POSIX Threads Programming

 

Abstract

In shared memory multiprocessor architectures, such as SMPs, threads can be used to implement parallelism. Historically, hardware vendors have implemented their own proprietary versions of threads, making portability a concern for software developers. For UNIX systems, a standardized C language threads programming interface has been specified by the IEEE POSIX 1003.1c standard. Implementations that adhere to this standard are referred to as POSIX threads, or Pthreads.

The tutorial begins with an introduction to concepts, motivations, and design considerations for using Pthreads. Each of the three major classes of routines in the Pthreads API are then covered: Thread Management, Mutex Variables, and Condition Variables. Example codes are used throughout to demonstrate how to use most of the Pthreads routines needed by a new Pthreads programmer. The tutorial concludes with a discussion and examples of how to develop hybrid MPI/Pthreads programs in an IBM SMP environment. A lab exercise, with numerous example codes (C Language) is also included.

Level/Prerequisites: Ideal for those who are new to parallel programming with threads. A basic understanding of parallel programming in C is assumed. For those who are unfamiliar with Parallel Programming in general, the material covered in EC3500: Introduction To Parallel Computing would be helpful.

Pthreads Overview

What is a Thread?

• Technically, a thread is defined as an independent stream of instructions that can be scheduled to run as such by the operating system. But what does this mean?

• To the software developer, the concept of a "procedure" that runs independently from its main program may best describe a thread.

• To go one step further, imagine a main program (a.out) that contains a number of procedures. Then imagine all of these procedures being able to be scheduled to run simultaneously and/or independently by the operating system. That would describe a "multi-threaded" program.

• How is this accomplished?

• Before understanding a thread, one first needs to understand a UNIX process. A process is created by the operating system, and requires a fair amount of "overhead". Processes contain information about program resources and program execution state, including:
  • Process ID, process group ID, user ID, and group ID
  • Environment
  • Working directory.
  • Program instructions
  • Registers
  • Stack
  • Heap
  • File descriptors
  • Signal actions
  • Shared libraries
  • Inter-process communication tools (such as message queues, pipes, semaphores, or shared memory).

  UNIX PROCESS	THREADS WITHIN A UNIX PROCESS

• Threads use and exist within these process resources, yet are able to be scheduled by the operating system and run as independent entities largely because they duplicate only the bare essential resources that enable them to exist as executable code.

• This independent flow of control is accomplished because a thread maintains its own:
  • Stack pointer
  • Registers
  • Scheduling properties (such as policy or priority)
  • Set of pending and blocked signals
  • Thread specific data.

• So, in summary, in the UNIX environment a thread:
  • Exists within a process and uses the process resources
  • Has its own independent flow of control as long as its parent process exists and the OS supports it
  • Duplicates only the essential resources it needs to be independently schedulable
  • May share the process resources with other threads that act equally independently (and dependently)
  • Dies if the parent process dies - or something similar
  • Is "lightweight" because most of the overhead has already been accomplished through the creation of its process.

• Because threads within the same process share resources:
  • Changes made by one thread to shared system resources (such as closing a file) will be seen by all other threads.
  • Two pointers having the same value point to the same data.
  • Reading and writing to the same memory locations is possible, and therefore requires explicit synchronization by the programmer.
  
  
  

  Pthreads Overview

  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 API's.

  • Pthreads are defined as a set of C language programming types and procedure calls, implemented with a pthread.h header/include file and a thread library - though the this library may be part of another library, such as libc.

  • There are several drafts of the POSIX threads standard. It is important to be aware of the draft number of a given implementation, because there are differences between drafts that can cause problems.
  
  
  

  Pthreads Overview

  Why Pthreads?

  • The primary motivation for using Pthreads is to realize potential program performance gains.
  • When compared to the cost of creating and managing a process, a thread can be created with much less operating system overhead. Managing threads requires fewer system resources than managing processes.

    For example, the following table compares timing results for the fork() subroutine and the pthreads_create() subroutine. Timings reflect 50,000 process/thread creations, were performed with the time utility, and units are in seconds, no optimization flags.

    Note: don't expect the sytem and user times to add up to real time, because these are SMP systems with multiple CPUs working on the problem at the same time. At best, these are approximations.

    Platform	fork()			pthread_create()
    	real	user	sys	real	user	sys
    IBM 375 MHz POWER3	61.94	3.49	53.74	7.46	2.76	6.79
    IBM 1.5 GHz POWER4	44.08	2.21	40.27	1.49	0.97	0.97
    INTEL 1.4 GHz Xeon	23.46	3.92	11.59	2.70	0.55	1.66
    INTEL 1.4 GHz Itanium 2	15.98	0.38	8.93	8.21	1.84	0.96

    fork_vs_thread.txt

  • All threads within a process share the same address space. Inter-thread communication is more efficient and in many cases, easier to use than inter-process communication.

  • Threaded applications offer potential performance gains and practical advantages over non-threaded applications in several other ways:
    • Overlapping CPU work with I/O: For example, a program may have sections where it is performing a long I/O operation. While one thread is waiting for an I/O system call to complete, CPU intensive work can be performed by other threads.
    • Priority/real-time scheduling: tasks which are more important can be scheduled to supersede or interrupt lower priority tasks.
    • Asynchronous event handling: tasks which service events of indeterminate frequency and duration can be interleaved. For example, a web server can both transfer data from previous requests and manage the arrival of new requests.

  • The primary motivation for considering the use of Pthreads on an SMP architecture is to achieve optimum performance. In particular, if an application is using MPI for on-node communications, there is a potential that performance could be greatly improved by using Pthreads for on-node data transfer instead.

  • For example:
    • Both Quadrics MPI and IBM's MPI can implement on-node task communication via shared memory, which involves at least one memory copy operation (process to process).
    • For Pthreads there is no intermediate memory copy required because threads share the same address space within a single process. There is no data transfer, per se. It becomes more of a cache-to-CPU or memory-to-CPU bandwidth (worst case) situation. These speeds are much higher.

      Platform	MPI Shared Memory Bandwidth (GB/sec)	Pthreads Worst Case Memory-to-CPU Bandwidth (GB/sec)
      IBM 375 MHz POWER3	0.5	16
      IBM 1.5 GHz POWER4	2.1	11
      Intel 1.4 GHz Xeon	0.3	4.3
      Intel 1.4 GHz Itanium 2	1.8	6.4

    
    
    

    Pthreads Overview

    Designing Threaded Programs

    • In order for a program to take advantage of Pthreads, it must be able to be organized into discrete, independent tasks which can execute concurrently. For example, if routine1 and routine2 can be interchanged, interleaved and/or overlapped in real time, they are candidates for threading.

    • Tasks that may be suitable for threading include tasks that:
      • Block for potentially long waits
      • Use many CPU cycles
      • Must respond to asynchronous events
      • Are of lesser or greater importance than other tasks
      • Are able to be performed in parallel with other tasks

    • Be careful if your application uses libraries or other objects that don't explicitly guarantee thread-safeness. When in doubt, assume that they are not thread-safe until proven otherwise.

      Thread-safeness: in a nutshell, refers an application's ability to execute multiple threads simultaneously without "clobbering" shared data or creating "race" conditions. For example, suppose that you use a library routine that accesses/modifies a global structure or location in memory. If two threads both call this routine it is possible that they may try to modify this global structure/memory location at the same time. If the routine does not employ some sort of synchronization constructs to prevent data corruption, then it is not thread-safe. The implication to users of external library routines is that if you aren't 100% certain the routine is thread-safe, then you take your chances with problems that could arise.

    • Several common models for threaded programs exist:

      • Manager/worker: a single thread, the manager assigns work to other threads, the workers. Typically, the manager handles all input and parcels out work to the other tasks. At least two forms of the manager/worker model are common: static worker pool and dynamic worker pool.

      • Pipeline: a task is broken into a series of suboperations, each of which is handled in series, but concurrently, by a different thread. An automobile assembly line best describes this model.

      • Peer: similar to the manager/worker model, but after the main thread creates other threads, it participates in the work.
      
      
      

      The Pthreads API

      
      

      • The Pthreads API is defined in the ANSI/IEEE POSIX 1003.1 - 1995 standard. Unlike MPI, this standard is not freely available on the Web - it must be purchased from IEEE.

      • The subroutines which comprise the Pthreads API can be informally grouped into three major classes:

        1. Thread management: The first class of functions work directly on threads - creating, detaching, joining, etc. They include functions to set/query thread attributes (joinable, scheduling etc.)

        2. Mutexes: The second class of functions deal with synchronization, called a "mutex", which is an abbreviation for "mutual exclusion". Mutex functions provide for creating, destroying, locking and unlocking mutexes. They are also supplemented by mutex attribute functions that set or modify attributes associated with mutexes.

        3. Condition variables: The third class of functions address communications between threads that share a mutex. They are based upon programmer specified conditions. This class includes functions to create, destroy, wait and signal based upon specified variable values. Functions to set/query condition variable attributes are also included.
        4. Naming conventions: All identifiers in the threads library begin with pthread_

           Routine Prefix	Functional Group
           pthread_	Threads themselves and miscellaneous subroutines
           pthread_attr_	Thread attributes objects
           pthread_mutex_	Mutexes
           pthread_mutexattr_	Mutex attributes objects.
           pthread_cond_	Condition variables
           pthread_condattr_	Condition attributes objects
           pthread_key_	Thread-specific data keys

        5. The concept of opaque objects pervades the design of the API. The basic calls work to create or modify opaque objects - the opaque objects can be modified by calls to attribute functions, which deal with opaque attributes.

        6. The Pthreads API contains over 60 subroutines. This tutorial will focus on a subset of these - specifically, those which are most likely to be immediately useful to the beginning Pthreads programmer.

        7. The pthread.h header file must be included in each source file using the Pthreads library. For some implementations, such as IBM's AIX, it may need to be the first include file.

        8. The current POSIX standard is defined only for the C language. Fortran programmers can use wrappers around C function calls. Also, the IBM Fortran compiler provides a Pthreads API. See the XLF Language Reference, located with IBM's Fortran documentation for more information.

        9. A number of excellent books about Pthreads are available. Several of these are listed in the References section of this tutorial.

           Compiling Threaded Programs

           
           

           • Some of the more commonly used compile commands for pthreads codes are listed in the table below.

             Platform	Compiler Command	Description
             IBM AIX	xlc_r  /  cc_r	C (ANSI  /  non-ANSI)
             	xlC_r	C++
             	xlf_r -qnosave xlf90_r -qnosave	Fortran - using IBM's Pthreads API (non-portable)
             INTEL LINUX	icc -pthread	C / C++
             COMPAQ Tru64	cc -pthread	C
             	cxx -pthread	C++
             All Above Platforms	gcc -pthread	GNU C
             	g++ -pthread	GNU C++
             	guidec -pthread	KAI C
             	KCC -pthread	KAI C++

           
           
           

           Thread Management

           Creating and Terminating Threads

           Routines:

           pthread_create (thread,attr,start_routine,arg) pthread_exit (status) pthread_attr_init (attr) pthread_attr_destroy (attr)

           Creating Threads:

           • Initially, your main() program comprises a single, default thread. All other threads must be explicitly created by the programmer.

           • pthread_create creates a new thread and makes it executable. Typically, threads are first created from within main() inside a single process. Once created, threads are peers, and may create other threads.

           • pthread_create arguments:
             • thread: An opaque, unique identifier for the new thread returned by the subroutine.
             • attr: An opaque attribute object that may be used to set thread attributes. You can specify a thread attributes object, or NULL for the default values.
             • start_routine: the C routine that the thread will execute once it is created.
             • arg: A single argument that may be passed to start_routine. It must be passed by reference as a pointer cast of type void. NULL may be used if no argument is to be passed.

             • The maximum number of threads that may be created by a process is implementation dependent.

               Question: After a thread has been created, how do you know when it will be scheduled to run by the operating system?

               Thread Attributes:

               • By default, a thread is created with certain attributes. Some of these attributes can be changed by the programmer via the thread attribute object.

               • pthread_attr_init and pthread_attr_destroy are used to initialize/destroy the thread attribute object.

               • Other routines are then used to query/set specific attributes in the thread attribute object.

               • Some of these attributes will be discussed later.

               Terminating Threads:

               • There are several ways in which a Pthread may be terminated:
                 • The thread returns from its starting routine (the main routine for the initial thread).
                 • The thread makes a call to the pthread_exit subroutine (covered below).
                 • The thread is canceled by another thread via the pthread_cancel routine (not covered here).
                 • The entire process is terminated due to a call to either the exec or exit subroutines.

               • pthread_exit is used to explicitly exit a thread. Typically, the pthread_exit() routine is called after a thread has completed its work and is no longer required to exist.

               • If main() finishes before the threads it has created, and exits with pthread_exit(), the other threads will continue to execute. Otherwise, they will be automatically terminated when main() finishes.

               • The programmer may optionally specify a termination status, which is stored as a void pointer for any thread that may join the calling thread.

               • Cleanup: the pthread_exit() routine does not close files; any files opened inside the thread will remain open after the thread is terminated.

               • Discussion: In subroutines that execute to completion normally, you can often dispense with calling pthread_exit() - unless, of course, you want to pass a return code back. However, in main(), there is a definite problem if main() completes before the threads it spawned. If you don't call pthread_exit() explicitly, when main() completes, the process (and all threads) will be terminated. By calling pthread_exit() in main(), the process and all of its threads will be kept alive even though all of the code in main() has been executed.

               ---

               Example: Pthread Creation and Termination

               • This simple example code creates 5 threads with the pthread_create() routine. Each thread prints a "Hello World!" message, and then terminates with a call to pthread_exit().

                 Example Code - Pthread Creation and Termination #include <pthread.h>#include <stdio.h>#define NUM_THREADS 5void *PrintHello(void *threadid){ printf("/n%d: Hello World!/n", threadid); pthread_exit(NULL);}int main (int argc, char *argv[]){ pthread_t threads[NUM_THREADS]; int rc, t; for(t=0; t<NUM_THREADS; t++){ printf("Creating thread %d/n", t); rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t); if (rc){ printf("ERROR; return code from pthread_create() is %d/n", rc); exit(-1); } } pthread_exit(NULL);}

               Passing Arguments to Threads

               • The pthread_create() routine permits the programmer to pass one argument to the thread start routine. For cases where multiple arguments must be passed, this limitation is easily overcome by creating a structure which contains all of the arguments, and then passing a pointer to that structure in the pthread_create() routine.

               • All arguments must be passed by reference and cast to (void *).

               Question: How can you safely pass data to newly created threads, given their non-deterministic start-up and scheduling?

               Example 1 - Thread Argument Passing This code fragment demonstrates how to pass a simple integer to each thread. The calling thread uses a unique data structure for each thread, insuring that each thread's argument remains intact throughout the program. int *taskids[NUM_THREADS];for(t=0; t<NUM_THREADS; t++){ taskids[t] = (int *) malloc(sizeof(int)); *taskids[t] = t; printf("Creating thread %d/n", t); rc = pthread_create(&threads[t], NULL, PrintHello, (void *) taskids[t]); ...}

               Example 2 - Thread Argument Passing This example shows how to setup/pass multiple arguments via a structure. Each thread receives a unique instance of the structure. struct thread_data{ int thread_id; int sum; char *message;};struct thread_data thread_data_array[NUM_THREADS];void *PrintHello(void *threadarg){ struct thread_data *my_data; ... my_data = (struct thread_data *) threadarg; taskid = my_data->thread_id; sum = my_data->sum; hello_msg = my_data->message; ...}int main (int argc, char *argv[]){ ... thread_data_array[t].thread_id = t; thread_data_array[t].sum = sum; thread_data_array[t].message = messages[t]; rc = pthread_create(&threads[t], NULL, PrintHello, (void *) &thread_data_array[t]); ...}

               Example 3 - Thread Argument Passing (Incorrect) This example performs argument passing incorrectly. The loop which creates threads modifies the contents of the address passed as an argument, possibly before the created threads can access it. int rc, t;for(t=0; t<NUM_THREADS; t++) { printf("Creating thread %d/n", t); rc = pthread_create(&threads[t], NULL, PrintHello, (void *) &t); ...}

               
               
               

               Thread Management

               Joining and Detaching Threads

               Routines:

               pthread_join (threadid,status) pthread_detach (threadid,status) pthread_attr_setdetachstate (attr,detachstate) pthread_attr_getdetachstate (attr,detachstate)

               Joining:

               • "Joining" is one way to accomplish synchronization between threads. For example:

                 

               • The pthread_join() subroutine blocks the calling thread until the specified threadid thread terminates.

               • The programmer is able to obtain the target thread's termination return status if it was specified in the target thread's call to pthread_exit().

               • Two other synchronization methods, mutexes and condition variables, will be discussed later.

                 Joinable or Not?

                 • When a thread is created, one of its attributes defines whether it is joinable or detached. Only threads that are created as joinable can be joined. If a thread is created as detached, it can never be joined.

                 • The final draft of the POSIX standard specifies that threads should be created as joinable. However, not all implementations may follow this.

                 • To explicitly create a thread as joinable or detached, the attr argument in the pthread_create() routine is used. The typical 4 step process is:
                   1. Declare a pthread attribute variable of the pthread_attr_t data type
                   2. Initialize the attribute variable with pthread_attr_init()
                   3. Set the attribute detached status with pthread_attr_setdetachstate()
                   4. When done, free library resources used by the attribute with pthread_attr_destroy()
                 Detaching:

                 • The pthread_detach() routine can be used to explicitly detach a thread even though it was created as joinable.

                 • There is no converse routine.

                 Recommendations:

                 • If a thread requires joining, consider explicitly creating it as joinable. This provides portability as not all implementations may create threads as joinable by default.

                 • If you know in advance that a thread will never need to join with another thread, consider creating it in a detached state. Some system resources may be able to be freed.

                 ---

                 Example: Pthread Joining

                 Example Code - Pthread Joining This example 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. #include <pthread.h>#include <stdio.h>#define NUM_THREADS3void *BusyWork(void *null){ int i; double result=0.0; for (i=0; i<1000000; i++) { result = result + (double)random(); } printf("result = %e/n",result); pthread_exit((void *) 0);}int main (int argc, char *argv[]){ pthread_t thread[NUM_THREADS]; pthread_attr_t attr; int rc, t, status; /* Initialize and set thread detached attribute */ pthread_attr_init(&attr); pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE); for(t=0; t<NUM_THREADS; t++) { printf("Creating thread %d/n", t); rc = pthread_create(&thread[t], &attr, BusyWork, NULL); if (rc) { printf("ERROR; return code from pthread_create() is %d/n", rc); exit(-1); } } /* Free attribute and wait for the other threads */ pthread_attr_destroy(&attr); for(t=0; t<NUM_THREADS; t++) { rc = pthread_join(thread[t], (void **)&status); if (rc) { printf("ERROR; return code from pthread_join() is %d/n", rc); exit(-1); } printf("Completed join with thread %d status= %d/n",t, status); } pthread_exit(NULL);}

                 
                 
                 

                 Thread Management

                 Stack Management

                 

                 pthread_attr_getstacksize (attr, stacksize) pthread_attr_setstacksize (attr, stacksize) pthread_attr_getstackaddr (attr, stackaddr) pthread_attr_setstackaddr (attr, stackaddr)

                 Preventing Stack Problems:

                 • The POSIX standard does not dictate the size of a thread's stack. This is implementation dependent and varies.

                 • Exceeding the default stack limit is often very easy to do, with the usual results: program termination and/or corrupted data.

                 • Safe and portable programs do not depend upon the default stack limit, but instead, explicitly allocate enough stack for each thread by using the pthread_attr_setstacksize routine.

                 • The pthread_attr_getstackaddr and pthread_attr_setstackaddr routines can be used by applications in an environment where the stack for a thread must be placed in some particular region of memory.

                 Some Practical Examples at LC:

                 • Default thread stack size varies. The maximum size that can be obtained also varies.

                   System	Architecture	#CPUs	Memory (GB)	Default Batch Stack Size (bytes)	Batch System Maximum Thread Stack Size (MB) by #Threads/Node
                   					2	4	8	16	32
                   THUNDER	Intel IA64	4	8	33554432	25000	25000	25000	25000	25000
                   MCR,ALC,LILAC	Intel IA32	2	4	2097152	1072	712	352	182	92
                   UM,UV	IBM Power4	8	16	196608	260	260	260	260	260
                   FROST,WHITE	IBM Power3	16	16	98304	260	260	260	260	260

                 ---

                 Example: Stack Management

                 Example Code - Stack Management This example demonstrates how to query and set a thread's stack size. #include <pthread.h>#include <stdio.h>#define NTHREADS 4#define N 1000#define MEGEXTRA 1000000 pthread_attr_t attr; void *dowork(void *threadid){ double A[N][N]; int i,j; size_t mystacksize; pthread_attr_getstacksize (&attr, &mystacksize); printf("Thread %d: stack size = %d bytes /n", threadid, mystacksize); for (i=0; i<N; i++) for (j=0; j<N; j++) A[i][j] = ((i*j)/3.452) + (N-i); pthread_exit(NULL);} int main(int argc, char *argv[]){ pthread_t threads[NTHREADS]; size_t stacksize; int rc, t; pthread_attr_init(&attr); pthread_attr_getstacksize (&attr, &stacksize); printf("Default stack size = %d/n", stacksize); stacksize = sizeof(double)*N*N+MEGEXTRA; printf("Amount of stack needed per thread = %d/n",stacksize); pthread_attr_setstacksize (&attr, stacksize); printf("Creating threads with stack size = %d bytes/n",stacksize); for(t=0; t<NTHREADS; t++){ rc = pthread_create(&threads[t], &attr, dowork, (void *)t); if (rc){ printf("ERROR; return code from pthread_create() is %d/n", rc); exit(-1); } } printf("Created %d threads./n", t); pthread_exit(NULL);}

                 
                 
                 

                 Thread Management

                 Miscellaneous Routines

                 pthread_self () pthread_equal (thread1,thread2)

                 • pthread_self returns the unique, system assigned thread ID of the calling thread.

                 • pthread_equalcompares two thread IDs. If the two IDs are different 0 is returned, otherwise a non-zero value is returned.

                 • Note that for both of these routines, the thread identifier objects are opaque and can not be easily inspected. Because thread IDs are opaque objects, the C language equivalence operator == should not be used to compare two thread IDs against each other, or to compare a single thread ID against another value.

                   pthread_once (once_control, init_routine)

                 • pthread_once executes the init_routine exactly once in a process. The first call to this routine by any thread in the process executes the given init_routine, without parameters. Any subsequent call will have no effect.

                 • The init_routine routine is typically an initialization routine.

                 • The once_control parameter is a synchronization control structure that requires initialization prior to calling pthread_once. For example:

                   pthread_once_t once_control = PTHREAD_ONCE_INIT;

                   pthread_yield ()

                 • pthread_yield forces the calling thread to relinquish use of its processor, and to wait in the run queue before it is scheduled again.
                   
                   
                   

                   Mutex Variables

                   Overview

                   • Mutex is an abbreviation for "mutual exclusion". Mutex variables are one of the primary means of implementing thread synchronization and for protecting shared data when multiple writes occur.

                   • A mutex variable acts like a "lock" protecting access to a shared data resource. The basic concept of a mutex as used in Pthreads is that only one thread can lock (or own) a mutex variable at any given time. Thus, even if several threads try to lock a mutex only one thread will be successful. No other thread can own that mutex until the owning thread unlocks that mutex. Threads must "take turns" accessing protected data.

                   • Mutexes can be used to prevent "race" conditions. An example of a race condition involving a bank transaction is shown below:

                     Thread 1	Thread 2	Balance
                     Read balance: 00		00
                     	Read balance: 00	00
                     	Deposit 0	00
                     Deposit 0		00
                     Update balance 00+0		00
                     	Update balance 00+0	00

                   • In the above example, a mutex should be used to lock the "Balance" while a thread is using this shared data resource.

                   • Very often the action performed by a thread owning a mutex is the updating of global variables. This is a safe way to ensure that when several threads update the same variable, the final value is the same as what it would be if only one thread performed the update. The variables being updated belong to a "critical section".

                   • A typical sequence in the use of a mutex is as follows:
                     • Create and initialize a mutex variable
                     • Several threads attempt to lock the mutex
                     • Only one succeeds and that thread owns the mutex
                     • The owner thread performs some set of actions
                     • The owner unlocks the mutex
                     • Another thread acquires the mutex and repeats the process
                     • Finally the mutex is destroyed

                   • When several threads compete for a mutex, the losers block at that call - an unblocking call is available with "trylock" instead of the "lock" call. When protecting shared data, it is the programmer's responsibility to make sure every thread that needs to use a mutex does so. For example, if 4 threads are updating the same data, but only one uses a mutex, the data can still be corrupted.
                     
                     
                     

                     Mutex Variables

                     Creating and Destroying Mutexes

                     Routines:

                     pthread_mutex_init (mutex,attr) pthread_mutex_destroy (mutex) pthread_mutexattr_init (attr) pthread_mutexattr_destroy (attr)

                     Usage:

                     • Mutex variables must be declared with type pthread_mutex_t, and must be initialized before they can be used. There are two ways to initialize a mutex variable:

                       1. Statically, when it is declared. For example:
                          pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;

                       2. Dynamically, with the pthread_mutex_init() routine. This method permits setting mutex object attributes, attr.

                       The mutex is initially unlocked.

                     • The attr object is used to establish properties for the mutex object, and must be of type pthread_mutexattr_t if used (may be specified as NULL to accept defaults). The Pthreads standard defines three optional mutex attributes:
                       • Protocol: Specifies the protocol used to prevent priority inversions for a mutex.
                       • Prioceiling: Specifies the priority ceiling of a mutex.
                       • Process-shared: Specifies the process sharing of a mutex.

                       Note that not all implementations may provide the three optional mutex attributes.

                     • The pthread_mutexattr_init() and pthread_mutexattr_destroy() routines are used to create and destroy mutex attribute objects respectively.

                     • pthread_mutex_destroy() should be used to free a mutex object which is no longer needed.
                     
                     
                     

                     Mutex Variables

                     Locking and Unlocking Mutexes

                     Routines:

                     pthread_mutex_lock (mutex) pthread_mutex_trylock (mutex) pthread_mutex_unlock (mutex)

                     Usage:

                     • The pthread_mutex_lock() routine is used by a thread to acquire a lock on the specified mutex variable. If the mutex is already locked by another thread, this call will block the calling thread until the mutex is unlocked.

                     • pthread_mutex_trylock() will attempt to lock a mutex. However, if the mutex is already locked, the routine will return immediately with a "busy" error code. This routine may be useful in preventing deadlock conditions, as in a priority-inversion situation.

                     • pthread_mutex_unlock() will unlock a mutex if called by the owning thread. Calling this routine is required after a thread has completed its use of protected data if other threads are to acquire the mutex for their work with the protected data. An error will be returned if:
                       • If the mutex was already unlocked
                       • If the mutex is owned by another thread

                     • There is nothing "magical" about mutexes...in fact they are akin to a "gentlemen's agreement" between participating threads. It is up to the code writer to insure that the necessary threads all make the the mutex lock and unlock calls correctly. The following scenario demonstrates a logical error:

                           Thread 1     Thread 2     Thread 3    Lock         Lock             A = 2        A = A+1      A = A*B    Unlock       Unlock