Member mutex causes SegFault

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.

@KerrekSB can you explain to me what's wrong with it? Other than not initialing currentPushQueue? — DrVonTrap
@user1137058: It's not wrong, it's just pointless. Almost every line of code you wrote does the same as implicit default behaviour and is thus redundant. — Kerrek SB

Michael A. Michael A. · Accepted Answer · 2015-07-11T22:56:36

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);
}

Member mutex causes SegFault

1 Answers