Topic : Multi-Threaded Programming
Author : LUPG
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Multi-Threaded Programming With POSIX Threads

Before We Start...

This tutorial is an attempt to help you become familiar with multi-threaded programming with the POSIX threads (pthreads) library, and attempts to show how its features can be used in "real-life" programs. It explains the different tools defined by the library, shows how to use them, and then gives an example of using them to solve programming problems. There is an implicit assumption that the user has some theoretical familiarity with paralell programming (or multi-processing) concepts. Users without such background might find the concepts harder to grasp. A seperate tutorial will be prepared to explain the theoreticl background and terms to those who are familiar only with normal "serial" programming.

I would assume that users which are familiar with asynchronous programming models, such as those used in windowing environments (X, Motif), will find it easier to grasp the concepts of multi-threaded programming.

When talking about POSIX threads, one cannot avoid the question "Which draft of the POSIX threads standard shall be used?". As this threads standard has been revised over a period of several years, one will find that implementations adhering to different drafts of the standard have a different set of functions, different default values, and different nuances. Since this tutorial was written using a Linux system with the kernel-level LinuxThreads library, v0.5, programmers with access to other systems, using different versions of pthreads, should refer to their system's manuals in case of incompatibilities. Also, since some of the example programs are using blocking system calls, they won't work with user-level threading libraries (refer to our parallel programming theory tutorial for more information -
Having said that, i'd try to check the example programs on other systems as well (Solaris 2.5 comes to mind), to make it more "cross-platform".

What Is a Thread? Why Use Threads

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).

The advantage of using a thread group instead of a normal serial program is that several operations may be carried out in parallel, and thus events can be handled immediately as they arrive (for example, if we have one thread handling a user interface, and another thread handling database queries, we can execute a heavy query requested by the user, and still respond to user input while the query is executed).

The advantage of using a thread group over using a process group is that context switching between threads is much faster then context switching between processes (context switching means that the system switches from running one thread or process, to running another thread or process). Also, communications between two threads is usually faster and easier to implement then communications between two 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. Another advantage of using processes is that they can run on different machines, while all the threads have to run on the same machine (at least normally).

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 */
do_loop(void* data)
    int i;

    int i;   /* counter, to print numbers */
    int j;   /* counter, for delay        */
    int me = *((int*)data);     /* thread identifying number */

    for (i=0; i<10; i++) {
 for (j=0; j<500000; j++) /* delay loop */
        printf("'%d' - Got '%d'\n", me, i);

    /* terminate the thread */

/* like any C program, program's execution begins in main */
main(int argc, char* argv[])
    int        thr_id;         /* thread ID for the newly created thread */
    pthread_t  p_thread;       /* thread's structure                     */
    int        a         = 1;  /* thread 1 identifying number            */
    int        b         = 2;  /* thread 2 identifying number            */

    /* create a new thread that will execute 'do_loop()' */
    thr_id = pthread_create(&p_thread, NULL, do_loop, (void*)&a);
    /* run 'do_loop()' in the main thread as well */
    /* NOT REACHED */
    return 0;

A few notes should be mentioned about this program:

1. Note that the main program is also a thread, so it executes the do_loop() function in parallel to the thread it creates.
2. pthread_create() gets 4 parameters. The first parameter is used by pthread_create() to supply the program with information about the thread. The second parameter 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. The third parameter is the name of the function that the thread will start executing. The forth parameter is an argument to pass to this function. Note the cast to a 'void*'. It is not required by ANSI-C syntax, but is placed here for clarification.
3. The delay loop inside the function is used only to demonstrate that the threads are executing in parallel. Use a larger delay value if your CPU runs too fast, and you see all the printouts of one thread before the other.
4. The call to pthread_exit() causes the current thread to exit and free any thread-specific resources it is taking. There is no need to use this call at the end of the thread's top function, since when it returns, the thread would exit automatically anyway. This function is useful if we want to exit a thread in the middle of its execution.

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 pthread_create.c -o pthread_create -lpthread

Note that for some of the programs later on this tutorial, one may need to add a '-D_GNU_SOURCE' flag to this compile line, to get the source compiled.

The source code for this program may be found in the pthread_create.c file.

Synchronizing Threads With Mutexes

One of the basic problems when running several threads that use the same memory space, is making sure they don't "step on each other's toes". By this we refer to the problem of using a data structure from two different threads.

For instance, consider the case where two threads try to update two variables. One tries to set both to 0, and the other tries to set both to 1. If both threads would try to do that at the same time, we might get with a situation where one variable contains 1, and one contains 0. This is because a context-switch (we already know what this is by now, right?) might occur after the first tread zeroed out the first variable, then the second thread would set both variables to 1, and when the first thread resumes operation, it will zero out the second variable,

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