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Description

A semaphore is signaled by calling OSSemPost(). If the semaphore value is 0 or more, it is incremented, and OSSemPost() returns to its caller. If tasks are waiting for the semaphore to be signaled, OSSemPost() removes the highest-priority task pending for the semaphore from the waiting list and makes this task ready-to-run. The scheduler is then called to determine if the awakened task is now the highest-priority task that is ready-to-run.

Files

os.h/os_sem.c

Prototype

OS_SEM_CTR  OSSemPost (OS_SEM  *p_sem,
                       OS_OPT   opt,
                       OS_ERR  *p_err)

Arguments

p_sem

is a pointer to the semaphore.

...

If OS_CFG_ARG_CHK_EN is set to 1 DEF_ENABLED in os_cfg.h: if p_sem is a NULL pointer.

...

If OS_CFG_OBJ_TYPE_CHK_EN is set to 1 in DEF_ENABLED in os_cfg.h: if p_sem is not pointing to a semaphore.

OS_ERR_OPT_INVALID

If OS_CFG_ARG_CHK_EN is set to DEF_ENABLED in os_cfg.h: if a valid option is not specified.

OS_ERR_OS_NOT_RUNNING

If OS_CFG_INVALID_OS_CALLS_CHK_EN is set to DEF_ENABLED in os_cfg.h: if µC/OS-III is not running yet.

OS_ERR_SEM_OVF

If the post would have caused the semaphore counter to overflow.

Returned Value

The current value of the semaphore count

Required Configuration

OS_CFG_SEM_EN must be enabled in os_cfg.h. Refer to uCµC-OS-III Configuration Manual.

Callers

Application and ISRs.

Notes/Warnings

1. Semaphores must be created before they are used.
2. You can also post to a semaphore from an ISR but the semaphore must be used as a signaling mechanism and not to protect a shared resource.

Example Usage

          OS_SEM  SwSem;
           
           
          void  TaskX (void  *p_arg)
          {
              OS_ERR      err;
              OS_SEM_CTR  ctr;
           
           
              (void)&p_arg;
              while (DEF_ON) {
                  :
                  :
                  ctr = OSSemPost(&SwSem,
                                   OS_OPT_POST_1 + OS_OPT_POST_NO_SCHED,
                                  &err);
                  /* Check "err" */
                  :
                  :
              }
          }