[solved] Application behaving in 3 different ways on 3 different machines
Hello.
During the holidays I've been developing an application on my desktop computer at home.
I setup a repository on github, so when I got back to work I cloned the repo to my laptop.
It wouldn't work.
The app is comprised of a client and a server, strangely enough the server would segfault at a strcpy at the very beginning, while the client would bug me about not supplying a command line parameter (it's supposed to work anyway).
So I ssh-ed into an office machine we use to test things out, cloned the repo and the problems are inverted!
Now it's the client that would segfault while the server pretends a parameter!
My machines are
desktop - i7 2600k with Ubuntu 12.04 x64 eng
laptop - core2duo with Ubuntu 12.04 x64 eng
test pc - core2duo with Ubuntu 10.04 ita (dunno if x86 or x64)
Now I could bear that an app developed on a single pc would require some tinkering on other pcs, but the same app displaying exactly symmetrical behaviour on two different pcs I can't understand.
Anyway the specific code that seems to be the problems is the following
You code did not work, it just happened not to crash. Because those pointers are never initialized they could be pointing to any bit of memory left over from a previous program. If you rebooted one of the PC's you may get different behavior. It is purely random, so it may not be something you can duplicate.
Last edited by jim mcnamara; 01-07-2013 at 08:16 AM..
There is a set of standards for C. They dictate what will or will not happen in the language.
Doing what you did created something that has undefined behavior. I'll make one pass at this.
When you run a C program:
Code:
1 - the OS creates a stack frame for main.
2 - the os simply overlays that stack on top of existing garbage in memory
3 - it does this for efficiency reasons and because that memory is no longer part of any process.
4 - when your program ran, those pointers were parked on top of memory that had some existing values in it.
5 - what was in the memory depends on the program that lived in that exact memory before
6 - it could be all 00000000, it could literally be anything.
7 - since it could be anything, it is possible that the memory pointed to (0xfaaa0000) - let's pretend.
8 - 0xfaaa0000 just HAPPENED to be by random chance an OS allocated location on your existing stack frame.
9 - Now we can use the memory for our program - no crash.
10 Why? because the memory is part of the process so you can what you want to it
11 What if 0xfaaa0000 was NOT part of allocated memory? Boom, program crash.
12 Therefore there is no known way to predict the behavior of the code, it is undefined
There is a set of standards for C. They dictate what will or will not happen in the language.
Doing what you did created something that has undefined behavior. I'll make one pass at this.
When you run a C program:
Code:
1 - the OS creates a stack frame for main.
2 - the os simply overlays that stack on top of existing garbage in memory
3 - it does this for efficiency reasons and because that memory is no longer part of any process.
4 - when your program ran, those pointers were parked on top of memory that had some existing values in it.
5 - what was in the memory depends on the program that lived in that exact memory before
6 - it could be all 00000000, it could literally be anything.
7 - since it could be anything, it is possible that the memory pointed to (0xfaaa0000) - let's pretend.
8 - 0xfaaa0000 just HAPPENED to be by random chance an OS allocated location on your existing stack frame.
9 - Now we can use the memory for our program - no crash.
10 Why? because the memory is part of the process so you can what you want to it
11 What if 0xfaaa0000 was NOT part of allocated memory? Boom, program crash.
12 Therefore there is no known way to predict the behavior of the code, it is undefined
I will disagree VERY slightly with Jim concerning #3 in the above list. On any UNIX system, data allocated to one process will never be given to another process for use as the stack of a new process. Doing so would create a security hole (covert channel).
Code may be shared between processes (using shared libraries); data may be shared using shared libraries, shared memory segments, mmap()ed files, etc. But address space that will be used as data (including the stack) that is not explicitly shared, will be cleared by the OS before handing it to any user-level process.
There is a lot of code run by a process when a C program starts executing before you get to the first line of code in main(). Shared libraries have to be linked in; the locale has to be initialized; the STDIO stdin, stdout, and stderr streams have to be initialized; etc. Any of these can leave random data in what will eventually become the stack frame allocated to main(), and some of them may leave different things on the stack depending on the time/date when the program was run, the version of the OS or shared libraries being used, etc.
So, the end result is the same. Uninitialized data on the stack of main() can vary from run to run. And if you have other uninitialized pointers being used by argp_parse() or s_catch_signals(), they may also be overwriting anything in your address space and may get segmentation faults on some future execution of your code.
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