Advanced Debugging with GDB

Advanced Debugging with gdbDavid KhosidSept 21, 2009david.kh@gmail.com

AgendaTechniques for debugging big, modern software:STL containers and algorithms, Boost Ex: how to see containersSignalsMulti-threaded (ex.: how to follow a thread?)Repetitive tasks on the almost unchanging code baseRemote debuggingExamples2

GDB was first written by Richard Stallman in 1986 as part of his GNU systemRichard Stallman, “Debugging with gdb”www.gnu.org/software/gdb/documentationHelp: $gdb –h (gdb) h (gdb) aproposCommand names may be truncated if the abbreviation is unambiguous. TAB completion. Command Cheat Sheetwww.yolinux.com/TUTORIALS/GDB-Commands.htmlLast GDB version is 6.8, new 7.0 soon: 2009-09-233Sources of information

Item #1: C++ and STL - ContainersHow to see container’s content?Commands file, ex. .gdbinithttp://www.yolinux.com/TUTORIALS/src/dbinit_stl_views-1.03.txtLimitations: a littlelibstdc++ compiled in debug modeLimitations: - different product , not for QA, not for client, not in performance tuning stage- performance 4

Item #1: C++ and STL - ContainersHow to see container’s content?Auxiliary functionstypedef map<string, float> MapStringFloat;void mapPrint(const MapStringFloat& m){ for(MapStringFloat::const_iterator pos = m.begin(); pos != m.end(); ++pos){ cout << pos->first << " : " << pos->second << "\n"; }Limitations: - you can’t do that without a process to debug (investigating core files)- optimization of unused functions. Solution: ‘volatile’Pretty-printing of STL containers in future versions of GDB5

Item #2: Extending GDB – User-defined commands(gdb) show user commandnameExample: (gdb)define adder print $arg0 + $arg1 + $arg2 end(gdb) adder 1 2 3 6

Item #3: Automating repetitive tasks What GDB Does During Startup1. Executes all commands from system init file 2. Executes all the commands from ~/.gdbinit3. Process command line options and operands 4. Executes all the commands from ./.gdbinit5. reads command files specified by the `-x' option 6. …7

Automating tasks - history, recordingcontinueWhat GDB Does During Startup… 6. Reads the command history recorded in the history file. (gdb) set history filename fname(gdb) set history save on/off(gdb) show history(gdb) show commands8

Item #4: Signals‘i handle’ or ‘i signals’Print a table of all the signals and how gdb has been told to handle each one.handle signal [keywords...]keywords: nostop|stop, print|noprint and pass|nopassEx: handle SIG35 nostop print pass handle SIG36 stop (implies the ‘print’ as well) handle SIG37 nostop print nopass handle SIG38 nostopnoprintnopass9

Item #5: Multi-threadsUse case: debugging specific thread, while controlling behavior of others.facilities for debugging multi-thread programs:• automatic notification of new threads• ‘thread threadno’, to switch among threads• ‘info threads’, to inquire about existing threads• thread-specific breakpoints• set mode for locking scheduler during execution (gdb) set scheduler-locking step/on/offothers: Interrupted System CallsExample:(gdb) i threads(gbd) b foo.cpp:13 thread 28 if x > lim10

Item #5: Remote debuggingUse case: - GDB runs on one machine (host) and the program being debugged (exe.verXYZ.stripped ) runs on another (target). - GDB communicates via Serial or TCP/IP.- Host and target: exactly match between the executables and libraries, with one exception: stripped on the target.- Complication: compiling on one machine (CC view), keeping code in different place (ex. /your/path/verXYZ)Solution: - Connect gdb to source in the given place:(gdb) set substitute-path /usr/src /mnt/cross (gdb) dir /your/path/verXYZ11

Remote debugging - exampleUsing gdbserver through TCP connection: remote (10.10.0.225)> gdbserver :9999 program_strippedor remote> ./gdbserver :9999 –attach <pid>host> gdb programhost>(gdb) handle SIGTRAP nostopnoprint pass to avoid pausing when launching the threadshost> (gdb) target remote 10.10.0.225:9999TARGET (Android Dev phone) HOST (Fedora Linux)12

Item #6: Back to the past Convenience variables are used to store values that you may want to refer later. Any string preceded by $ is regarded as a convenience variable.Ex.:set $table = *table_ptr (gdb) show convCheckpoint - a snapshot of a program’s state(gdb) checkpoint(gdb) i checkpoint(gdb) restartcheckpoint-idValue history- values printed by the print command.13

Small Items: #7, #8 #7. How to see macros? $ g++ -gdwarf-2 -g3 a.cpp -o prog#8. 64 bit .vs. 32bit -m32 flagOn 64-bit machine, install another 32-bit version of GDB $ ls -l `which gdb32` /usr/bin/gdb32 -> ‘/your/install/path’14

Lightweight how-to'sHow to remove a symbol table from a file?A: stripHow to supply arguments to your program in GDB?A1: With --args option #sudo gdb -silent --args /bin/ping google.comA2: As arguments to run: (gdb) runarg1 arg2run without arguments uses the same arguments used by the previous run. A3: With set args command: (gdb) set args arg1 arg2(gdb) show args set args without arguments – removes all arguments. How to know where you are (file, next execution line)?A: (gdb) f15

Lightweight how-to's - continueHow to find out the crash file executable? A1: #file core.1234A2: #gdb core.1234A3: use /proc/sys/kernel/core_pattern#echo "core_%e.%p" > /proc/sys/kernel/core_pattern if the program foo dumps its core, the core_foo.1234 will be created.How to find out why your program stopped?A: (gdb) iprogWhich command(s) can be used to exit from loops?A:(gdb)until lineNo‘print’, ‘info’, ‘show’- what is a difference?‘print’ – print value of expression‘info’ – showing things about the program being debugged‘show’ – showing things about the debugger16

Advanced Debugging with GDB

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Advanced Debugging with GDB