register; go figure) for EACH OPCODE. This will make your eyes bug out in a hurry.
But even then--nothing was wrong! The opcodes being loaded into the processor were exactly what they should be. But on this one particular test-and-branch instruction, the processor would just start to go crazy (address and data lines full of random noise; had to be powered down).
I dug out the processor manual and triple-checked the opcode name and number, addressing mode, and operands. Every bit was correct.
In utter frustration, we decided to call Motorolla to see if we could get some assistance from them. After going through a small maze of transfers, we finally ended up talking to the right person who knew (and quickly told us) that:
That particular addressing mode, when used with that particular
opcode, was known to throw the DSP into a hosed state.
It was a bug in the processor itself. The solution was simply to change my code to use a different addressing mode, and all was well.
Cute :).
Laurent
I purchased a library to support multiple serial ports under DOS, way back in 1989/90. I purchased the library because I didn't know assembler well enough, and I specificly wanted flexible support for multi-port serial devices (Digiboards come to mind).
I wrote up my program, testing it with a single serial port and had success quickly. I expanded it to support multiple ports and had it working, up until it was supposed to actually communicate with both ports at the same time.
The company which released the library failed to reference anything except the first port in their interrupt handler. Leaving me to trace through unknown assembler code trying to figure out what was wrong with code that worked, but not correctly.
I sent them a letter complaining about the problem, they sent back disks with that bug patched (exactly the same as my patch) and a couple of other bugs fixed (which didn't effect me).
And vowed at that point to never trust thrid party code. (I did have full sourcecode though, which was nice).
Come back and ask me in about twenty-four hours. I've been wrestling with an absolute doozy for the last six, and somehow I get the impression this is going to be the one ;)
We use a 3rd party software package. It has no documentation. It takes in legacy basic code and spits out horribly ugly C code. I have 2 bugs that I have yet to fix cause of their software. I know it is their stuff but cannot figure out where and their solution is to upgrade their software which I don't have time to test and debug. It is NOT open source so it is blackbox debugging. I can say for sure it is there stuff cause the code is transpiled through their stuff and works on our other two platforms.
Only 'flamers' flame!
As others have noticed, this one is somewhat your own fault for failing to RTFM; the standard assert() and MS-specific ASSERT() and VERIFY() features all quite deliberately have this property.
This does raise an interesting question, though: should you have a separate "debug" build? I'm coming around to the view that, for most applications today, the answer is probably no. A separate debug build has traditionally been used to provide extra diagnostic information and checks during development, which are then stripped out before the program is released by build in "release" mode. On the other hand, this has the following downsides.
So, I hereby begin the campaign to abolish separate debug and release builds in non-performance-critical applications. :-)
If you disagree, post your argument. (-1, Overrated) isn't your personal censorship tool for views you don't like.
Two situations come to mind.
The first is not so much a specific bug that was difficult to find, as it was the general means that I was forced to use to locate bugs. (Yes, I found quite a few.) Back in the early 80's I was working at IBM in the QA group responsible for testing their VM operating system. We were tasked with taking the existing VM OS and not only were we to improve its performance on multi-processor systems, we were also to improve its reliability by doing extensive reviews and testing. I was responsible for testing the free storage allocator.
Some background: For those who may not be aware, that whole operating system was written in IBM BAL (Basic Assembler Language) using the 370 instruction set. The VM operating system created a virtual machine environment for each user - thus producing the appearance of a machine, identical in [almost] all respects to running on the bare hardware. Those few differences pertained to some optimizations in the the virtual memory management's use of PTLBs (page table look-aside buffers) among others.
So, I needed to test memory allocation on the bare hardware to make sure that it worked okay. Once that was nailed down, I had to test memory allocation when VM was running on VM. But, there was yet another set of optimizations that I needed to test when a VM was running on VM running on VM (i.e. a "3rd level" VM).
It was not possible to just issue VM commands to test the various code paths. So, each test consisted of setting hardware breakpoints at the appropriate hex offset, and single stepping through these allocations. By the time I got to testing the 3rd level VM code, I was tracing and debugging these PTLB calculations and allocations, single stepping through instructions in hexadecimal and verifying multiple levels of indirection to memory pages where those calculations were also in hex. Those were the days! Just a year or so out of college, and I had all to myself a multi-million dollar mainframe computer that could support several hundred people!
The other bug was actually a specific bug that caused much early hair loss. I was working at a place where there was a lot of new employees come on board. Along with that, new departments were being formed, people were being promoted and moving to different groups, and there was a great deal of office moves as a result. So, it soon became a problem finding somebody's office. "Gee, wasn't Mary here just last week?"
Sensing a need, I wrote up a quick REXX program (yes, this was back in the early 80's, too) which did data aquisition through forms and supported the generation of reports sorted by various categories: Name, Department, Room Number, etc. This was pretty straightforward and in a couple days I'd gotten it coded, tested, and all the data populated. As there were only a few hundred people I used a flat file (the other alternative was creating a DB2 database and disk space was very dear back then!)
Rolled it out and received much positive feedback. Except, there was one person who noticed there was an error in the ordering of room numbers. See the format was: "Floor: 1, 2, or 3"; then the "building wing: compass direction: N, S, E, or W", and lastly, "room number: 2-digit number". As this was a rapidly growing organization, managers would be allocated several empty offices in advance for the people they'd hire during the next quarter. Also, the building had just been constructed and some areas and sometimes whole wings were still not yet ready for use so there were many gaps in the data. Here is a selection of the kinds of results I saw for the room numbers, in ascending order:
Why are the rooms in the South wing nicely ordered, but things are really messed up for the East wing? I spent HOURS and HOURS trying to figure this one out. See if you can tell what the problem is before reading ahead.
The problem? There is a single set of comparison operators <, =, and > and they just did the right kind of comparison depending on the data type of the operands. Well, here I was thinking the data was text, but the program was making the comparisons as if these were numbers that contained an exponent part
My trickiest bug fix was very confusing.
TI 6701 DSP, custom embedded hardware, eprom for first and second boot stage, third boot stage is on flash.
The second boot stage had to be compressed to fit in the eprom. The first boot stage was limited in what it could do. The second boot stage in eprom was a subset of the flash code.
We would be fixing bugs and adding features in the firmware, but once in a while we would add a line of code and the firmware would crash in an entirely different spot, sometimes even on boot up.
Add one line of code elsewhere and the problem goes away. After a while we realized it did not matter what the line of code was that we added! it mattered where, though. We quadruple checked our cinit linker section and almost everything concievable including lost pointers and trashed code segments. The in-circuit-emulator proved almost useless with major crashes and special case things. So no debugger access.
What happened?
There are write-only latches that the boot loader needs to write to in order to control the front panel. At some point, the flash code was updated so that the last written pattern to the latches would be stored so you could query the value. When an eprom was made based on this code, the problem occured.
After the flash code was read from flash, and relocated to the appropriate spot by code safely residing in internal SRAM of the DSP, the internal code blinked a front panel LED. The latch buffer was still residing in DRAM, in the same area that the new code was loaded into!
So if one specific bit in the flash code was supposed to be a 1, the bootloader would change it to a zero and we would get a spurious crash.
If the bit was supposed to be zero, there would be no problems!!!!
--jeff++
ipv6 is my vpn
mythread(void){
...
char buffer[0x10000];
and it died right away. Finally, I realized I was blowing away the stack by allocating too large a buffer (I only wanted 0x1000). PITA to figure out.
Do you even lift?
These aren't the 'roids you're looking for.
It's a simple one, but the first time (it happened to me a couple years ago), you really search for it:
...
#define square(x) x*x
value = 1/square(x);
Ever since, any C macro I'd write has a dozen ('s in them... That's why I like inline functions...
Opus: the Swiss army knife of audio codec
Writing a threaded app in C++.
...
... m = new MyThread(); ... }
Was having the constructor of the thread class automatically starting to execute the thread. i.e...
class Mythread : public Thread {
MyThread() {
run()
}
}
Used via something like:
class MyMainThread : public Thread
{
MyThread *m;
MyApp() {
~MyApp() { delete m; }
}
As part of MyThread's run was a "quit" function for the application... which would send a kill signal to all of the other threads.
The kill signal was "safe" so that it would call destructors.
Problem?
m's constructor tells MyMainThread to kill itself, which invokes MyMainThread's destructor.
Calls delete on m.
But at this point, m is still being constructed, so you're deleting an uninitialized pointer.
Talk about a sucky night.
...debugging a Beta-smartcard driver for Windows CE and "hacking" it so it works with particular smartcards it wasn't capable of until then =).
Matz
Well, it isn't quite a bug, but it comes pretty damn close. ASP (VBScript) doesn't handle SQL queries longer than 255 characters. link This is the only place I could find it referenced on the web. Lots of hair-pulling on this one. Still haven't fixed it, and any workarounds I can think of are extremely ugly, hairy and otherwise full of cruftiness.
My greatest debugging was on the first production run of the upgraded PATRIOT radar transmitter I was working on. The particular unit in question would start fine, run up, warm up the humoungus amplifier tubes for the radar, switch into high power (for long range operation and tracking) and BAM! reset itself.
Looked like a S/W bug to everyone, so I (as the last 'surviving' member of the S/W team at that point) was called in to find it and fix it.
Well, gathering data was the hard part - I needed to figure out what was happening with scope probes (tracing didn't work, and I couldn't rewrite all the firmware to do any logging or checkpointing). Small catch - the cycle running the system up to high power (where the bug was seen) takes 8+ minutes. Each time.
So I basically had 7 tries per hour (max) to figure out where to hang a scope probe off a backplane of about 4000 wires to figure out what the heck was going on. While at the same time leafing through 40K of assembler code trying to eye-ball the problem.
Three solid days of doing that (about 10-12 hours per day) with my boss constantly pestering me for a accurate estimate of how long it would take to fix it (Gee, thanks for that). Did I mention that I was 3 years on this project at this point - and that it was the first project I was on right out of school - and that I'd 'inherited' 2/3rds of the firmware from other developers who'd moved on? Way to be supportive boss.
Anyhow, I finally figured out it was a H/W fault, not the S/W at all. Turns out a 24 volt PS was "weak". When the 208 3-phase power that runs the transmitter dipped from the load of switching to high power, the 24 volt PS would drop it's voltage. Just enough that the 5 volt PS running the logic detected the drop in 24 volt PS voltage, and due to the fail-safes to protect the circuitry - shut itself off!
Which resets the control logic, brings down the power, steadies the 208 3-phase, bringing the 24 volt PS back in line, starting up the 5 volt PS, and away we go again.
All found with a couple of scopes. Boy that was fun.
Um, Microsoft rules eh; if that's not ms-troll I don't know what is...
However you are missing the point; setting aside it should have been implemented as assert and not Assert, it was the none standard behaviour in what was supposed to be an ANSI compliant C++ that is the real issue. So yes, that behaviour is none standard, and since practically every C++ programmer I know, who used VC++ including some world class acts and apparently plenty of slashdotters, fell for it. It is the Microsoft convention that was/is wrong, it is counter intuitive, so yes it was/is YAMB (Yet Another Microsoft Bug). The MASM segmentation alignment issue is the another example of the same attitude.
We are always right and that is the way it works,
that is the way it works, so you are wrong,
if you are wrong, we are right;
we are always right. ad nausium..
So that is why when Microsoft break a convention or standard and the fault is everybody elses.
Well frankly you need to grow up and down scale your ego, think freely instead engaging in group think and start listerning to others. Don't you know the customer is always right. So if we raise something, you should appologise for wasting my time, thank me for the contribution, and don't under any circumstance imply I'm stupid simply because my opinion differs. That really pisses me off about Microsoft consultants ops I mean 'evangelists'. I wonder what prat though it a good idea to send out 'evangelists' to preach 'belief' to 'Engineers'?
The best laugh I ever got at an evangelists expense was telling him that nature provided him one mouth and two ears and perhaps he should use them in that ratio and then he may understand our requirements. Did he shut up ? Well for about 3 seconds, before launching into his spiel.
So yes, none standard behaviour is YAMB (Yet Another Microsoft Bug), it is not RTFM.
Just yesterday I spent a great deal of time trying to figure out why our application that never leaked memory before was now leaking after a conversion to VC++.NET with /CLR.
The problem ended up being that while DllMain does get a call with DLL_PROCESS_ATTACH, it no longer gets a call with DLL_PROCESS_DETACH.
So if you are, like me, allocating global tables when the DLL loads and deallocating them when the DLL unloads, you've got problems!
To see my (unanswered) post along with code to reproduce it, goto microsoft.public.dotnet.vc.
I do everything the voices in my head tell me to...
why was this modded as off-topic?
The modder was an idiot.
AC
The weirdest bug I have encountered was... Last month.
The app I'm developing right now, must wait for data to appear in a table, then read it, process it and then send it.
I did not want to constantly poll the DB, so I used the Oracle package DBMS_ALERT. Basically, you do a dbms_alert.wait on a named alert and then, in my trigger, the alert is notified, so my thread in the java application unlocks and can go read the data.
Well, everything was running fine, we did some test and new code all week long. But then, one day, it started to behave bizzarly...
We were getting notified "randomly", even when we were not inserting any data into the table! (Was an on insert trigger). I was pretty amazed, I must admit.
So after a whole day trying to figure out what the hell was going on, our no-more-appreciated dba came to us and told us he had created a new schema for our test on the database, with all the triggers and indexes and etc etc... I immediatly took a look at the table using the alert trigger, and there was something like 200 rows in there...
It turned out that the alerts were propagating accross all the different schema, and it was not specified in the documentation the dba gave me... So we just added dynamic alert names and the problem was solved!
I'd rather be sailing...
The research project I'm working on uses a Graph library called LEDA which I don't fully understand and which won't compile when our source files have a .cpp extension. *.c works fine (using g++ as the compiler)
A couple days ago, I started adding a polymorphic class heirarchy to the project, and the first time I tried to compile it, I got a bunch of strange linker errors telling me "undefined reference to [classname] type_info function". I, of course, blamed LEDA and made another test program to see if I could isolate "LEDA's bug." Of course, I couldn't get the same thing to break in another program.
"When in doubt, turn to Google": A search turned up that this error meant I hadn't put the "= 0" in a pure virtual function's declaration. I didn't want any pure virtual functions, but it turned out that I had forgotten the [classname]:: in front of one of my functions.
Talk about dumb.
This was not my bug, but it was a legendary story at Burroughs in the late 70's. A user program had an intermittent bug that was dependent on the day of the week that the COMPILER was created, i.e. if the compiler was recompiled on a Tuesday, then the user code manifested this bug, but if the compiler was built on another day the user code would be fine. The Burroughs machines had byte string arithmetic hardware, and it turned out that a date string containing field was overflowing into an adjacent memory location that caused the compiler to emit bad code. I have no idea how they found the problem.
... and I'm already beginning to feel that tingling sensation I get in my fingers and on my scalp when I'm about to loose my temper in a big way. It's funny how my hearing fades away when I'm really angry and I can only hear the blood rushing through my ears... My conscious mind has likewise blotted out most of that excruciatingly painful experience of trying to fix some extremely incapable butthole's code on Microsoft NT. I guess this is probably related to the postwar-stress symptoms some soldiers had after returning from Vietnam. Anyway... my psychiatrist says that by forgetting all about it the conscious mind tries to protect itself because if it didn't blot out all that literally mind-blowing pain my mind just couldn't go on functioning. Anyway... just forgetting about it works for me, but sometimes I get sudden flashbacks of stepping through the MFC code and the fantasies I have about torturing the Microborgs who wrote this piece of crap to death with red hot iron pokers and electrical stunners (they'd put me away if I really told you what I would love to do to them). That was in my rookie years, now you will excuse me I think I have a major flashback coming on and need to be alone...
I once had to debug a program in an embedded controller - the company I
worked for had this as a product that we'd written 10 years earlier - the
computer that had the assembler and debugger for that controller had been
hauled off for scrap years earlier and the only known copies of the source
code for the program was on that machine's hard drive and on a tape backup
for which we had no functioning tape drive.
Then a bug was found in a mode of operation that we'd hardly ever used
before...just one day before we were planning to demo it to a customer. I
had 24 hours to write a disassembler, an assembler and a simulator for the
CPU - then disassemble the program, find the bug, fix it, reassemble it, test
and burn new EPROM's.
The assembler was essentially just the disassembler run backwards and the
disassembler was little more than a lookup table for the opcodes.
Fortunately, the bug was easy to find and I made it with just one all-nighter.
www.sjbaker.org
This was a problem with an Algol program I wrote on an ancient Singer mainframe when I was in college back in the mid 1970's. I don't remember enough Algol to write it out - but in C++ it would be something like:
...and then later...
...and *much* later...
;
void foo ( double &bar ) { bar = bar + 1.0 ; }
foo ( 6.0 ) ;
printf ( "%f", 6.0 )
Well, in C++, this is illegal because you can't take the address of a constant like 6.0 - but on this old mainframe, the Algol compiler optimised the use of storage by keeping one copy of each large constant that you used (like 6.0) in a table somewhere in memory. Being a crappy compiler, it didn't error-detect my little faux-pas.
Hence the value of the constant 'six' was invisibly changed to seven - everywhere in the program from the point where foo() was called onwards.
Of course this happened in a 3,000 line program that was much more complicated than the example above. Finding that problem was a *BITCH*. You just don't suspect things like simple numeric constants!
www.sjbaker.org
I was writing a serial port driver for a 68000 CPU (it might have been
;
;
/* Send a null byte */
...but the 68000 has a bizarre quirk in it's instruction set ... unless the location you are reading from
a 68010 or an '020 - I forget) - it worked byte-by-byte - no special
line-by-line reading. It echoed everything that came into the serial
port directly to the output.
Trouble was, the first character on every line of keyboard
input would get 'lost'...despite the fact that the application
was reading one character at a time and the return key wasn't
treated as anything special.
The dumb terminals of those days couldn't scroll an entire screenful
of text at full baud rate - so you were advised to send a null byte
after every linefeed to give the terminal an extra millisecond or so
to do scrolling in case the cursor was on the last line of the screen.
My serial port output code (which seemed to work perfectly)
said something like:
char *UART_data_port = SOME_HARDWARE_ADDRESS
*UART_data_port = character
if ( character == '\n' )
*UART_data_port = 0 ;
The problem was that the compiler silently optimised the
'UART_data_port=0' to:
CLR UART_data_port
that when you clear a memory location, it *reads* that address
first. Normally, it doesn't matter a damn that you read something
when you needn't have
is memory mapped hardware...like my UART.
Since the data port was memory-mapped and the same address was
used for READING the serial port as for WRITING to it, whenever
my code wrote a newline, the ensuing null byte would cause the
CPU to read a character from the input stream and throw it away!
Argh! That was hard to find...the output code was in a separate
module from the (suspected) input code - even when I looked at the
compiler output, it looked OK - in the end I had to hang a logic
analyser on the UART and the CPU address bus to find out where it
was executing when the character was 'eaten'. Then reading the
Motorola data book revealed the awful horror.
www.sjbaker.org
I once had to write some Delphi code to call into a VC++ dll that worked as a data only smart card driver.
Basically this smart card had something like 8Kbit addressable space (it was the early days of smart cards as storage devices).
I had to write code to detect card insertion, then detec card version, talk to the smart card reader (on the serial port) and read/write data to the smart card.
Got it all to work fine, except for one thing. The first letter of any string written to the smart card always got hosed.
The cause? The manufacture of the smart card used the first 2 bits of the addressable space on the card as some form of control bits. (this was burried in a foot note somewhere in the documentation). Anyways after finally finding this problem I figured out that I would always write a space char infront of any string going into the card, and always discarded the first char coming out of the card.
Problem solved.
Reflection is a nice feature in Java except they made it a pain in the ass to use. I work on a product that is a Java application running on Windows, Linux, Solaris, and Mac (both OSX and Mac OS 9). Because we are still supporting Mac OS 9, we cannot use a Java 2 compiler at all- so we are squeezing the entire tree through Sun's 1.1.8 compiler every night. (So we're still writing Java 1.1 code! In this day and age! If you call any Java 2 method, like add() on a Vector, it breaks the nightly build.)
Now there are some things that our customers want that absolutely require Java 2, like drag and drop. If you are running Mac OS 9, drag and drop won't work in our program. Sorry. But we have it working for everybody else on all other platforms- by using reflection to access the DnD classes! And the code looks horrible. One line of ordinary code balloons to five lines of incomprehensible gibberish when you use reflection.
The way I see it there are two primary uses for reflection. One is the use that Sun originally intended- for people writing IDEs, bean containers, debuggers, profilers, etc. The other is for people like us, who are compiling against a fossilized version of the JDK but need to introduce some forward-compatibility and access classes we know are usually there but we can't compile against statically. Sun's attitude is always to tell all customers to upgrade to their latest and greatest version of Java. (Sun's inability to take on the backward-compatibility issue from either a design or a policy perspective is really annoying. It's what killed the whole applet idea. And now their JDK 1.4 compiler is spitting out classes with version numbers that make old software freak out. I still have to find the compiler switch that turns that off.)
I think it would be cool if Java had a "reflection" keyword with which you could declare a block of code as being dynamically and not statically compiled- so you could write ordinary code in there and the compiler would break it down during a preprocessing step into the required Class/Method/Field gibberish and let you catch something like an "UnsupportedApiException" in a catch block underneath. Of course, the chance of that happening is zero, and even if it did happen, the 1.1.8 compiler wouldn't understand it anyway. Does anyone know if Sun has any plans for introducing a standard for compiler extensions? It strikes me as a move that would involve relinquishing too much control.
When the users pressed a specific (valid) key sequence rapidly enough, the keys stopped working.
The monitor used a message passing RTOS and it turned out that a small utility function was the culrpit. This function aquired a message buffer, filled it in and sent the message to a specific task. That task was then responsible for releasing the buffer. This worked fine - until someone used this utility function in that very same task. When the key sequence was entered, one task would send a message that in turn would require a call to the utility function in that task. But before that message was processed, another task would call the utility and thus aquire the message buffer. So when the offending task became running, it would have two messages in its queue: The first message would require it to try to aquire the message buffer already used in the second message.
Result: deadlock.
I should add: this was in 8086 assembly...
My opinion? See above.
I was developing some code for Atmel's 89x2051 microcontroller, it was to be used for steering a small train for miners in an ore mine. Basicaly just go forwards, backwards, accelerate and brake, all handled by infrared remote control. Easy job, with great disaster potential :-) (Engine was 15 tons + several carts loaded with ore) The trains were custom made, we were replacing just the control system which was originaly some horrible design around Z80 CPU on 5 PCB's ...
... Thus, when removing lines from the code, it sometimes worked, when we managed to cut the code size under 512 bytes (it was a simple program). But adding an instruction more thrashed the binary ... Grrrrr ...
But to the bug - I wrote the software for the controller, it worked fine in the emulator, but when loaded into the controller, it was locking up and acting badly. And it seemed, that adding and removing lines at random positions in the source was changing the behaviour. WTF ?!?!?!
After a lot of hair pulling and loathing, a friend of mine got an idea to load the resulting binary code into the hex editor. Guess what ! The problem was in the linker we used - it had a bug, that if you had a resulting code longer than 512 bytes, it would wrap it around from zero again
A developer I know was writing documentation ('programming in word') and found that she had typed something like 'once this product has been vaselined'!
Turns out that the v and the b are next door to each other on the keyboard.
Similarly, I used to use int prefixes to identify integers and once came up with a funny when mistyping intCount by swapping the t and the c.
All things in moderation; including moderation
if ( bSomething == TRUE )
...
Once upon a time (1994), in a far away place called "BBN" (now 'Genuity', but my division is not part of Brooks Automation), there lived some statistical software called "RS/1". This software spent its days happily compiling and linking and running self tests on VMS (VAX only; the Alpha port is another story), on Unix (HP/UP, Ultrix, AIX, SunOS, and Solaris), and reminising about the "old" days when it ran on PDP-11s and IBM /370s.
And then the good witch of marketing (Hi peggy!) said, "run on Windows, too. All our customers want it". And the bad old troll called "billg" said, "win32s works just like windows NT or Windows 95 -- it's a single, stable base of 32 bit code".
So we get the build elves to build the code, and it all compiles and links, and we run the self checks, and everything works on NT. So we try under win32s. And a moldy, dusty, forgotton old self-test, which had spent a decade waking up, saying "pass", and quietly going to sleep, roused itself, and with a rusty, creaky voice, said, "fail!".
But that test never fails! It's an old "can we sort a table" test; it uses table (think 'spreadsheet') routines that had, over the years, been build to Keep On Working And Never Stop. And the tables weren't sorting.
In the end, we put two (wow!) PCs into one office and ran NT on one, and Win3.1 with win32s on the other, and stepped and checked, stepped and checked, until the answer came back.
When traced down, we learned that the table-sort routines worked by sorting into a "temporary" table -- a table that hopefully is kept in memory, but might be saved on disk. Once the table is sorted, the original on-disk table was deleleted, and the new table dropped into place using a 'rename' call. If there was no on-disk version of the original table, then a bunch of buffer magic was made to happen; otherwise different buffer magic happened.
unlink is defined to return a fail code if you try to delete a file that doesn't exist. On all version of windows it was correct except win32s. Our test case, of course, made a temporary table to sort, it never existed on disk, so when the code tried to unlink it, the wrong result code was set and then the wrong buffer magic happened - and then the table wouldn't appear sorted.
It never occured to us that that's where the problem might lie -- who would think that a bad 'unlink' call would result in table sort failure?
Peter Smith
then: BBN Software Products.
now: WildTangent.com
Small print: I don't remember if it was the 'unlink' call or the 'rename' call. I seem to remember it was doing an 'unlink'.
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I was working on a program that did some video-conferencing stuff using OpenH323 this past spring for senior project, and I had one of the most bizzarre bugs I've ever encountered.
The program would mostly run fine, but now & then one of the threads would crash horribly. GDB was (almost) no help. I could see enough from the backtrace that the stack was being smashed, so I guessed that there was some null pointer deref going on (since there were no arrays that could overflow in this bit of code). However, single stepping through that thread almost never produced errors, and when it did, they were non-deterministic1
Finally, after much staring at code, I started to have a clue. I started single stepping through other threads, and finally found my problem.
Thread B was smashing the stack of Thread A (throug a null pointer deref as I had originally guessed). It did this in such a way that Thread B didn't get messed up at all, and Thread A could often continue running for some time. Furthermore, the way in which it smashed the stack let Thread A get several levels deep in bogus function calls!
-Cheetah
I was once writing a quasi-shell program that had a "code window" in which commands would be typed, and then the whole block of text would be parsed into separate instructions, ordered, memory allocated, etc, and then the program would automatically carry out whatever functions were required without the user having to navigate through endless menus. Now, I'm not going to go into all the details for why it was done this way, and why the certain feature that had the bug to be described was needed, because that's irrelevant, and it's a long story anyway. It was there, and that's all you need to know. Anyway, onto the bug itself: the sort-of-script-language that was developed for this had a basic implementation of variables. Now, figuring out how to store and retrieve user-defined-at-runtime variables, in VB, [Hey, I had no choice! I use MS, I don't have to like it.] was hard enough. [I eventually had to use a library.ocx object that I found somewhere on my harddrive that acted sort of like a hash.] But then I was faced with the problem of having the parser recognize the variables when they came up in the code, AND having to integrate the same variable recognition PLUS replacement in the math engine. I ended up with a very convoluted string of crazy Regular Expressions [How I wish i could've used Perl!] to find the variables and chop out whitespace, and replace them with their current values retrieved from the library object, which itself was not an easy task, with some VB string handling functions thrown in for a bit of clarity. But, for some weird reason, and in mathematical expresions only, it wouldn't find variables at the end of a line, or sometimes it would see "foo" at the end of one line, and "bar" at the beginning of the next, and think that it was one variable named "foobar". Of course, this led to lots of "undefined variable" errors in the latter case, and lots of syntax errors in the former. I then had to look through EVERY regular expression that had anything to do with parsing variables, in the entire program, due to the fact that some of the variable matching for the math engine was contained in the main parser, so a problem in the main parser would also cause problems with the math engine. Eventually, after 3 [or maybe it was only 2...] days, I discovered one single misplaced ^ in a regular expression that determined when to hand off control to the math engine! Man, was I mad. And don't get me started on the virtual memory stack I had to write all the code for...
Never underestimate the power of human stupidity.
This was a mainframe. A physically huge mainframe. The two CPUs and memories alone took three rows of cabinets, each row about 25 feet long, connected by cross-cabinets. Then there were about forty cabinets of peripheral gear, including many drums, tape drives, printers, and two desk-sized consoles, one per CPU. All of this gear, though, delivered only 1.25 MIPS, and there was only about 1MB of memory (256K of 36 bit words.)
The system kept crashing. For each crash, a dump was produced - a stack of paper about two inches thick, with some of the major data structures decoded at the front, followed by the entire contents of memory, in octal. When I arrived for the job, there were two stacks of these six feet high waiting for me.
So I started in on this, figuring out what had caused each crash, tracking pointers with multicolored pens, and fixing the bugs in the operating system, which was all in assembly. After a while, the most common crashes had been fixed, and I was then spending time on the more difficult problems.
Some problems required software workarounds for hardware problems. The system clock would make errors when its fan filters were clogged. (Yes, electronics was so big back then that the system clock had multiple muffin fans.) Code was written to deal with this.
Occasionally, code overlays would be misread from the drums. A checksum crashed the system when this happened, but reread support was added to make that error recoverable.
The most intractable problem involved data that seemed to be corrupted when written by one processor and read by the other. We looked and looked for race conditions, but even additional locking didn't help. Finally, a hardware consultant was brought in, and he built a custom hardware device that checked that certain bits matched between the processor and one of the memories. This was used during operation, and finally, after several days, the device triggered, the whole system froze with its clock stopped, and we could verify that the neon lamps at the processor end of the data path didn't match those at the memory end.
Eventually, we had that beast running reliably, with a month or so between crashes. Gradually, the operation was expanded, until there were five mainframes crunching away.
I think most compilers can produce java 1.1 compliant code. So why don't you just compile under 1.2, write everything normally and just don't use the dnd stuff if a Class.forName on some DnD specific class fails?