POSIX Thread

llnl上关于pthread的一篇tutorial，原文链接https://computing.llnl.gov/tutorials/pthreads/

relative exercise：https://computing.llnl.gov/tutorials/pthreads/exercise.html#Exercise2

部分内容备份：

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:

#include 
 #include 
 #include 

 /*   
 The following structure contains the necessary information  
 to allow the function "dotprod" to access its input data and 
 place its output into the structure.  
 */

 typedef struct 
  {
    double      *a;
    double      *b;
    double     sum; 
    int     veclen; 
  } DOTDATA;

 /* Define globally accessible variables and a mutex */

 #define NUMTHRDS 4
 #define VECLEN 100
    DOTDATA dotstr; 
    pthread_t callThd[NUMTHRDS];
    pthread_mutex_t mutexsum;

 /*
 The function dotprod is activated when the thread is created.
 All input to this routine is obtained from a structure 
 of type DOTDATA and all output from this function is written into
 this structure. The benefit of this approach is apparent for the 
 multi-threaded program: when a thread is created we pass a single
 argument to the activated function - typically this argument
 is a thread number. All  the other information required by the 
 function is accessed from the globally accessible structure. 
 */  

 void *dotprod(void *arg)
 {

    /* Define and use local variables for convenience */

    int i, start, end, len ;
    long offset;
    double mysum, *x, *y;
    offset = (long)arg;
     
    len = dotstr.veclen;
    start = offset*len;
    end   = start + len;
    x = dotstr.a;
    y = dotstr.b;

    /*
    Perform the dot product and assign result
    to the appropriate variable in the structure. 
    */

    mysum = 0;
    for (i=start; i

Condition Variables

Overview

 

• Condition variables provide yet another way for threads to synchronize. While mutexes implement synchronization by controlling thread access to data, condition variables allow threads to synchronize based upon the actual value of data.

   

• Without condition variables, the programmer would need to have threads continually polling (possibly in a critical section), to check if the condition is met. This can be very resource consuming since the thread would be continuously busy in this activity. A condition variable is a way to achieve the same goal without polling.

   

• A condition variable is always used in conjunction with a mutex lock.

   

• A representative sequence for using condition variables is shown below.

   

  Main Thread Declare and initialize global data/variables which require synchronization (such as "count") Declare and initialize a condition variable object  Declare and initialize an associated mutex  Create threads A and B to do work
  Thread A Do work up to the point where a certain condition must occur (such as "count" must reach a specified value) Lock associated mutex and check value of a global variable Call pthread_cond_wait() to perform a blocking wait for signal from Thread-B. Note that a call to pthread_cond_wait() automatically and atomically unlocks the associated mutex variable so that it can be used by Thread-B. When signalled, wake up. Mutex is automatically and atomically locked. Explicitly unlock mutex Continue	Thread B Do work Lock associated mutex Change the value of the global variable that Thread-A is waiting upon.  Check value of the global Thread-A wait variable. If it fulfills the desired condition, signal Thread-A. Unlock mutex.  Continue
  Main Thread Join / Continue

   

 

Condition Variables

Creating and Destroying Condition Variables

 Routines:

 

pthread_cond_init (condition,attr) pthread_cond_destroy (condition) pthread_condattr_init (attr) pthread_condattr_destroy (attr)

 

 Usage:

 

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

   

  1. Statically, when it is declared. For example: 
     pthread_cond_t myconvar = PTHREAD_COND_INITIALIZER;

      

  2. Dynamically, with the pthread_cond_init() routine. The ID of the created condition variable is returned to the calling thread through the condition parameter. This method permits setting condition variable object attributes, attr.

   

• The optional attr object is used to set condition variable attributes. There is only one attribute defined for condition variables: process-shared, which allows the condition variable to be seen by threads in other processes. The attribute object, if used, must be of type pthread_condattr_t (may be specified as NULL to accept defaults).

  Note that not all implementations may provide the process-shared attribute.

   

• The pthread_condattr_init() and pthread_condattr_destroy() routines are used to create and destroy condition variable attribute objects.

   

• pthread_cond_destroy() should be used to free a condition variable that is no longer needed.

 

Condition Variables

Waiting and Signaling on Condition Variables

 Routines:

 

pthread_cond_wait (condition,mutex) pthread_cond_signal (condition) pthread_cond_broadcast (condition)

 

 Usage:

 

• pthread_cond_wait() blocks the calling thread until the specified condition is signalled. This routine should be called while mutex is locked, and it will automatically release the mutex while it waits. After signal is received and thread is awakened, mutex will be automatically locked for use by the thread. The programmer is then responsible for unlocking mutex when the thread is finished with it.

  Recommendation: Using a WHILE loop instead of an IF statement (see watch_count routine in example below) to check the waited for condition can help deal with several potential problems, such as:

  • If several threads are waiting for the same wake up signal, they will take turns acquiring the mutex, and any one of them can then modify the condition they all waited for.
  • If the thread received the signal in error due to a program bug
  • The Pthreads library is permitted to issue spurious wake ups to a waiting thread without violating the standard. 

   

• The pthread_cond_signal() routine is used to signal (or wake up) another thread which is waiting on the condition variable. It should be called after mutex is locked, and must unlock mutex in order for pthread_cond_wait() routine to complete.

   

• The pthread_cond_broadcast() routine should be used instead of pthread_cond_signal() if more than one thread is in a blocking wait state.

   

• It is a logical error to call pthread_cond_signal() before calling pthread_cond_wait().

   

Proper locking and unlocking of the associated mutex variable is essential when using these routines. For example:  Failing to lock the mutex before calling pthread_cond_wait() may cause it NOT to block.    Failing to unlock the mutex after calling pthread_cond_signal() may not allow a matching pthread_cond_wait() routine to complete (it will remain blocked).

 

---

 

Example: Using Condition Variables

• This simple example code demonstrates the use of several Pthread condition variable routines. 
• The main routine creates three threads. 
• Two of the threads perform work and update a "count" variable. 
• The third thread waits until the count variable reaches a specified value.

  #include 
   #include 
   #include 
  
   #define NUM_THREADS  3
   #define TCOUNT 10
   #define COUNT_LIMIT 12
  
   int     count = 0;
   int     thread_ids[3] = {0,1,2};
   pthread_mutex_t count_mutex;
   pthread_cond_t count_threshold_cv;
  
   void *inc_count(void *t) 
   {
     int i;
     long my_id = (long)t;
  
     for (i=0; i