Thread-related Error with mosquitto_lib_init in a Multi-Threaded Environment

[KudoWu]

Hello,

First, let me provide you with information about my operating environment:

system: Linux xxx 4.4.60 3, SMP PREEMPT, armv7l GNU/Linux
libmosquitto version: release/1.6(libmosquitto.so.1.6.15)
libssl version: OpenSSL 1.1.1g

I encountered a problem when using mosquitto_lib_init in my code. To give you some context: I created a separate thread to handle MQTT operations, including initialization, connection, and publishing. I initiate the MQTT client using mosquitto_lib_init, and after a successful connection, I call mosquitto_loop_start. Importantly, there are no other threads to operate it.

When running the code, I encountered the following issue, as identified through gdb:

#0  0x76fd7580 in pthread_rwlock_trywrlock () from /lib/ld-musl-arm.so.1
#1  0x76fd7430 in pthread_rwlock_timedwrlock () from /lib/ld-musl-arm.so.1
#2  0x76ebee00 in CRYPTO_STATIC_MUTEX_lock_write () from /usr/lib/libcrypto.so
#3  0x76e66bd0 in CRYPTO_get_ex_new_index () from /usr/lib/libcrypto.so
#4  0x76f438a4 in net__init () at /home/xxxx/Work/workspace/cross_dynamiclink_lib_src/mosquitto/lib/net_mosq.c:149
#5  0x76f417cc in mosquitto_lib_init () at /home/xxxx/Work/workspace/cross_dynamiclink_lib_src/mosquitto/lib/mosquitto.c:77
#6  0x54f78aa8 in xx::module::ModuleMqttTsp::Start() ()
#7  0x54f455c4 in void std::_Mem_fn_base<void (xx::module::ModuleMqttTsp::*)(), true>::operator()<, void>(xx::module::ModuleMqttTsp*) const ()
#8  0x54f45258 in void std::_Bind_simple<std::_Mem_fn<void (xx::module::ModuleMqttTsp::*)()> (xx::module::ModuleMqttTsp*)>::_M_invoke<0u>(std::_Index_tuple<0u>) ()
#9  0x54f44fc0 in std::_Bind_simple<std::_Mem_fn<void (xx::module::ModuleMqttTsp::*)()> (xx::module::ModuleMqttTsp*)>::operator()() ()
#10 0x54f44df0 in std::thread::_Impl<std::_Bind_simple<std::_Mem_fn<void (xx::module::ModuleMqttTsp::*)()> (xx::module::ModuleMqttTsp*)> >::_M_run() ()
#11 0x76c432fc in ?? () from /usr/lib/libstdc++.so.6
Cannot access memory at address 0x7d0f00

Upon inspecting the mosquitto.h documentation, I found the following hint:

 *	If your application uses threads you must use <mosquitto_threaded_set> to
 *	tell the library this is the case, otherwise it makes some optimisations
 *	for the single threaded case that may result in unexpected behaviour for
 *	the multi threaded case.

I should mention that even though my application operates in a multi-threaded environment, I only have one thread interacting with the MQTT client. Therefore, it appears that using mosquitto_lib_init is not incorrect in my scenario.

However, I experimented with an alternative approach in my code:

-    mosquitto_lib_init();
+    // mosquitto_lib_init();
     mosq = mosquitto_new(token.c_str(), true, NULL);
+    mosquitto_threaded_set(mosq, true);

Surprisingly, this modification resulted in successful execution without any errors. Although there is currently a problem that the server cannot receive it in publish, I think this is my own problem.

I could not find any relevant entries in the ChangeLog.txt regarding changes to mosquitto_lib_init in subsequent versions of the library. Therefore, I am seeking your assistance in understanding why this problem occurred and whether my modification is the correct solution.

Thank you very much for your support!

Bye

KudoWu