C ++空指针

Question

pthread_mutex_t mutexREAD;
int main(int argc, char *argv[]){
    pthread_t READERthreads;
    pthread_mutex_init(&mutexREAD, NULL);
    string *fname;
    cin>> *fname;
    pthread_create(&READERthreads, NULL, reader_thread, (void*) fname);
}
void *reader_thread(void *param){
    string fname = *(string *) param;
    cout<<"filename is "<< fname<<endl;
    ifstream myfile(fname.c_str());
    return NULL;
 }

The code above throw segmentation fault. I probably messed up with my pointers, but I do not know where went wrong and how can I fix it?

Answer 1

Two problems: The first and probably cause of the crash is that you have a pointer fname but it doesn't actually point anywhere. That leads to undefined behavior when you dereference it.

Unless you need to pass different strings to different threads, it doesn't need to be a pointer, just use the address-of operator & when calling pthread_create .

You can of course use std::thread instead, and pass the string by value and not have to worry about pointers at all:

...
std::string fname;
std::cin >> fname;

std::thread READERthread{&reader_thread, fname);
READERthread.join();
...

And

void reader_thread(std::string fname)
{
    ...
}

---

The second problem is that you don't wait for the thread to finish before you exit the process. That will kill the thread. You either need to join the thread, which waits for it to exit. Or you can detach it, and only exit the main thread (not the process), which leaves the other thread running in the background.

Answer 2

You declared a pointer to string , just use a string and pass its address.

pthread_mutex_t mutexREAD;
int main(int argc, char *argv[]){
    pthread_t READERthreads;
    pthread_mutex_init(&mutexREAD, NULL);
    string fname;
    cin>> fname;
    pthread_create(&READERthreads, NULL, reader_thread, (void*) &fname);
    pthread_join(&READERthreads,NULL);
}
void *reader_thread(void *param){
    string fname = *(string *) param;
    cout<<"filename is "<< fname<<endl;
    ifstream myfile(fname.c_str());
    return NULL;
 }

The other problem is that you don't wait for thread termination, then the stack allocated string may be deallocated before the thread had time to use it... Use pthread_join in the spawning thread.