View source for Debug

When compiling {{VLC}}, you can compile a debug binary using <code>--enable-debug</code> on the ./configure script.

Those debug facts are common to every programs, not only VLC.

== 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 <code>CFLAGS="-g" CXXFLAGS="-g" --enable-debug</code> parameters at the [[configure|./configure]] stage.

For stepping into code, better is to do <code>CFLAGS="-g -Og" CXXFLAGS="-g -Og"</code> also add <code>--disable-optimizations</code> and not use <code>--enable-release</code>. Replace <code>-Og</code> with <code>-O0</code> 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.



== See also ==

* [[Report Bugs]]
* [[Tutorial for gdb debug under Win32|Tutorial for GDB with Win32]]

[[Category:Building]]
[[Category:Coding]]