Segmentation fault in C++ thread

Question

I'm trying to set up a basic threaded class in C++, but I'm getting a seg fault when I try to create a thread. Here's what GDB reports:

Program received signal SIGSEGV, Segmentation fault.
0x0000000000401b68 in StartThread (pFunction=
    0x401ad2 <FindPrimesThread(void*)>, pLimit=5000000) at Thread.cpp:35
35          state->mLimit = pLimit;

when I try to call it like this:

ThreadState *primesState = StartThread(FindPrimesThread, 5000000);

Here's my code:

Thread.hpp

#ifndef THREAD_HPP
#define THREAD_HPP

#include <pthread.h>
#include "Types.hpp"

typedef struct {
    ulong       mLimit;     // Upper limit of numbers to test 
    int         mStarted;   // True if the thread started successfully
    int         mExitCode;  // Thread exit code
    pthread_t   mThreadId;  // Thread ID
} ThreadState;

// Defines a type named ThreadFunction which is a pointer to a function with void * as the parameter and
// void * as the return value.
typedef void *(*ThreadFunction)(void *);

ThreadState *StartThread
    (
    ThreadFunction const pFunction,  // Pointer to the thread function
    ulong const          pLimit      // Upper limit of numbers to test
    );

#endif

Thread.cpp

#include "Amicable.hpp"
#include "Keith.hpp"
#include "Main.hpp"
#include "Prime.hpp"
#include "Thread.hpp"

ThreadState *StartThread
    (
    ThreadFunction const pFunction,  // Pointer to the thread function
    ulong const          pLimit      // Upper limit of numbers to test
    ) {
        ThreadState *state;
        state->mLimit = pLimit;
        pthread_t threadId;
        state->mStarted = pthread_create(&threadId, NULL, pFunction, (void *)state);
        if(state->mStarted == 0){
            state->mThreadId = threadId;
        }
        return state;
    }

Any idea on what's going wrong here?

Answer 1

ThreadState *state;
state->mLimit = pLimit;

You are writing to memory you haven't allocated

Answer 2

You have an uninitialized pointer in ThreadState. On line 35, you create a pointer to a ThreadState, but you never assign that pointer to point to any ThreadState object.

Remember, a pointer is just a memory address. "ThreadState*" just means, "this is a memory address, and we can interpret the data in memory at the address I hold to be a ThreadState object."

Perhaps you meant to do "ThreadState *state = new ThreadState();"? Don't forget, someone will need to go and delete that ThreadState object when they're done using it so as not to leak memory!