Date: Wed, 30 Jan 2002 12:17:13 +0100 From: Robert Suetterlin <robert@mpe.mpg.de> To: freebsd-questions@freebsd.org Cc: freebsd-hackers@freebsd.org Subject: HOW to debug memory corruption efficiently? Message-ID: <20020130111713.GA25203@robert2.mpe-garching.mpg.de>
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Hello everyone! I have a problem using some third party C++ library. After returning from a seemingly innocent function call my stack frame gets destroyed step by step --- and maybe other parts of memory as well. Unfortunately I'm neither proficient in C++ nor efficient in debugging, so I stumble around the problem rather blindfolded. Once I blamed the compiler, systemlibraries and operating system, I knew I needed some help. The problem appeared on a linux system, but I guess the techniques I might need will be the same under Linux, FreeBSD or any un*x. I checked the program execution using gdb and found that in the cleanup code after the function call (this is under Linux, so I will not present the assembler, as I guess clean up will be different under freebsd) something strange happens. When displaying the backtrace with gdb, the debugger suddenly complains: 6 card = new DAQ::SISO::Grabber(1); (gdb) bt #0 DAQ::SISO::Slave::Slave (this=0xbffff65b) at Slave.cc:6 #1 0x0804a352 in main () at test.cc:10 #2 0x400df65f in __libc_start_main () from /lib/libc.so.6 (gdb) n 10 card->LoadFramegrabberDesign( const_cast<char *>(name), 26, 52 ); (gdb) bt #0 DAQ::SISO::Slave::Slave (this=0xbffff65b) at Slave.cc:10 #1 0x0804a352 in main () at test.cc:10 #2 0x400df65f in __libc_start_main () from /lib/libc.so.6 Cannot access memory at address 0xbf080506 I stepped (s) and single stepped (si) over ther card = new ... command. The 'Cannot access memory' appears four single steps after finish returns from the Constructor. So not while within the Constructor or any function called from there. The assembler command after which the strange message appears seems to restore the basepointer (i386). Later during program execution (everytime after a method call using card->method(); my stack frame gets corrupted slowly or the message 'Cannot access memory' changes to complain about different adresses. I guess that somewhere within the Constructor or some function called therein my stack and registers stored there get corrupted due to a memory leak, freak pointer, etc. Is there an efficient way to find out which part of the program is responsible for the corruption. I.e. is there an efficient way to fix the blame either to my code or the third party libraries :)? Or is there another likely cause of this problem, which has nothing to do with freak pointers or the libraries, but with using C++ wrong. I compiled using -Wall and got no warnings. Sincerely, Ronbert S. -- Robert Suetterlin (robert@mpe.mpg.de) phone: (+49)89 / 30000-3546 fax: (+49)89 / 30000-3950 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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