Member mutex causes SegFault - c++

Question

I can't figure out why the code below is causing Segmentation Faults.
If I remove the call to pushLock.lock() and .unlock(), it runs fine.
#include <mutex>
#include <queue>
class FunctionQueue{
public:
FunctionQueue();
~FunctionQueue();
void pushInt(int);
private:
std::mutex pushLock;
int currentPushQueue;
std::queue<int> instructionQueues[2];
};
FunctionQueue::FunctionQueue(){
instructionQueues[0] = std::queue<int>();
instructionQueues[1] = std::queue<int>();
// pushLock.unlock();
}
FunctionQueue::~FunctionQueue(){}
void FunctionQueue::pushInt(int newArgument){
pushLock.lock();
instructionQueues[currentPushQueue].push(newArgument);
pushLock.unlock();
}
int main(int argc, char* argv[]){
FunctionQueue testQueue;
testQueue.pushInt(10);
}
The output from a gdb BackTrace was the very unhelpful:
Program received signal SIGSEGV, Segmentation fault.
0x0000000000000000 in ?? ()
(gdb) bt
#0 0x0000000000000000 in ?? ()
#1 0x00007ffff347a291 in ?? () from /lib/x86_64-linux-gnu/libdl.so.2
#2 0x00007ffff347a6d7 in ?? () from /lib/x86_64-linux-gnu/libdl.so.2
#3 0x00007ffff347a198 in dlsym () from /lib/x86_64-linux-gnu/libdl.so.2
#4 0x00007ffff7904b3e in ?? () from /usr/lib/nvidia-331/libGL.so.1
#5 0x00007ffff78e8db4 in ?? () from /usr/lib/nvidia-331/libGL.so.1
#6 0x00007ffff7dea0fd in ?? () from /lib64/ld-linux-x86-64.so.2
#7 0x00007ffff7dea223 in ?? () from /lib64/ld-linux-x86-64.so.2
#8 0x00007ffff7ddb30a in ?? () from /lib64/ld-linux-x86-64.so.2
#9 0x0000000000000001 in ?? ()
#10 0x00007fffffffe8a6 in ?? ()
#11 0x0000000000000000 in ?? ()
Any help you can give would be excellent.
Thanks in advance.

Answer 1

Completely remove the commented out code in the constructor of your class as that shouldn't be there in the first place since you haven't locked anything. Problems with this are:
1. You haven't initialized or assigned the member variable 'currentPushQueue' to any value so this code:
instructionQueues[currentPushQueue].push(newArgument);
is completely wrong unless currentPushQueue is assigned.
2. You aren't using mutexs as they are meant to be used, which is with the provided wrappers (std::unique_lock/std::lock_guard).
Try this code and respond please:
#include <mutex>
#include <queue>
class FunctionQueue
{
public:
FunctionQueue();
~FunctionQueue();
void pushInt(int);
private:
std::mutex pushLock;
int currentPushQueue = 0; // Set this variable somehow
std::queue<int> instructionQueues[2];
};
FunctionQueue::FunctionQueue()
{
instructionQueues[0] = std::queue<int>();
instructionQueues[1] = std::queue<int>();
}
FunctionQueue::~FunctionQueue() {}
void FunctionQueue::pushInt(int newArgument)
{
std::unique_lock<std::mutex> mutexLock(pushLock);
instructionQueues[currentPushQueue].push(newArgument);
// Unlocks automatically
}
int main(int argc, char* argv[])
{
FunctionQueue testQueue;
testQueue.pushInt(10);
}