cpp send_from segfault about liblo

[rbarreiros]

Same code crashes in linux

Thread 2 "XControl_Proxy" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7ffff7a43700 (LWP 9162)]
send_data (a=0x55555558aee0, from=0x7fffffffe300, data=0x7ffff0000ca0 "/xinfo", data_len=12) at .... /ext/liblo/src/send.c:474
474             sock = from->sockets[0].fd;
(gdb) bt
#0  send_data (a=0x55555558aee0, from=0x7fffffffe300, data=0x7ffff0000ca0 "/xinfo", data_len=12) at ... /ext/liblo/src/send.c:474
#1  0x0000555555569ec5 in lo_send_message_from (a=0x55555558aee0, from=0x7fffffffe300, path=0x7ffff0010b70 "/xinfo", msg=0x7ffff0000b60) at ... /ext/liblo/src/send.c:576
#2  0x00005555555633d9 in lo::Address::send_from (this=0x555555578170 <to>, from=0x7fffffffe300, path=..., m=0x7ffff0000b60) at ... /ext/liblo/lo/lo_cpp.h:258
#3  0x0000555555562c6a in AtoB_generic_handler (path=0x7ffff0010b70 "/xinfo", types=0x7ffff0000bc1 "", argv=0x0, argc=0, m=0x7ffff0000b60, data=0x7fffffffe300) at ... /main.cpp:15
#4  0x000055555556e06c in dispatch_method (s=0x55555558afa0, path=0x7ffff0010b70 "/xinfo", msg=0x7ffff0000b60, sock=-1) at ... /ext/liblo/src/server.c:1942
#5  0x000055555556dcc9 in dispatch_data (s=0x55555558afa0, data=0x7ffff0010b70, size=12, sock=-1) at ... /ext/liblo/src/server.c:1866
#6  0x000055555556d471 in lo_server_recv (s=0x55555558afa0) at ... /ext/liblo/src/server.c:1693
#7  0x000055555556d143 in lo_servers_recv_noblock (s=0x7ffff7a42ea8, recvd=0x7ffff7a42eb4, num_servers=1, timeout=10) at ... /ext/liblo/src/server.c:1577
#8  0x000055555556d1b0 in lo_server_recv_noblock (s=0x55555558afa0, timeout=10) at ... /ext/liblo/src/server.c:1587
#9  0x000055555556f857 in thread_func (data=0x55555558b2a0) at ... /ext/liblo/src/server_thread.c:278
#10 0x00007ffff7fa3609 in start_thread (arg=<optimized out>) at pthread_create.c:477
#11 0x00007ffff7b7f103 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

[radarsat1]

Hm, so this seems to be a weird C++ "type level" bug, if you want. What's happening is that there are overrides for send_from that take lo_server, so what should ideally happen is that the lo_server() operator of lo::Server (from which lo::ServerThread is derived) should be called to get a lo_server.

But, since on the C level, both lo_server and lo_server_thread are typedef'd as void*, the C++ compiler finds the more direct route of calling lo::ServerThread::lo_server_thread(). It then dispatches the send_data(lo_server, ..), erroneously passing the lo_server_thread as the lo_server.

(By the way you should be passing your lo::ServerThread to send_data by reference, not by pointer as you have it here.)

From looking at lo_cpp.h, I tried to handle this by providing a specific override for lo::ServerThread&. But, even if I fix your code to pass in a lo::ServerThread&, it's not getting called, instead choosing to always call the variant that takes lo_server, and I don't know why.

One fix might be to redefine lo_server and lo_server_thread to be pointers to empty, distinct structs, instead of to void*, so that the compiler doesn't see them as the same type. Theoretically that shouldn't break existing code? I'll have to give it a try.

[radarsat1]

In the meantime, this seems to work:

lo::ServerThread& server = *reinterpret_cast<lo::ServerThread*>(data);
toB.send_from(lo_server_thread_get_server(server), path, m);

[radarsat1]

I took the leap and redefined the liblo types to fix this problem: 2c1ef1c

Maybe this will warrant a major version bump. Let's see if people report incompatibilities.

[peternewman]

Maybe this will warrant a major version bump. Let's see if people report incompatibilities.

Just FYI, unfortunately this does break compatibility with our existing code @radarsat1 :
OpenLightingProject/ola#1949