How to “multithread” C code

Question

I have a number crunching application written in C. It is kind of a main loop that for each value calls, for increasing values of \"i\", a function that performs some calcul

Answer 1

a good exercise for learning concurrent programming in any language would be to work on a thread pool implementation.
In this pattern you create some threads in advance. Those threads are treated as an resource. A thread pool object/structure is used to assign user defined task to those threads for execution. When the task is finished you can collect it's results. You can use thread pool as a general purpose design pattern for concurrency. The main idea could look similar to

#define number_of_threads_to_be_created 42
// create some user defined tasks
Tasks_list_t* task_list_elem = CreateTasks();
// Create the thread pool with 42 tasks
Thpool_handle_t* pool = Create_pool(number_of_threads_to_be_created);

// populate the thread pool with tasks
for ( ; task_list_elem; task_list_elem = task_list_elem->next) {
   add_a_task_to_thpool (task_list_elem, pool);
}
// kick start the thread pool
thpool_run (pool);

// Now decide on the mechanism for collecting the results from tasks list.
// Some of the candidates are:
// 1. sleep till all is done (naive)
// 2. pool the tasks in the list for some state variable describing that the task has
//    finished. This can work quite well in some situations
// 3. Implement signal/callback mechanism that a task can use to signal that it has 
//    finished executing.

The mechanism for collecting data from tasks and the amount of threads used in pool should be chosen to reflect your requirements and the capabilities of the hardware and runtime environment.
Also please note that this pattern does not say anything how you should "synchronize" your tasks with each other/outside surroundings. Also error handling can be a bit tricky (example: what to do when one task fails). Those two aspects need to be thought in advance - they can restrict usage of thread pool pattern.

About thread pool:
http://en.wikipedia.org/wiki/Thread_pool_pattern
http://docs.oracle.com/cd/E19253-01/816-5137/ggedn/index.html

A good literature about pthreads to get going:
http://www.advancedlinuxprogramming.com/alp-folder/alp-ch04-threads.pdf