LINUX Troubleshooting

Troubleshooting

C++ compilation - crash - gdb debugging - pvm

As you have noticed in class, a computer is not perfect and not smart enough to understand all your needs...
That's why we constructed this page!
This page will be updated regularly, if you still have problems, contact me, so we can add it to this page!!

Try to report the problem (what is the problem, error messages, ...) as exact as possible

Jan Lemeire: jan.lemeire@vub.ac.be 4K227 02/629.2997

C++ compilation & linking

check whether the error is at a compile or link step
always remember your last changes, which will be causing the problem (back up working versions at least every day!)
'filter' the error by commenting parts of your code (in emacs)
print out intermediate results
some compile errors (especially with included header files) are caused by the previous line, so check it.

Problem	Possible solutions
warning: clock skew detected...	this is no real problem, don't worry
warning: implicit declaration of function xxx (compilation)	the header file where this function is declared, is not included or you mistyped the functions name
cannot find function at link time	be sure the file in the header and c-file or exactly the same
"multiple definitions of ..."	compiler or linker found 2 times the same function: - or you reimplemented an existing function - or you linked the same object file multiple times
linking of libraries	when a library is using another, the link order is important!! If `lib2` is using `lib1`, you should link in this order: `-llib2 -llib1`
program crashes for big problem sizes	the sizes of static arrays (eg: int arr[10000]) are limited! Use dynamic arrays (arr = new int[10000])
link error "undefined reference to vtable for class x"	your class is not complete (not all methods are implemented...) put each qt QObject class (widgets etc) in a separate file! with qt signals & slots, you sometimes need to compile it a second time (so that the moc file is generated)
problems with templates	read this important comment
crash with a glibc error while using PAPI or our PAPIprofiler	Don't put your code in a directory with a name containing spaces

QT

Problem	Possible solutions
while debugging, gdb stops regularly Program received signal SIG32, Real-time event 32	execute handle SIG32 pass noprint nostop to ignore these stops

Crash errors

Most common reasons for a C++ program to crash (segmentation fault):

be carefull with pointers and memory allocation!!

delete or free commands must refer to an allocated object!
for arrays with dynamic size, look here how to do it in C++

index goes out of array range : this is not checked by C++ (unlike Modula-2)

when you changes an non-existing element of the array, the memory gets corrupted, but your program continues! However, it will cause strange crash errors further on.

Run your program with the debugger and inspect the call stack after the crash with 'up'.

gdb debugging

Problem	Possible solutions
can't find file (when trying to put a breakpoint)	the `-g` compile flag must be set (makefile) or `debug` for the tmake CONFIG
Warning: source file is more recent than executable	it means that you should recompile the executable!

pvm

Problem	Possible solutions

failed to add machine to pvm	the computer you are working on should be added to .rhosts file
problems adding machines on infopc4	the pvm of infopc4 got a lot of errors... run your program on a different computer, eg infopc23 with `rsh infopc23`(do this for the 3 consoles)
.

Problem	Possible solutions