Pthread Mutex Init Sample Cover Letter

pthread_mutex_lock(3) - Linux man page

Prolog

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

Name

pthread_mutex_lock, pthread_mutex_trylock, pthread_mutex_unlock - lock and unlock a mutex

Synopsis

#include <pthread.h>

int pthread_mutex_lock(pthread_mutex_t *mutex);
int pthread_mutex_trylock(pthread_mutex_t *mutex);
int pthread_mutex_unlock(pthread_mutex_t *mutex);

Description

The mutex object referenced by mutex shall be locked by calling pthread_mutex_lock(). If the mutex is already locked, the calling thread shall block until the mutex becomes available. This operation shall return with the mutex object referenced by mutex in the locked state with the calling thread as its owner.

If the mutex type is PTHREAD_MUTEX_NORMAL, deadlock detection shall not be provided. Attempting to relock the mutex causes deadlock. If a thread attempts to unlock a mutex that it has not locked or a mutex which is unlocked, undefined behavior results.

If the mutex type is PTHREAD_MUTEX_ERRORCHECK, then error checking shall be provided. If a thread attempts to relock a mutex that it has already locked, an error shall be returned. If a thread attempts to unlock a mutex that it has not locked or a mutex which is unlocked, an error shall be returned.

If the mutex type is PTHREAD_MUTEX_RECURSIVE, then the mutex shall maintain the concept of a lock count. When a thread successfully acquires a mutex for the first time, the lock count shall be set to one. Every time a thread relocks this mutex, the lock count shall be incremented by one. Each time the thread unlocks the mutex, the lock count shall be decremented by one. When the lock count reaches zero, the mutex shall become available for other threads to acquire. If a thread attempts to unlock a mutex that it has not locked or a mutex which is unlocked, an error shall be returned.

If the mutex type is PTHREAD_MUTEX_DEFAULT, attempting to recursively lock the mutex results in undefined behavior. Attempting to unlock the mutex if it was not locked by the calling thread results in undefined behavior. Attempting to unlock the mutex if it is not locked results in undefined behavior.

The pthread_mutex_trylock() function shall be equivalent to pthread_mutex_lock(), except that if the mutex object referenced by mutex is currently locked (by any thread, including the current thread), the call shall return immediately. If the mutex type is PTHREAD_MUTEX_RECURSIVE and the mutex is currently owned by the calling thread, the mutex lock count shall be incremented by one and the pthread_mutex_trylock() function shall immediately return success.

The pthread_mutex_unlock() function shall release the mutex object referenced by mutex. The manner in which a mutex is released is dependent upon the mutex's type attribute. If there are threads blocked on the mutex object referenced by mutex when pthread_mutex_unlock() is called, resulting in the mutex becoming available, the scheduling policy shall determine which thread shall acquire the mutex.

(In the case of PTHREAD_MUTEX_RECURSIVE mutexes, the mutex shall become available when the count reaches zero and the calling thread no longer has any locks on this mutex.)

If a signal is delivered to a thread waiting for a mutex, upon return from the signal handler the thread shall resume waiting for the mutex as if it was not interrupted.

Return Value

If successful, the pthread_mutex_lock() and pthread_mutex_unlock() functions shall return zero; otherwise, an error number shall be returned to indicate the error.

The pthread_mutex_trylock() function shall return zero if a lock on the mutex object referenced by mutex is acquired. Otherwise, an error number is returned to indicate the error.

Errors

The pthread_mutex_lock() and pthread_mutex_trylock() functions shall fail if:

EINVAL
The mutex was created with the protocol attribute having the value PTHREAD_PRIO_PROTECT and the calling thread's priority is higher than the mutex's current priority ceiling.

The pthread_mutex_trylock() function shall fail if:

EBUSY
The mutex could not be acquired because it was already locked.

The pthread_mutex_lock(), pthread_mutex_trylock(), and pthread_mutex_unlock() functions may fail if:

EINVAL
The value specified by mutex does not refer to an initialized mutex object.
EAGAIN
The mutex could not be acquired because the maximum number of recursive locks for mutex has been exceeded.

The pthread_mutex_lock() function may fail if:

EDEADLK
The current thread already owns the mutex.

The pthread_mutex_unlock() function may fail if:

EPERM
The current thread does not own the mutex.

These functions shall not return an error code of [EINTR].

The following sections are informative.

Examples

None.

Application Usage

None.

Rationale

Mutex objects are intended to serve as a low-level primitive from which other thread synchronization functions can be built. As such, the implementation of mutexes should be as efficient as possible, and this has ramifications on the features available at the interface.

The mutex functions and the particular default settings of the mutex attributes have been motivated by the desire to not preclude fast, inlined implementations of mutex locking and unlocking.

For example, deadlocking on a double-lock is explicitly allowed behavior in order to avoid requiring more overhead in the basic mechanism than is absolutely necessary. (More "friendly" mutexes that detect deadlock or that allow multiple locking by the same thread are easily constructed by the user via the other mechanisms provided. For example, pthread_self() can be used to record mutex ownership.) Implementations might also choose to provide such extended features as options via special mutex attributes.

Since most attributes only need to be checked when a thread is going to be blocked, the use of attributes does not slow the (common) mutex-locking case.

Likewise, while being able to extract the thread ID of the owner of a mutex might be desirable, it would require storing the current thread ID when each mutex is locked, and this could incur unacceptable levels of overhead. Similar arguments apply to a mutex_tryunlock operation.

Future Directions

None.

See Also

pthread_mutex_destroy(), pthread_mutex_timedlock(), the Base Definitions volume of IEEE Std 1003.1-2001, <pthread.h>

Copyright

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright © 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Referenced By

pthreads(7)

In the Linux threads series, we discussed on the ways in which a thread can terminate and how the return status is passed on from the terminating thread to its parent thread. In this article we will throw some light on an important aspect known as thread synchronization.

Linux Threads Series: part 1, part 2, part 3, part 4 (this article).

Thread Synchronization Problems

Lets take an example code to study synchronization problems :

#include<stdio.h> #include<string.h> #include<pthread.h> #include<stdlib.h> #include<unistd.h> pthread_t tid[2]; int counter; void* doSomeThing(void *arg) { unsigned long i = 0; counter += 1; printf("\n Job %d started\n", counter); for(i=0; i<(0xFFFFFFFF);i++); printf("\n Job %d finished\n", counter); return NULL; } int main(void) { int i = 0; int err; while(i < 2) { err = pthread_create(&(tid[i]), NULL, &doSomeThing, NULL); if (err != 0) printf("\ncan't create thread :[%s]", strerror(err)); i++; } pthread_join(tid[0], NULL); pthread_join(tid[1], NULL); return 0; }

The above code is a simple one in which two threads(jobs) are created and in the start function of these threads, a counter is maintained through which user gets the logs about job number which is started and when it is completed. The code and the flow looks fine but when we see the output :

$ ./tgsthreads Job 1 started Job 2 started Job 2 finished Job 2 finished

If you focus on the last two logs, you will see that the log ‘Job 2 finished’ is repeated twice while no log for ‘Job 1 finished’ is seen.

Now, if you go back at the code and try to find any logical flaw, you’ll probably not find any flaw easily. But if you’ll have a closer look and visualize  the execution of the code, you’ll find that :

  • The log ‘Job 2 started’ is printed just after ‘Job 1 Started’  so it can easily be concluded that while thread 1 was processing the scheduler scheduled the thread 2.
  • If the above assumption was true then the value of the ‘counter’ variable got incremented again before job 1 got finished.
  • So, when Job 1 actually got finished, then the wrong value of counter produced the log ‘Job 2 finished’ followed by the ‘Job 2 finished’  for the actual job 2 or vice versa as it is dependent on scheduler.
  • So we see that its not the repetitive log but the wrong value of the ‘counter’ variable that is the problem.

The actual problem was the usage of the variable ‘counter’ by second thread when the first thread was using or about to use it. In other words we can say that lack of synchronization between the threads while using the shared resource ‘counter’ caused the problems or in one word we can say that this problem happened due to ‘Synchronization problem’ between two threads.

Mutexes

Now since we have understood the base problem, lets discuss the solution to it. The most popular way of achieving thread synchronization is by using Mutexes.

A Mutex is a lock that we set before using a shared resource and release after using it. When the lock is set, no other thread can access the locked region of code. So we see that even if thread 2 is scheduled while thread 1 was not done accessing the shared resource and the code is locked by thread 1 using mutexes then thread 2 cannot even access that region of code. So this ensures a synchronized access of shared resources in the code.

Internally it works as follows :

  • Suppose one thread has locked a region of code using mutex and is executing that piece of code.
  • Now if scheduler decides to do a context switch, then all the other threads which are ready to execute the same region are unblocked.
  • Only one of all the threads would make it to the execution but if this thread tries to execute the same region of code that is already locked then it will again go to sleep.
  • Context switch will take place again and again but no thread would be able to execute the locked region of code until the mutex lock over it is released.
  • Mutex lock will only be released by the thread who locked it.
  • So this ensures that once a thread has locked a piece of code then no other thread can execute the same region until it is unlocked by the thread who locked it.
  • Hence, this system ensures synchronization among the threads while working on shared resources.

A mutex is initialized and then a lock is achieved by calling the following two functions :

int pthread_mutex_init(pthread_mutex_t *restrict mutex, const pthread_mutexattr_t *restrict attr); int pthread_mutex_lock(pthread_mutex_t *mutex);

The first function initializes a mutex and through second function any critical region in the code can be locked.

The mutex can be unlocked and destroyed by calling following functions :

int pthread_mutex_unlock(pthread_mutex_t *mutex); int pthread_mutex_destroy(pthread_mutex_t *mutex);

The first function above releases the lock and the second function destroys the lock so that it cannot be used anywhere in future.

A Practical Example

Lets see a piece of code where mutexes are used for thread synchronization

#include<stdio.h> #include<string.h> #include<pthread.h> #include<stdlib.h> #include<unistd.h> pthread_t tid[2]; int counter; pthread_mutex_t lock; void* doSomeThing(void *arg) { pthread_mutex_lock(&lock); unsigned long i = 0; counter += 1; printf("\n Job %d started\n", counter); for(i=0; i<(0xFFFFFFFF);i++); printf("\n Job %d finished\n", counter); pthread_mutex_unlock(&lock); return NULL; } int main(void) { int i = 0; int err; if (pthread_mutex_init(&lock, NULL) != 0) { printf("\n mutex init failed\n"); return 1; } while(i < 2) { err = pthread_create(&(tid[i]), NULL, &doSomeThing, NULL); if (err != 0) printf("\ncan't create thread :[%s]", strerror(err)); i++; } pthread_join(tid[0], NULL); pthread_join(tid[1], NULL); pthread_mutex_destroy(&lock); return 0; }

In the code above :

  • A mutex is initialized in the beginning of the main function.
  • The same mutex is locked in the ‘doSomeThing()’ function while using the shared resource ‘counter’
  • At the end of the function ‘doSomeThing()’ the same mutex is unlocked.
  • At the end of the main function when both the threads are done, the mutex is destroyed.

Now if we look at the output, we find :

$ ./threads Job 1 started Job 1 finished Job 2 started Job 2 finished

So we see that this time the start and finish logs of both the jobs were present. So thread synchronization took place by the use of Mutex.

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Tagged as:pthread_mutex_destroy Examples, pthread_mutex_init Examples, pthread_mutex_lock Examples, pthread_mutex_unlock Examples

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