Short version of question: How can I get gdb to use the debugging symbols for libc?
Longer version: I am debugging a program with gdb and I want to see information about a futex used by libc. However, at some point during debugging I get output such as:
Catchpoint 2 (call to syscall futex), 0x00007ffff772b73e in ?? () from /lib/libc.so.6 (gdb) bt #0 0x00007ffff772b73e in ?? () from /lib/libc.so.6 #1 0x00007ffff767fb90 in ?? () from /lib/libc.so.6 #2 0x00007ffff767a4c0 in vfprintf () from /lib/libc.so.6 #3 0x00007ffff768565a in printf () from /lib/libc.so.6 ....
When I run
info sharedlibrary in gdb at the breakpoint I see:
(gdb) info sharedlibrary From To Syms Read Shared Object Library 0x00007ffff7dddaf0 0x00007ffff7df6704 Yes (*) /lib64/ld-linux-x86-64.so.2 0x00007ffff7bc53e0 0x00007ffff7bd1388 Yes (*) /lib/libpthread.so.0 0x00007ffff79ba190 0x00007ffff79bd7d8 Yes (*) /lib/librt.so.1 0x00007ffff76538c0 0x00007ffff7766c60 Yes (*) /lib/libc.so.6 0x00007ffff6c1fd80 0x00007ffff6c303c8 Yes (*) /lib/libgcc_s.so.1 (*): Shared library is missing debugging information.
And when I run
ldd I see:
linux-vdso.so.1 => (0x00007ffff7fde000) libpthread.so.0 => /lib/libpthread.so.0 (0x00007ffff7dbf000) librt.so.1 => /lib/librt.so.1 (0x00007ffff7bb6000) libc.so.6 => /lib/libc.so.6 (0x00007ffff7833000) /lib64/ld-linux-x86-64.so.2 (0x00007ffff7fdf000)
I am using Ubuntu 10.04 and I think that the version of libc with debug symbols is in
/usr/lib/debug/lib. I tried setting my
LD_LIBRARY_PATH variable to have this at the front of the path but that did not seem to make a difference.
I'm not completely clear on how the program chooses which shared libraries to load, whether this is set at runtime or compile time (I sort of assumed runtime but now I'm not sure). So information on how to get gdb to use the debug version of libc is appreciated.
I think that the version of libc with debug symbols is in /usr/lib/debug/lib. I tried setting my LD_LIBRARY_PATH variable to have this at the front of the path but that did not seem to make a difference.
These are not the droids you are looking for.
The libraries in /usr/lib/debug are not real libraries. Rather, the contain only debug info, but do not contain
.data sections of the real
libc.so.6. You can read about the separate debuginfo files here.
The files in
/usr/lib/debug come from
libc6-dbg package, and GDB will load them automatically, so long as they match your installed
libc6 version. If your
libc6-dbg do not match, you should get a warning from GDB.
You can observe the files GDB is attempting to read by setting
set verbose on. Here is what you should see when
libc6-dbg do match:
(gdb) set verbose on (gdb) run thread_db_load_search returning 0 Reading symbols from /lib64/ld-linux-x86-64.so.2...Reading symbols from /usr/lib/debug/lib/ld-2.11.1.so...done. thread_db_load_search returning 0 done. thread_db_load_search returning 0 Loaded symbols for /lib64/ld-linux-x86-64.so.2 Reading symbols from system-supplied DSO at 0x7ffff7ffb000...done. WARNING: no debugging symbols found in system-supplied DSO at 0x7ffff7ffb000. thread_db_load_search returning 0 Reading in symbols for dl-debug.c...done. Reading in symbols for rtld.c...done. Reading symbols from /lib/librt.so.1...Reading symbols from /usr/lib/debug/lib/librt-2.11.1.so...done. thread_db_load_search returning 0 ... etc ...
For instance I see
Reading symbols from /lib/libc.so.6...(no debugging symbols found)...done
That implies that your GDB is not searching
/usr/lib/debug. One way that could happen is if you set
debug-file-directory in your
Here is the default setting:
(gdb) show debug-file-directory The directory where separate debug symbols are searched for is "/usr/lib/debug".
debug-files-directorywas not set correctly but setting it correctly in
.gdbinitseems to fix the problem - Gabriel Southern 2012-04-10 23:19
directorycommand with the path to the source. If you're running a distro that patches libc (such as Debian), then be sure to apply the same patches (such as by running
debuild), else the source line numbers mayn't match - Craig M. Brandenburg 2013-11-05 16:40
Make sure you've installed the debug symbols for libc:
sudo apt-get install libc6-dbg
And if you're on an x64 system debugging x86 code:
sudo apt-get install libc6:i386 sudo apt-get install libc6-dbg:i386
/usr/lib/debugfor the symbols for libc. For instance I see
Reading symbols from /lib/libc.so.6...(no debugging symbols found)...done.So I've still got some problem, but your answer is very helpful for understanding what the real problem is - Gabriel Southern 2012-04-06 20:54