Semaphore operations are the same in both the Oracle Solaris Operating Environment and the POSIX environment. The function name changed from sema_ in the Oracle Solaris Operating Environment to sem_ in pthreads. This section discusses the following topics:
Initializing a semaphore
Incrementing a semaphore
Blocking on a semaphore count
Decrementing a semaphore count
Destroying the semaphore state
Use sema_init(3C) to initialize the semaphore variable pointed to by sp by count amount.
#include <thread.h>
int sema_init(sema_t *sp, unsigned int count, int type,
void *arg);
type can be one of the following values:
USYNC_PROCESS. The semaphore can be used to synchronize threads in this process and other processes. Only one process should initialize the semaphore. arg is ignored.
USYNC_THREAD. The semaphore can be used to synchronize threads in this process, only. arg is ignored.
Multiple threads must not initialize the same semaphore simultaneously. A semaphore must not be reinitialized while other threads might be using the semaphore.
#include <thread.h> sema_t sp; int ret; int count; count = 4; /* to be used within this process only */ ret = sema_init(&sp, count, USYNC_THREAD, 0);
#include <thread.h> sema_t sp; int ret; int count; count = 4; /* to be used among all the processes */ ret = sema_init (&sp, count, USYNC_PROCESS, 0);
sema_init() returns 0 if successful. When any of the following conditions is detected, sema_init() fails and returns the corresponding value.
EINVAL
Description: sp refers to an invalid semaphore.
EFAULT
Description: Either sp or arg point to an illegal address.
Use sema_post(3C) to atomically increment the semaphore pointed to by sp. When any threads are blocked on the semaphore, one thread is unblocked.
#include <thread.h> int sema_post(sema_t *sp);
sema_post() returns 0 if successful. When any of the following conditions is detected, sema_post() fails and returns the corresponding value.
EINVAL
Description: sp refers to an invalid semaphore.
EFAULT
Description: sp points to an illegal address.
EOVERFLOW
Description: The semaphore value pointed to by sp exceeds SEM_VALUE_MAX.
Use sema_wait(3C) to block the calling thread until the count in the semaphore pointed to by sp becomes greater than zero. When the count becomes greater than zero, atomically decrement the count.
#include <thread.h> int sema_wait(sema_t *sp);
sema_wait() returns 0 if successful. When any of the following conditions is detected, sema_wait() fails and returns the corresponding value.
EINVAL
Description: sp refers to an invalid semaphore.
EINTR
Description: The wait was interrupted by a signal.
Use sema_trywait(3C) to atomically decrement the count in the semaphore pointed to by sp when the count is greater than zero. This function is a nonblocking version of sema_wait().
#include <thread.h> int sema_trywait(sema_t *sp);
sema_trywait() returns 0 if successful. When any of the following conditions is detected, sema_trywait() fails and returns the corresponding value.
EINVAL
Description: sp refers to an invalid semaphore.
EBUSY
Description: The semaphore pointed to by sp has a zero count.
Use sema_destroy(3C) to destroy any state that is associated with the semaphore pointed to by sp. The space for storing the semaphore is not freed.
#include <thread.h> int sema_destroy(sema_t *sp);
sema_destroy() returns 0 if successful. When the following condition is detected, sema_destroy() fails and returns the corresponding value.
EINVAL
Description: sp refers to an invalid semaphore.