NAME

pthread_mutexattr_gettype, pthread_mutexattr_settype - get or set a mutex type

 SYNOPSIS

#include <pthread.h>

int pthread_mutexattr_gettype(const pthread_mutexattr_t *attr, int *type);
int pthread_mutexattr_settype(pthread_mutexattr_t *attr, int type);

 DESCRIPTION

The pthread_mutexattr_gettype() and pthread_mutexattr_settype() functions respectively get and set the mutex type attribute. This attribute is set in the type parameter to these functions. The default value of the type attribute is PTHREAD_MUTEX_DEFAULT.

The type of mutex is contained in the type attribute of the mutex attributes. Valid mutex types include:

PTHREAD_MUTEX_NORMAL

This type of mutex does not detect deadlock. A thread attempting to relock this mutex without first unlocking it will deadlock. Attempting to unlock a mutex locked by a different thread results in undefined behaviour. Attempting to unlock an unlocked mutex results in undefined behaviour.

PTHREAD_MUTEX_ERRORCHECK

This type of mutex provides error checking. A thread attempting to relock this mutex without first unlocking it will return with an error. A thread attempting to unlock a mutex which another thread has locked will return with an error. A thread attempting to unlock an unlocked mutex will return with an error.

PTHREAD_MUTEX_RECURSIVE

A thread attempting to relock this mutex without first unlocking it will succeed in locking the mutex. The relocking deadlock which can occur with mutexes of type PTHREAD_MUTEX_NORMAL cannot occur with this type of mutex. Multiple locks of this mutex require the same number of unlocks to release the mutex before another thread can acquire the mutex. A thread attempting to unlock a mutex which another thread has locked will return with an error. A thread attempting to unlock an unlocked mutex will return with an error.

PTHREAD_MUTEX_DEFAULT

Attempting to recursively lock a mutex of this type results in undefined behaviour. Attempting to unlock a mutex of this type which was not locked by the calling thread results in undefined behaviour. Attempting to unlock a mutex of this type which is not locked results in undefined behaviour. An implementation is allowed to map this mutex to one of the other mutex types.

 RETURN VALUE

If successful, the pthread_mutexattr_settype() function returns zero. Otherwise, an error number is returned to indicate the error.

Upon successful completion, the pthread_mutexattr_gettype() function returns zero and stores the value of the type attribute of attr into the object referenced by the type parameter. Otherwise an error is returned to indicate the error.

 ERRORS

The pthread_mutexattr_gettype() and pthread_mutexattr_settype() functions will fail if:

[EINVAL]

The value type is invalid.

The pthread_mutexattr_gettype() and pthread_mutexattr_settype() functions may fail if:

[EINVAL]

The value specified by attr is invalid.

 EXAMPLES

None.

 APPLICATION USAGE

It is advised that an application should not use a PTHREAD_MUTEX_RECURSIVE mutex with condition variables because the implicit unlock performed for a pthread_cond_wait() or pthread_cond_timedwait() may not actually release the mutex (if it had been locked multiple times). If this happens, no other thread can satisfy the condition of the predicate.

 FUTURE DIRECTIONS

None.

 SEE ALSO

pthread_cond_wait(), pthread_cond_timedwait(), <pthread.h>.