Ubuntu manuals

Provided by: freebsd-manpages_6.2-1_all

NAME

      sema, sema_init, sema_destroy, sema_post, sema_wait, sema_timedwait,
      sema_trywait, sema_value - kernel counting semaphore
 

SYNOPSIS

      #include <sys/types.h>
      #include <sys/lock.h>
      #include <sys/sema.h>
 
      void
      sema_init(struct sema *sema, int value, const char *description);
 
      void
      sema_destroy(struct sema *sema);
 
      void
      sema_post(struct sema *sema);
 
      void
      sema_wait(struct sema *sema);
 
      int
      sema_timedwait(struct sema *sema, int timo);
 
      int
      sema_trywait(struct sema *sema);
 
      int
      sema_value(struct sema *sema);
 

DESCRIPTION

      Counting semaphores provide a mechanism for synchronizing access to a
      pool of resources.  Unlike mutexes, semaphores do not have the concept of
      an owner, so they can also be useful in situations where one thread needs
      to acquire a resource, and another thread needs to release it.  Each
      semaphore has an integer value associated with it.  Posting (increment‐
      ing) always succeeds, but waiting (decrementing) can only successfully
      complete if the resulting value of the semaphore is greater than or equal
      to zero.
 
      Semaphores should not be used where mutexes and condition variables will
      suffice.  Semaphores are a more complex synchronization mechanism than
      mutexes and condition variables, and are not as efficient.
 
      Semaphores are created with sema_init(), where sema is a pointer to space
      for a struct sema, value is the initial value of the semaphore, and
      description is a pointer to a null-terminated character string that
      describes the semaphore.  Semaphores are destroyed with sema_destroy().
      A semaphore is posted (incremented) with sema_post().  A semaphore is
      waited on (decremented) with sema_wait(), sema_timedwait(), or
      sema_trywait().  The timo argument to sema_timedwait() specifies the min‐
      imum time in ticks to wait before returning with failure.  sema_value()
      is used to read the current value of the semaphore.
      The sema_value() function returns the current value of the semaphore.
 
      If decrementing the semaphore would result in its value being negative,
      sema_trywait() returns 0 to indicate failure.  Otherwise, a non-zero
      value is returned to indicate success.
 
      The sema_timedwait() function returns 0 if waiting on the semaphore suc‐
      ceeded; otherwise a non-zero error code is returned.
 

ERRORS

      The sema_timedwait() function will fail if:
 
      [EWOULDBLOCK]      Timeout expired.
      condvar(9), mtx_pool(9), mutex(9), sx(9)