pthread_create segmentation fault for file reading

Question

I have a class Class1 with a method Class1::Run that is called sequentially. Inside this method I want to load some text files, sometimes big ones, and perform some operations. Since this text files take some time to load, I would like to load them in a different thread and perform some alternative operations while waiting for them to be ready. In this thread I would like to call another class' method to load the files.

I have created the following struct:

struct args {
   int cmd;
   Class2 * object2;
}

This is the structure of Class1::Run :

pthread load_thread;
struct args thread_args;
thread_args.cmd = 0; //this is used to specify the kind of file to be loaded
thread_args.object2 = object2;
pthread_create( &load_thread, NULL, &ThreadHelper, (void*) &thread_args );

object2 has been declared in Class1 as Class2 * object2 and initialized somewhere else.

The ThreadHelper function has been declared as static inside Class1 and it is structured as follows:

void * Class1::ThreadHelper(void * thread_args) {
   struct args * targs = (struct args*) thread_args;
   targs->object2->LoadFile(targs->cmd);
}

All this is causing a segmentation fault. How can I solve? Also, since the Run function runs sequentially, could it be a problem if a new thread is created before the next one has finished?

Answer 1

The problem is that you pass to thread a pointer to local variable thread_args . You should make it either global variable - move it outside function, or allocate it on heap, ie:

pthread load_thread;
struct args* thread_args=new args;
thread_args->cmd = 0; //this is used to specify the kind of file to be loaded
thread_args->object2 = object2;
pthread_create( &load_thread, NULL, &ThreadHelper, (void*) thread_args );

and dont forget to delete it inside thread function after you are done with its work (you may use std::unique_ptr to make it automatic).

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now I see you could move struct args thread_args; to Class1 - same as object2 .