Debug

2017-03-10T14:55:22Z

Stephane-gourichon-lpad: Replace <code>-Og</code> with <code>-O0</code> to prevent compiler from optimizing out variables.

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:Coding]]

Debug

2017-03-10T12:54:24Z

Stephane-gourichon-lpad: Fix instructions about getting a meaningul backtrace. Not sure --disable-optimizations has any effect at all.

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>. 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:Coding]]

VideoLAN Wiki - User contributions [en]

Debug

Debug