Debug
When compiling VLC media player, you can compile a debug binary using --enable-debug on the ./configure script.
Those debug facts are common to every programs, not only VLC.
Contents
What it is used for
Of course, the binary compiled in debug mode will or should behave like the release one (more or less). Differences are
- developers can cause VLC to crash when it reached a suspicious state for development purpose, while release version will not in the same suspicious state
- binary backtraces are meaningful in this mode since symbols are embedded in it.
So its main usage is for developers to use debug-friendly binaries and to reproduce a crash and get its backtrace.
How to enable it
As said above, you basically just have to add CFLAGS="-g" CXXFLAGS="-g" --enable-debug parameters at the ./configure stage.
For stepping into code, better is to do CFLAGS="-g -Og" CXXFLAGS="-g -Og" also add --disable-optimizations and not use --enable-release. Replace -Og with -O0 to prevent compiler from optimizing out variables. For more information see VLC configure help.
Backtraces
A backtrace is a snapshot of a process at the time it is about to crash.
What does it look like?
Here is a debug-friendly backtrace:
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xb22e9b90 (LWP 17982)]
0xb7a965e8 in CDDAFormatTitle (p_access=0x842afe0, i_track=99 'c') at info.c:630
630 info.c: No such file or directory.
in info.c
(gdb) bt full
#0 0xb7a965e8 in CDDAFormatTitle (p_access=0x842afe0, i_track=99 'c') at
info.c:630
psz_name = <value optimized out>
config_varname = <value optimized out>
p_cdda = <value optimized out>
psz_mrl = 0x8277238 "cddax:///dev/sr0@T99"
#1 0xb7a94b0f in CDDAReadBlocks (p_access=0x842afe0) at access.c:264
psz_title = <value optimized out>
go_on = <value optimized out>
p_block = <value optimized out>
p_cdda = (cdda_data_t *) 0x84323c0
i_blocks = 20
__func__ = "CDDAReadBlocks"
[...]
#6 0xb6acfe9c in Demux (p_demux=0x833dcb0) at ../../include/vlc_stream.h:189
p_sys = (demux_sys_t *) 0x842eca8
i_pos = <value optimized out>
p_block = (block_t *) 0x859f380
#7 0xb7f34bae in MainLoop (p_input=0x8429c08) at input/input.c:538
b_force_update = 0
i_ret = <value optimized out>
i_type = 138588128
val = {i_int = 1064861644, b_b..........= 0 '\0', h = 0 '\0'}}
i_intf_update = 1214500117823853
#8 0xb7f362b2 in Run (p_input=0x8429c08) at input/input.c:444
No locals.
#9 0xb7e7917b in start_thread () from /lib/libpthread.so.0
No symbol table info available.
#10 0xb7ddb09e in clone () from /lib/libc.so.6
No symbol table info available.
And here is a not debug-friendly backtrace, unuseful in fact.
Program received signal SIGSEGV, Segmentation fault. [Switching to Thread -1342661712 (LWP 8792)] 0xb7db65ab in __module_Need () from /usr/lib/libvlc.so.0 (gdb) bt #0 0xb7db65ab in __module_Need () from /usr/lib/libvlc.so.0 #1 0xb7ec5c34 in ?? () from /usr/lib/libvlc.so.0 #2 0xb5487553 in ?? () from /usr/lib/vlc/stream_out/libstream_out_transcode_plugin.so #3 0x00000009 in ?? () #4 0xb7ec3c74 in ?? () from /usr/lib/libvlc.so.0 #5 0x0854bef8 in ?? () #6 0x08525a00 in ?? () #7 0xb7efaf04 in ?? () from /usr/lib/libvlc.so.0 #8 0xb7dce493 in __vlc_object_release () from /usr/lib/libvlc.so.0 #9 0x0000006f in ?? () #10 0x00000003 in ?? () #11 0x00000000 in ?? ()
As you can see, developers can't cope with last backtrace.
Is a backtrace enough to debug?
It is not enough for someone else than you to fix a bug. At least, you really should describe what you were doing, how to reproduce the bug if you can, etc.
If you try to fix a bug with someone else or if you are asking help on your backtrace, better is really to use an up-to-date VLC source-code. Otherwise, say so and maybe give the commit ID/VLC tag you are working on.
