pthread_mutex_lock, pthread_mutex_trylock
Prototype: 
#include <pthread.h>
int pthread_mutex_lock(pthread_mutex_t *mutex);
int pthread_mutex_trylock(pthread_mutex_t *mutex);
General Description: Lock or try to lock a semaphore for entering a critical section. The parameter is simply a variable that acts like a reservation ticket. If another thread tries to lock a reserved spot, it blocks until the reserving thread releases the semaphore.
Return Value: The call returns a zero on success and nonzero on error. You can find the exact code in errno.
Parameters
mutex The semaphore variable.
Possible Errors
EINVAL The mutex has not been properly initialized.
EDEADLK [try_lock] The calling thread has already locked the mutex (error checking mutexes only).
EBUSY [lock] The calling thread can't acquire, because it is currently locked.
Examples 
pthread_mutex_t mutex = fastmutex;
...
if ( pthread_mutex_lock(&mutex) == 0 )
{
    /**** work on critical data ****/
    pthread_mutex_unlock(&mutex);
}


pthread_mutex_t mutex = fastmutex;
...
/*---Do other processing while waiting for semaphore---*/
while ( pthread_mutex_trylock(&mutex) != 0  &&  errno == EBUSY )
{
    /**** Work on something else while waiting ****/
}
/*---Got the semaphore!  Now work on the critical section---*/
if ( errno != ENOERROR )
{
    /**** work on critical data ****/
    pthread_mutex_unlock(&mutex);
}