Semaphore

Semaphore abstraction for the QuantumRT Real-Time Kernel.

Part of QuantumRT Real-Time Kernel.

Defines

SEM_FAILED

Semaphore initialization failed.

Functions

int sem_init(sem_t *semaphore, int pshared, unsigned count)

Initialize a semaphore.

Error numbers:

• EINVAL if the semaphore is invalid.

  See also

  sem_post() for example.

Parameters:

• semaphore – Pointer to the semaphore.

Returns:

0 on success, -1 on error and errno set.

int sem_destroy(sem_t *semaphore)

Destroy the semaphore. Destroying a semaphore that has threads waiting on it results in undefined behavior.

Error numbers:

• EINVAL if the semaphore is invalid.

Parameters:

• semaphore – Pointer to the semaphore.

Returns:

int 0 on success, -1 on error and errno set.

int sem_post(sem_t *semaphore)

Unlock a semaphore, incrementing its count and potentially waking a waiting thread.

Error numbers:

• EINVAL if the semaphore is invalid.

Parameters:

• semaphore – Pointer to the semaphore.

Returns:

int 0 on success, -1 on error and errno set.

// Example:  PID Control Loop with ADC Feedback

#include "qrt.h"
#include "semaphore.h"

sem_t adc_sem;

void ADC_DeferredHandler(qrt_u32_t arg)
{
    sem_post((sem_t *)arg);     // Signal new sample ready
}

void ADC_IRQHandler(void)
{
    // Kernel calls not allowed in ISR context
    qrt_defercall(ADC_DeferredHandler, (qrt_u32_t)&adc_sem);
}

void *control_loop(void *)
{
    sem_init(&adc_sem, 0, 0u);

    adc_init();

    for (;;)
    {
        sem_wait(&adc_sem);      // Wait for ADC sample ready
        update_pid_controller();
    }
}

int sem_timedwait(sem_t *semaphore, const struct timespec *abstime)

Try to lock a semaphore with timed blocking.

Error numbers:

• EINVAL if the thread would have wait, and the abstime is invalid (less than 0 or greater than 1000 million).

• ETIMEDOUT if the wait timed out.

• EDEADLK if a deadlock condition is detected.

• EINVAL if the semaphore is invalid.

  See also

  sem_post() for example.

Parameters:

• semaphore – Pointer to the semaphore.

Returns:

0 on success, -1 on error and errno set.

int sem_clockwait(sem_t *semaphore, clockid_t clock_id, const struct timespec *abstime)

Try to lock a semaphore with blocking and a specific clock.

Error numbers:

• EINVAL if the thread would have wait, and the abstime is invalid (less than 0 or greater than 1000 million).

• ETIMEDOUT if the wait timed out.

• EDEADLK if a deadlock condition is detected.

• EINVAL if the semaphore is invalid.

• EINVAL if the clock_id is other than CLOCK_MONOTONIC or CLOCK_SYSTIMER.

Parameters:

• semaphore – Pointer to the semaphore.

Returns:

0 on success, -1 on error and errno set.

int sem_trywait(sem_t *semaphore)

Try to lock a semaphore without blocking.

Error numbers:

• EAGAIN if the semaphore is already held.

• EINVAL if the semaphore is invalid

Parameters:

• semaphore – Pointer to the semaphore.

Returns:

0 on success, -1 on error and errno set.

int sem_wait(sem_t *semaphore)

Try to lock a semaphore with blocking.

Error numbers:

• ETIMEDOUT if the wait timed out.

• EDEADLK if a deadlock condition is detected.

• EINVAL if the semaphore is invalid.

  See also

  sem_post() for example.

Parameters:

• semaphore – Pointer to the semaphore.

Returns:

0 on success, -1 on error and errno set.

int sem_getvalue(sem_t *semaphore, int *sval)

Get the current value of the semaphore.

Error numbers:

• EINVAL if the semaphore is invalid

Parameters:

• semaphore – Pointer to the semaphore.

• sval – Pointer to the location where the current value will be stored.

Returns:

int 0 on success, -1 on error and errno set.

sem_t *sem_open(const char *name, int oflag, mode_t mode, unsigned int value)

Not supported.

Returns:

Always -1 and sets errno to ENOSYS.

int sem_close(sem_t *semaphore)

Not supported.

Returns:

Always -1 and sets errno to ENOSYS.

int sem_unlink(const char *name)

Not supported.

Returns:

Always -1 and sets errno to ENOSYS.

struct sem_t