man pages section 9: DDI and DKI Kernel Functions

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Updated: July 2014



semaphore, sema_init, sema_destroy, sema_p, sema_p_sig, sema_v, sema_tryp - semaphore functions


#include <sys/ksynch.h>

void sema_init(ksema_t *sp, uint_t val, char *name, ksema_type_t type,
      void *arg);
void sema_destroy(ksema_t *sp);
void sema_p(ksema_t *sp);
void sema_v(ksema_t *sp);
int sema_p_sig(ksema_t *sp);
int sema_tryp(ksema_t *sp);

Interface Level

Solaris DDI specific (Solaris DDI).



A pointer to a semaphore, type ksema_t.


Initial value for semaphore.


Descriptive string. This is obsolete and should be NULL. (Non-NULL strings are legal, but they are a waste of kernel memory.)


Variant type of the semaphore. Currently, only SEMA_DRIVER is supported.


Type-specific argument; should be NULL.


These functions implement counting semaphores as described by Dijkstra. A semaphore has a value which is atomically decremented by sema_p() and atomically incremented by sema_v(). The value must always be greater than or equal to zero. If sema_p () is called and the value is zero, the calling thread is blocked until another thread performs a sema_v() operation on the semaphore.

Semaphores are initialized by calling sema_init(). The argument, val, gives the initial value for the semaphore. The semaphore storage is provided by the caller but more may be dynamically allocated, if necessary, by sema_init(). For this reason, sema_destroy() should be called before deallocating the storage containing the semaphore.

The sema_p_sig() function decrements the semaphore, as does sema_p(). However, if the semaphore value is zero, sema_p_sig() will return without decrementing the value if a signal (that is, from kill(2)) is pending for the thread.

The sema_tryp() function will decrement the semaphore value only if it is greater than zero, and will not block.

Return Values


sema_tryp() could not decrement the semaphore value because it was zero.


sema_p_sig() was not able to decrement the semaphore value and detected a pending signal.


These functions can be called from user, interrupt, or kernel context, except for sema_init() and sema_destroy(), which can be called from user or kernel context only. None of these functions can be called from a high-level interrupt context. In most cases, sema_v() and sema_p() should not be called from any interrupt context.

If sema_p() is used from interrupt context, lower-priority interrupts will not be serviced during the wait. This means that if the thread that will eventually perform the sema_v() becomes blocked on anything that requires the lower-priority interrupt, the system will hang.

For example, the thread that will perform the sema_v() may need to first allocate memory. This memory allocation may require waiting for paging I/O to complete, which may require a lower-priority disk or network interrupt to be serviced. In general, situations like this are hard to predict, so it is advisable to avoid waiting on semaphores or condition variables in an interrupt context.

See Also

kill(2), condvar(9F), mutex(9F)

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