CSCI 3500: Studio 9

Threads

In this studio, you will:

1. Create threads using Pthreads
2. Join with (wait for) threads to finish their work
3. Create a structure to hold thread parameters
4. Pass parameters to and from threads using void* pointers

Please complete the required exercises below, as well as any optional enrichment exercises that you wish to complete.

As you work through these exercises, please record your answers in a text file. When finished, submit your work by sending your text file and source code to dferry_submit@slu.edu with the phrase Threads in the subject line.

Make sure that the name of each person who worked on these exercises is listed in the first answer, and make sure you number each of your responses so it is easy to match your responses with each exercise.

Required Exercises

1. As the answer to the first exercise, list the names of the people who worked together on this studio.

2. Open up the manual page for pthread_create(). The third parameter to this function specifies a new thread's entry point- the location at which the new thread begins execution.This parameter will look a little unusual, because it's a special data type called a function pointer. The function pointer type is given as:

   return_type (* function_name ) ( argument_types )

   What is the return type and the expected arguments for the thread entry function?

3. Create a new progam. Before writing your main function, write a function called thread_entry with the signature specified in the previous exercise. This function should print a message and then return NULL.

4. Now write your main() function and make a call to the function pthread_create(). For the time being, leave the second and fourth parameters of this function NULL. Be sure to check the return value for errors. Note that this function does not set errno and you cannot use perror().

   After you make a call to pthread_create(), make a call to pthread_join() to wait for the created thread to finish. The first argument of this function needs the value from parameter one of pthread_create(). The second argument of this function can be left NULL.

   In order to compile your program you will need to add the -pthread option to GCC. Copy and paste your program output.

5. Now you can create and run code inside a thread, but this really isn't useful unless you can communicate with the thread. There are a variety of ways to pass data between threads, but we will use a specific method called void* passing. To do this we need to define a struct, which is a collection of variables that are grouped together. The syntax for defining a struct is as follows:

   struct struct_name{
  arg1_type arg1;
  arg2_type arg2;
  ...
  argN_type argN;
};

   And you can then declare a struct for use in your program such:

     struct struct_name variable_name
  variable_name.arg1 = value1; //direct access
  variable_name.arg2 = value2; //direct access


   Keep in mind that you can also access the values of this struct through a pointer to the structure as well. This will be important in your thread entry function:
     struct struct_name *struct_ptr = &variable_name;
  struct_ptr->arg1 = value1; //pointer access
  struct_ptr->arg2 = value2; //pointer access


   Define a struct with three members: an integer called arg1, a character buffer called arg2, and another integer called ret. Copy and paste your struct definition.

6. Now create a struct in your main program and fill it with values. Your thread can access this struct by passing a pointer through the fourth argument of pthread_create(). Create a pointer to your struct as illustrated in the last exercise and do so.

   A pointer given to the pthread_create() function in this way will be available to your thread entry function as the void* parameter. However, the compiler doesn't know how to access the data pointed to by a void* pointer. To access the data inside the struct we need to cast the pointer type back to a struct pointer, as so:

   void* thread_entry( void* args ){
  struct struct_name* arg_ptr = (struct struct_name*) args;
  printf("thread arg1: %d\n", arg_ptr->arg1);
  printf("thread arg2: %s\n", arg_ptr->arg2);
  ...
}

   Print out the values of your struct arguments in the newly created thread. Copy and paste the program output.

7. Now use the same structure to return a value to the main() function. Write a value to the third member of your structure somewhere in your thread, and then read that value in main() function after you have executed pthread_join().

8. Finally, parameterize your program to create N threads. This means you will need to create an array of N pthread_t identifiers, and an array of N struct struct_name structures to pass arguments. Then, call pthread_create() for N iterations, and then pthread_join() for N iterations.

   You can easily write multiple, different strings to each thread using the snprintf() function, which works just like the printf() function. For example:

   unsigned bufSize = 100;
struct struct_name args[num_threads];
for( i = 0; i < num_threads; i++ ){
snprintf(args[i].arg2, bufSize, "This is thread %d", i); }

   Copy and paste your program output for five threads when you have finished.

9. Run your prgram (with five threads) several times. What do you notice about the output ordering? Why do you think this is?

Optional Enrichment Exercises

1. No optional exercises