jack_interposer

jack_interposer is a small library that can be used for troubleshooting
misbehaving JACK applications. No recompile of the application under test is 
neccessary, though you get more useful backtraces when the application is
compiled with debugging symbols in.

== Building ==

$ make

== Usage ==

You can simply add this library to your LD_PRELOAD environment:

$ export LD_PRELOAD=/home/arnouten/dev/jack_interposer/jack_interposer.so

Then run the program you wish to analyze: it will abort as soon as a 
non-rt-safe operation is performed from the jack process callback thread:

$ lmms
cannot connect output ports. you'll have to do connections at your own!
malloc(24) is called while in rt section
Aborted
$ 

To get a more useful backtrace, run the application in gdb:

$ gdb lmms
*snip*
(gdb) run
*snip*
malloc(24) is called while in rt section

Program received signal SIGABRT, Aborted.
[Switching to Thread 0xb35bdb70 (LWP 18358)]
0xb7fe1424 in __kernel_vsyscall ()
(gdb) 

Finally, use 'bt' to produce the actual backtrace:

(gdb) bt
#0  0xb7fe1424 in __kernel_vsyscall ()
#1  0xb6b138c0 in raise () from /lib/i686/cmov/libc.so.6
#2  0xb6b16df5 in abort () from /lib/i686/cmov/libc.so.6
#3  0xb7fddbf9 in malloc () from /home/arnouten/dev/jack_interposer/jack_interposer.so
#4  0xb72bee1d in qMalloc(unsigned int) () from /usr/lib/libQtCore.so.4
#5  0x080da586 in audioJACK::processCallback(unsigned int, void*) ()
#6  0x080dae08 in audioJACK::staticProcessCallback(unsigned int, void*) ()
#7  0xb7fddc4d in interposed_process_callback () from /home/arnouten/dev/jack_interposer/jack_interposer.so
#8  0xb708659e in ?? () from /usr/lib/libjack.so.0
#9  0xb708a34f in ?? () from /usr/lib/libjack.so.0
#10 0xb725b585 in start_thread () from /lib/i686/cmov/libpthread.so.0
#11 0xb6bb818e in clone () from /lib/i686/cmov/libc.so.6
(gdb) 

Looks like LMMS is using some QT helper classes that are performing malloc()'s!

== Limitations ==

Unless compiled with GCC 3.3 or later, jack_interposer only works for 
single-CPU machines, or machines configured to run the application under test 
on a single CPU.

This tool only works for single-client applications.