Hands-on; Shared Memory I; Threads

1. Creation and Termination Threads, This example http://siber.cankaya.edu.tr/ozdogan/ParallelComputing/cfiles/code41.c program creates 5 threads with the pthread_create() routine. Each thread prints a ``Hello World!'' message, and then terminates with a call to pthread_exit(). Compile as

   gcc -o code41 code41.c -lpthread
   

2. Passing Arguments to Threads 1, This example http://siber.cankaya.edu.tr/ozdogan/ParallelComputing/cfiles/code42.c program demonstrates how to pass a simple integer to each thread.
3. Passing Arguments to Threads 2, This example http://siber.cankaya.edu.tr/ozdogan/ParallelComputing/cfiles/code43.c program shows how to setup/pass multiple arguments via a structure. Each thread receives a unique instance of the structure.
4. Passing Arguments to Threads 3 - Incorrectly, This example http://siber.cankaya.edu.tr/ozdogan/ParallelComputing/cfiles/code44.c program performs argument passing incorrectly.
   • It passes the address of variable $t$, which is shared memory space and visible to all threads.
   • The loop which creates threads modifies the contents of the address passed as an argument, possibly before the created threads can access it.

5. Joining Threads, This example http://siber.cankaya.edu.tr/ozdogan/ParallelComputing/cfiles/code45.c program demonstrates how to ``wait'' for thread completions by using the Pthread join routine. Since some implementations of Pthreads may not create threads in a joinable state, the threads in this example are explicitly created in a joinable state so that they can be joined later. Compile as

   gcc -o code45 code45.c -lpthread -lm
   

6. Exercise: Complete this http://siber.cankaya.edu.tr/ozdogan/ParallelComputing/cfiles/code57_template.c template; which is a thread program for finding the minimum of a list of integers;
   • The list is partitioned equally among the threads.
   • The size of each thread's partition is stored in the variable (partial_list_size).
   • The pointer to the start of each thread's partial list is passed to it as the pointer (list_ptr).
   • The test-update operation for minimum_value is protected by the mutex-lock minimum_value_lock.
   • Threads execute pthread_mutex_lock to gain exclusive access to the variable minimum_value.
   • Once this access is gained, the value is updated as required, and the lock subsequently released.
   • Since at any point of time, only one thread can hold a lock, only one thread can test-update the variable.