POSIX Threads

  1. Threads are fun !
  2. What are threads?
  3. What are pthreads?
  4. Threads are efficient ...
  5. Threads are useful - example applications
  6. Creating and destroying threads
  7. Exercises   1   2   3   4   5


Threads are fun !

Knowing how to properly use threads should be part of every computer science and engineering student repertoire. This tutorial is an attempt to help you become familiar with multi-threaded programming with the POSIX (Portable Operating System Interface) threads, or pthreads. This tutorial explains the different tools defined by the pthread library, shows how to use them, and gives examples of using them to solve real life programming problems.


What are threads?

Technically, a thread is defined as an independent stream of instructions that can be scheduled to run as such by the operating system.

A thread is a semi-process that has its own stack, and executes a given piece of code. Unlike a real process, the thread normally shares its memory with other threads (where as for processes we usually have a different memory area for each one of them). A Thread Group is a set of threads all executing inside the same process. They all share the same memory, and thus can access the same global variables, same heap memory, same set of file descriptors, etc. All these threads execute in parallel (i.e. using time slices, or if the system has several processors, then really in parallel).

          Single- and Multi-Threaded Processes


What are pthreads?

Historically, hardware vendors have implemented their own proprietary versions of threads. These implementations differed substantially from each other, making it difficult for programmers to develop portable threaded applications.

In order to take full advantage of the capabilities provided by threads, a standardized programming interface was required. For UNIX systems, this interface has been specified by the IEEE POSIX 1003.1c standard (1995). Implementations which adhere to this standard are referred to as POSIX threads, or Pthreads. Most hardware vendors now offer Pthreads in addition to their proprietary threads.


Threads are efficient ...

If implemented correctly, threads have some advantages over processes. Compared to the standard fork(), threads carry a lot less overhead.

Remember that fork() produces a second copy of the calling process. The parent and the child are completely independent, each with its own address space, with its own copies of its variables, which are completely independent of the same variables in the other process.

Threads share a common address space, thereby avoiding a lot of the inefficiencies of multiple processes.

On the other hand, because threads in a group all use the same memory space, if one of them corrupts the contents of its memory, other threads might suffer as well. With processes, the operating system normally protects processes from one another, and thus if one corrupts its own memory space, other processes won't suffer.


Threads are useful - example applications

Example 1: A responsive user interface

One area in which threads can be very helpful is in user-interface programs. These programs are usually centered around a loop of reading user input, processing it, and showing the results of the processing. The processing part may sometimes take a while to complete, and the user is made to wait during this operation. By placing such long operations in a separate thread, while having another thread to read user input, the program can be more responsive. It may allow the user to cancel the operation in the middle.

Example 2: A graphical interface

In graphical programs the problem is more severe, since the application should always be ready for a message from the windowing system telling it to repaint part of its window. If it's too busy executing some other task, its window will remain blank, which is rather ugly. In such a case, it is a good idea to have one thread handle the message loop of the windowing systm and always ready to get such repain requests (as well as user input). Whenever this thread sees a need to do an operation that might take a long time to complete (say, more then 0.2 seconds in the worse case), it will delegate the job to a separate thread.

Example 3 : A Web server


Creating and destroying threads

When a multi-threaded program starts executing, it has one thread running, which executes the main() function of the program. This is already a full-fledged thread, with its own thread ID. In order to create a new thread, the program should use the pthread_create() function. Here is how to use it:



#include <stdio.h>       /* standard I/O routines                 */
#include <pthread.h>     /* pthread functions and data structures */

/* function to be executed by the new thread */
void* PrintHello(void* data)
{
    int my_data = (int)data;     	/* data received by thread */

    pthread_detach(pthread_self());
    printf("Hello from new thread - got %d\n", my_data);
    pthread_exit(NULL);			/* terminate the thread */
}

/* like any C program, program's execution begins in main */
int main(int argc, char* argv[])
{
    int        rc;         	/* return value                           */
    pthread_t  thread_id;     	/* thread's ID (just an integer)          */
    int        t         = 11;  /* data passed to the new thread          */

    /* create a new thread that will execute 'PrintHello' */
    rc = pthread_create(&thread_id, NULL, PrintHello, (void*)t);  
    if(rc)			/* could not create thread */
    {
        printf("\n ERROR: return code from pthread_create is %d \n", rc);
        exit(1);
    }
    printf("\n Created new thread (%d) ... \n", thread_id);
    
    pthread_exit(NULL);		/* terminate the thread */
}

Understanding the simple threaded program above. While it does not do anything useful, it will help you understand how threads work. Let us take a step by step look at what the program does.

  1. In main() we declare a variable called thread_id, which is of type pthread_t. This is basically an integer used to identify the thread in the system. After declaring thread_id, we call the pthread_create() function to create a real, living thread.

  2. pthread_create() gets 4 arguments The first argument is a pointer to thread_id, used by pthread_create() to supply the program with the thread's identifier. The second argument is used to set some attributes for the new thread. In our case we supplied a NULL pointer to tell pthread_create() to use the default values. Notice that PrintHello() accepts a void * as an argument and also returns a void * as a return value. This shows us that it is possible to use a void * to pass an arbitrary piece of data to our new thread, and that our new thread can return an arbitrary piece of data when it finishes. How do we pass our thread an arbitrary argument? Easy. We use the fourth argument to the pthread_create() call. If we do not want to pass any data to the new thread, we set the fourth argument to NULL. pthread_create() returns zero on success and a non-zero value on failure.

  3. After pthread_create() successfully returns, the program will consist of two threads. This is because the main program is also a thread and it executes the code in the main() function in parallel to the thread it creates. Think of it this way: if you write a program that does not use POSIX threads at all, the program will be single-threaded (this single thread is called the "main" thread).

  4. The call to pthread_exit() causes the current thread to exit and free any thread-specific resources it is taking.

In order to compile a multi-threaded program using gcc, we need to link it with the pthreads library. Assuming you have this library already installed on your system, here is how to compile our first program:

      gcc hello.c -o hello -lpthread
The source code for this program may be found in the hello.c file.


Exercise 1. Create a directory called pthreads in your class Unix directory and download hello.c into the pthreads directory. Compile the source code and run the hello executable. The ouput should be similar to
      Created new thread (4) ...
      Hello from new thread - got 11

Exercise 2. A thread can get its own thread id by calling pthread_self(), which returns the thread id:
      pthread_t pthread_self();
Use it as
      pthread_t tid;
      tid = pthread_self();
Modify the code for hello.c to print out the thread id for both threads. Recompile and run the hello executable. The new ouput should be similar to
      I am thread 1. Created new thread (4) ...
      Hello from new thread 4 - got 11
Now modify the code so that the main thread passes its own thread id to the new thread it creates. Recompile and run the hello executable. The ouput should be similar to
      I am thread 1. Created new thread (4) ...
      Hello from new thread 4 - got 1

Exercise 3. There are several ways for threads to terminate. One way to safely terminate is to call the pthread_exit routine (the equivalent of exit for processeas). In this exercise, modify the your hello.c program as follows. In the PrintHello routine, add a line before the printf call sleep(1);. This should be the first line of the function. In the main function, comment out the last statement line which contains the pthread_exit call. Recompile and run the hello executable.

What happens? Why?

Now, put the pthread_exit call back in the main program, but remove it from the PrintHello routine. Also add the sleep call to the main routine, just before the second printf call, and remove it from the PrintHello routine. Recompile and run the hello executable.

What happens? Why?

It is necessary to use pthread_exit at the end of the main program. Otherwise, when it exits, all running threads will be killed.


Exercise 4. The pthread_join() function for threads is the equivalent of wait() for processes. A call to pthread_join blocks the calling thread until the thread with identifier equal to the first argument terminates.

#include <stdio.h>       /* standard I/O routines                 */
#include <pthread.h>     /* pthread functions and data structures */

void* PrintHello(void* data)
{
    pthread_t tid = (pthread_t)data;    /* data received by thread */

    pthread_join(tid, NULL);            /* wait for thread tid     */
    printf("Hello from new thread %d - got %d\n", pthread_self(), data);
    pthread_exit(NULL);	                /* terminate the thread    */
}

/* like any C program, program's execution begins in main */
int main(int argc, char* argv[])
{
    int        rc;         	       /* return value                  */
    pthread_t  thread_id;     	       /* thread's ID (just an integer) */
    int        tid;

    tid = pthread_self();

    rc = pthread_create(&thread_id, NULL, PrintHello, (void*)tid);  
    if(rc)                             /* could not create thread */
    {
        printf("\n ERROR: return code from pthread_create is %d \n", rc);
        exit(1);
    }
    sleep(1);
    printf("\n Created new thread (%d) ... \n", thread_id);
    pthread_exit(NULL);		
}
The first argument to pthread_join() is the identifier of the thread to join. The second argument is a void pointer.
      pthread_join(pthread_t tid, void * return_value);
If the return_value pointer is non-NULL, pthread_join will place at the memory location pointed to by return_value, the value passed by the thread tid through the pthread_exit call. Since we don't care about return value of the main thread, we set it to NULL.

Recompile and run the executable for the above code. Is the otuput what you expected?


Note. At any point in time, a thread is either joinable or detached (default state is joinable). Joinable threads must be reaped or killed by other threads (using pthread_join) in order to free memory resources. Detached threads cannot be reaped or killed by other threads, and resources are automatically reaped on termination. So unless threads need to synchronize among themselves, it is better to call

      pthread_detach(pthread_self());
instead of pthread_join.


Exercise 5. Write a program hellomany.c that will create a number N of threads specified in the command line, each of which prints out a hello message and its own thread ID. To see how the execution of the threads interleaves, make the main thread sleep for 1 second for every 4 or 5 threads it creates. The output of your code should be similar to:
       I am thread 1. Created new thread (4) in iteration 0...
       Hello from thread 4 - I was created in iteration 0
       I am thread 1. Created new thread (6) in iteration 1...
       I am thread 1. Created new thread (7) in iteration 2...
       I am thread 1. Created new thread (8) in iteration 3...
       I am thread 1. Created new thread (9) in iteration 4...
       I am thread 1. Created new thread (10) in iteration 5...
       Hello from thread 6 - I was created in iteration 1
       Hello from thread 7 - I was created in iteration 2
       Hello from thread 8 - I was created in iteration 3
       Hello from thread 9 - I was created in iteration 4
       Hello from thread 10 - I was created in iteration 5
       I am thread 1. Created new thread (11) in iteration 6...
       I am thread 1. Created new thread (12) in iteration 7...
       Hello from thread 11 - I was created in iteration 6
       Hello from thread 12 - I was created in iteration 7
Do not look at the
solution. Try it on your own first.
Acknowledgement: Fragments of these notes have been taken from the developerWorks tutorial and the programming tutorial on Posix threads.