Chapter 6.1 - 信号量管理
Created by : Mr Dk.
2019 / 11 / 20 09:12
Nanjing, Jiangsu, China
6.1.1 概述
信号量可以使任务或中断服务向另一个任务发送一个变量。使用信号量之前，必须先建立信号量。信号量由两个部分组成：
- 信号量计数值
- 表示事件发生 - 初始值取 0 (暂时没有事件发生)
- 用于共享资源的访问 - 初始值取 1 (二值信号量) 或 n (多个相同资源)
- 等待信号量的任务组成的等待任务列表
信号量函数可以在编译前剪裁。
6.1.2 建立信号量 - OSSemCreate() 函数
建立一个信号量，并对信号量赋予初始计数值。调用者显然是任务或启动代码，是使用任何其它信号量函数的前提。
原理：
- 从空闲 ECB 链表中取一个 ECB
- 初始化 ECB 中的变量
- 将 ECB 的等待任务列表清 0
- 返回 ECB 指针
/*
*********************************************************************************************************
* CREATE A SEMAPHORE
*
* Description: This function creates a semaphore.
*
* Arguments : cnt is the initial value for the semaphore. If the value is 0, no resource is
* available (or no event has occurred). You initialize the semaphore to a
* non-zero value to specify how many resources are available (e.g. if you have
* 10 resources, you would initialize the semaphore to 10).
*
* Returns : != (void *)0 is a pointer to the event control block (OS_EVENT) associated with the
* created semaphore
* == (void *)0 if no event control blocks were available
*********************************************************************************************************
*/
OS_EVENT *OSSemCreate (INT16U cnt)
{
OS_EVENT *pevent;
#if OS_CRITICAL_METHOD == 3u /* Allocate storage for CPU status register */
OS_CPU_SR cpu_sr = 0u;
#endif
#ifdef OS_SAFETY_CRITICAL_IEC61508
if (OSSafetyCriticalStartFlag == OS_TRUE) {
OS_SAFETY_CRITICAL_EXCEPTION();
}
#endif
if (OSIntNesting > 0u) { /* See if called from ISR ... */
return ((OS_EVENT *)0); /* ... can't CREATE from an ISR */
}
OS_ENTER_CRITICAL();
pevent = OSEventFreeList; /* Get next free event control block */
if (OSEventFreeList != (OS_EVENT *)0) { /* See if pool of free ECB pool was empty */
OSEventFreeList = (OS_EVENT *)OSEventFreeList->OSEventPtr;
}
OS_EXIT_CRITICAL();
if (pevent != (OS_EVENT *)0) { /* Get an event control block */
pevent->OSEventType = OS_EVENT_TYPE_SEM;
pevent->OSEventCnt = cnt; /* Set semaphore value */
pevent->OSEventPtr = (void *)0; /* Unlink from ECB free list */
#if OS_EVENT_NAME_EN > 0u
pevent->OSEventName = (INT8U *)(void *)"?";
#endif
OS_EventWaitListInit(pevent); /* Initialize to 'nobody waiting' on sem. */
}
return (pevent);
}
6.1.3 删除信号量 - OSSemDel() 函数
删除一个信号量，调用者只能是任务。删除信号量时指定条件选项：
OS_DEL_NO_PEND- 已经没有任何任务等待该信号量时，才删除信号量OS_DEL_ALWAYS- 不管有无等待任务，立刻删除信号量；所有等待该信号量的任务立刻就绪
如果删除成功返回 NULL，删除不成功则返回原 ECB，并能通过错误代码查明原因。
原理：将 ECB 恢复空闲原始状态，归还给空闲链表。
/*
*********************************************************************************************************
* DELETE A SEMAPHORE
*
* Description: This function deletes a semaphore and readies all tasks pending on the semaphore.
*
* Arguments : pevent is a pointer to the event control block associated with the desired
* semaphore.
*
* opt determines delete options as follows:
* opt == OS_DEL_NO_PEND Delete semaphore ONLY if no task pending
* opt == OS_DEL_ALWAYS Deletes the semaphore even if tasks are waiting.
* In this case, all the tasks pending will be readied.
*
* perr is a pointer to an error code that can contain one of the following values:
* OS_ERR_NONE The call was successful and the semaphore was deleted
* OS_ERR_DEL_ISR If you attempted to delete the semaphore from an ISR
* OS_ERR_INVALID_OPT An invalid option was specified
* OS_ERR_TASK_WAITING One or more tasks were waiting on the semaphore
* OS_ERR_EVENT_TYPE If you didn't pass a pointer to a semaphore
* OS_ERR_PEVENT_NULL If 'pevent' is a NULL pointer.
*
* Returns : pevent upon error
* (OS_EVENT *)0 if the semaphore was successfully deleted.
*
* Note(s) : 1) This function must be used with care. Tasks that would normally expect the presence of
* the semaphore MUST check the return code of OSSemPend().
* 2) OSSemAccept() callers will not know that the intended semaphore has been deleted unless
* they check 'pevent' to see that it's a NULL pointer.
* 3) This call can potentially disable interrupts for a long time. The interrupt disable
* time is directly proportional to the number of tasks waiting on the semaphore.
* 4) Because ALL tasks pending on the semaphore will be readied, you MUST be careful in
* applications where the semaphore is used for mutual exclusion because the resource(s)
* will no longer be guarded by the semaphore.
*********************************************************************************************************
*/
#if OS_SEM_DEL_EN > 0u
OS_EVENT *OSSemDel (OS_EVENT *pevent,
INT8U opt,
INT8U *perr)
{
BOOLEAN tasks_waiting;
OS_EVENT *pevent_return;
#if OS_CRITICAL_METHOD == 3u /* Allocate storage for CPU status register */
OS_CPU_SR cpu_sr = 0u;
#endif
#ifdef OS_SAFETY_CRITICAL
if (perr == (INT8U *)0) {
OS_SAFETY_CRITICAL_EXCEPTION();
}
#endif
#if OS_ARG_CHK_EN > 0u
if (pevent == (OS_EVENT *)0) { /* Validate 'pevent' */
*perr = OS_ERR_PEVENT_NULL;
return (pevent);
}
#endif
if (pevent->OSEventType != OS_EVENT_TYPE_SEM) { /* Validate event block type */
*perr = OS_ERR_EVENT_TYPE;
return (pevent);
}
if (OSIntNesting > 0u) { /* See if called from ISR ... */
*perr = OS_ERR_DEL_ISR; /* ... can't DELETE from an ISR */
return (pevent);
}
OS_ENTER_CRITICAL();
if (pevent->OSEventGrp != 0u) { /* See if any tasks waiting on semaphore */
tasks_waiting = OS_TRUE; /* Yes */
} else {
tasks_waiting = OS_FALSE; /* No */
}
switch (opt) {
case OS_DEL_NO_PEND: /* Delete semaphore only if no task waiting */
if (tasks_waiting == OS_FALSE) {
#if OS_EVENT_NAME_EN > 0u
pevent->OSEventName = (INT8U *)(void *)"?";
#endif
pevent->OSEventType = OS_EVENT_TYPE_UNUSED;
pevent->OSEventPtr = OSEventFreeList; /* Return Event Control Block to free list */
pevent->OSEventCnt = 0u;
OSEventFreeList = pevent; /* Get next free event control block */
OS_EXIT_CRITICAL();
*perr = OS_ERR_NONE;
pevent_return = (OS_EVENT *)0; /* Semaphore has been deleted */
} else {
OS_EXIT_CRITICAL();
*perr = OS_ERR_TASK_WAITING;
pevent_return = pevent;
}
break;
case OS_DEL_ALWAYS: /* Always delete the semaphore */
while (pevent->OSEventGrp != 0u) { /* Ready ALL tasks waiting for semaphore */
(void)OS_EventTaskRdy(pevent, (void *)0, OS_STAT_SEM, OS_STAT_PEND_OK);
}
#if OS_EVENT_NAME_EN > 0u
pevent->OSEventName = (INT8U *)(void *)"?";
#endif
pevent->OSEventType = OS_EVENT_TYPE_UNUSED;
pevent->OSEventPtr = OSEventFreeList; /* Return Event Control Block to free list */
pevent->OSEventCnt = 0u;
OSEventFreeList = pevent; /* Get next free event control block */
OS_EXIT_CRITICAL();
if (tasks_waiting == OS_TRUE) { /* Reschedule only if task(s) were waiting */
OS_Sched(); /* Find highest priority task ready to run */
}
*perr = OS_ERR_NONE;
pevent_return = (OS_EVENT *)0; /* Semaphore has been deleted */
break;
default:
OS_EXIT_CRITICAL();
*perr = OS_ERR_INVALID_OPT;
pevent_return = pevent;
break;
}
return (pevent_return);
}
6.1.4 等待信号量 - OSSemPend() 函数
请求信号量，直到其它任务或中断置位信号量，或信号量超出等待的预期时间。若超时时间内信号量被置为，则最高优先级的任务取得信号量并转入就绪。一个被 OSTaskSuspend() 挂起的任务也可以接受信号量，但这个任务还是会保持挂起状态。该函数只能被任务调用，并给定一个超时时间。0 则表示无限期等待。调用时，如果信号量的值大于 0，则递减信号量并直接返回；否则将任务加入等待任务队列，并挂起任务。
原理：从 ECB 中获取成员变量
- 如果大于 0，则得到信号量
- 如果等于 0，则挂起任务
/*
*********************************************************************************************************
* PEND ON SEMAPHORE
*
* Description: This function waits for a semaphore.
*
* Arguments : pevent is a pointer to the event control block associated with the desired
* semaphore.
*
* timeout is an optional timeout period (in clock ticks). If non-zero, your task will
* wait for the resource up to the amount of time specified by this argument.
* If you specify 0, however, your task will wait forever at the specified
* semaphore or, until the resource becomes available (or the event occurs).
*
* perr is a pointer to where an error message will be deposited. Possible error
* messages are:
*
* OS_ERR_NONE The call was successful and your task owns the resource
* or, the event you are waiting for occurred.
* OS_ERR_TIMEOUT The semaphore was not received within the specified
* 'timeout'.
* OS_ERR_PEND_ABORT The wait on the semaphore was aborted.
* OS_ERR_EVENT_TYPE If you didn't pass a pointer to a semaphore.
* OS_ERR_PEND_ISR If you called this function from an ISR and the result
* would lead to a suspension.
* OS_ERR_PEVENT_NULL If 'pevent' is a NULL pointer.
* OS_ERR_PEND_LOCKED If you called this function when the scheduler is locked
*
* Returns : none
*********************************************************************************************************
*/
/*$PAGE*/
void OSSemPend (OS_EVENT *pevent,
INT32U timeout,
INT8U *perr)
{
#if OS_CRITICAL_METHOD == 3u /* Allocate storage for CPU status register */
OS_CPU_SR cpu_sr = 0u;
#endif
#ifdef OS_SAFETY_CRITICAL
if (perr == (INT8U *)0) {
OS_SAFETY_CRITICAL_EXCEPTION();
}
#endif
#if OS_ARG_CHK_EN > 0u
if (pevent == (OS_EVENT *)0) { /* Validate 'pevent' */
*perr = OS_ERR_PEVENT_NULL;
return;
}
#endif
if (pevent->OSEventType != OS_EVENT_TYPE_SEM) { /* Validate event block type */
*perr = OS_ERR_EVENT_TYPE;
return;
}
if (OSIntNesting > 0u) { /* See if called from ISR ... */
*perr = OS_ERR_PEND_ISR; /* ... can't PEND from an ISR */
return;
}
if (OSLockNesting > 0u) { /* See if called with scheduler locked ... */
*perr = OS_ERR_PEND_LOCKED; /* ... can't PEND when locked */
return;
}
OS_ENTER_CRITICAL();
if (pevent->OSEventCnt > 0u) { /* If sem. is positive, resource available ... */
pevent->OSEventCnt--; /* ... decrement semaphore only if positive. */
OS_EXIT_CRITICAL();
*perr = OS_ERR_NONE;
return;
}
/* Otherwise, must wait until event occurs */
OSTCBCur->OSTCBStat |= OS_STAT_SEM; /* Resource not available, pend on semaphore */
OSTCBCur->OSTCBStatPend = OS_STAT_PEND_OK;
OSTCBCur->OSTCBDly = timeout; /* Store pend timeout in TCB */
OS_EventTaskWait(pevent); /* Suspend task until event or timeout occurs */
OS_EXIT_CRITICAL();
OS_Sched(); /* Find next highest priority task ready */
OS_ENTER_CRITICAL();
switch (OSTCBCur->OSTCBStatPend) { /* See if we timed-out or aborted */
case OS_STAT_PEND_OK:
*perr = OS_ERR_NONE;
break;
case OS_STAT_PEND_ABORT:
*perr = OS_ERR_PEND_ABORT; /* Indicate that we aborted */
break;
case OS_STAT_PEND_TO:
default:
OS_EventTaskRemove(OSTCBCur, pevent);
*perr = OS_ERR_TIMEOUT; /* Indicate that we didn't get event within TO */
break;
}
OSTCBCur->OSTCBStat = OS_STAT_RDY; /* Set task status to ready */
OSTCBCur->OSTCBStatPend = OS_STAT_PEND_OK; /* Clear pend status */
OSTCBCur->OSTCBEventPtr = (OS_EVENT *)0; /* Clear event pointers */
#if (OS_EVENT_MULTI_EN > 0u)
OSTCBCur->OSTCBEventMultiPtr = (OS_EVENT **)0;
#endif
OS_EXIT_CRITICAL();
}
6.1.5 发送信号量 - OSSemPost() 函数
用于置位 (发送) 信号量，使信号量的计数值 +1。如果有任务在等待信号量，则最高优先级任务将得到信号量而就绪。
- 从任务中被调用，将进行任务调度或切换
- 从中断中被调用，不发生任务切换——所有嵌套中断都退出后，才会发生任务切换
原理：
- 查询 ECB 的等待列表
- 如果有任务在等待信号量，则将优先级最高的任务置为就绪，并重新调度
- 如果没有，则将信号量计数器 +1
/*
*********************************************************************************************************
* POST TO A SEMAPHORE
*
* Description: This function signals a semaphore
*
* Arguments : pevent is a pointer to the event control block associated with the desired
* semaphore.
*
* Returns : OS_ERR_NONE The call was successful and the semaphore was signaled.
* OS_ERR_SEM_OVF If the semaphore count exceeded its limit. In other words, you have
* signalled the semaphore more often than you waited on it with either
* OSSemAccept() or OSSemPend().
* OS_ERR_EVENT_TYPE If you didn't pass a pointer to a semaphore
* OS_ERR_PEVENT_NULL If 'pevent' is a NULL pointer.
*********************************************************************************************************
*/
INT8U OSSemPost (OS_EVENT *pevent)
{
#if OS_CRITICAL_METHOD == 3u /* Allocate storage for CPU status register */
OS_CPU_SR cpu_sr = 0u;
#endif
#if OS_ARG_CHK_EN > 0u
if (pevent == (OS_EVENT *)0) { /* Validate 'pevent' */
return (OS_ERR_PEVENT_NULL);
}
#endif
if (pevent->OSEventType != OS_EVENT_TYPE_SEM) { /* Validate event block type */
return (OS_ERR_EVENT_TYPE);
}
OS_ENTER_CRITICAL();
if (pevent->OSEventGrp != 0u) { /* See if any task waiting for semaphore */
/* Ready HPT waiting on event */
(void)OS_EventTaskRdy(pevent, (void *)0, OS_STAT_SEM, OS_STAT_PEND_OK);
OS_EXIT_CRITICAL();
OS_Sched(); /* Find HPT ready to run */
return (OS_ERR_NONE);
}
if (pevent->OSEventCnt < 65535u) { /* Make sure semaphore will not overflow */
pevent->OSEventCnt++; /* Increment semaphore count to register event */
OS_EXIT_CRITICAL();
return (OS_ERR_NONE);
}
OS_EXIT_CRITICAL(); /* Semaphore value has reached its maximum */
return (OS_ERR_SEM_OVF);
}
6.1.6 无等待地获取信号量 - OSSemAccept() 函数
查看资源是否可用或事件是否发生，不挂起任务，立即返回。调用者可以是任务，也可以是中断。
原理：
- 直接查询 ECB 中的
OSEventCnt实现信号量的查询 - 如果信号量的值大于 0，则返回信号量的值，将将信号量的值 -1
- 如果信号量的值小于 0，则返回 0
/*
*********************************************************************************************************
* ACCEPT SEMAPHORE
*
* Description: This function checks the semaphore to see if a resource is available or, if an event
* occurred. Unlike OSSemPend(), OSSemAccept() does not suspend the calling task if the
* resource is not available or the event did not occur.
*
* Arguments : pevent is a pointer to the event control block
*
* Returns : > 0 if the resource is available or the event did not occur the semaphore is
* decremented to obtain the resource.
* == 0 if the resource is not available or the event did not occur or,
* if 'pevent' is a NULL pointer or,
* if you didn't pass a pointer to a semaphore
*********************************************************************************************************
*/
#if OS_SEM_ACCEPT_EN > 0u
INT16U OSSemAccept (OS_EVENT *pevent)
{
INT16U cnt;
#if OS_CRITICAL_METHOD == 3u /* Allocate storage for CPU status register */
OS_CPU_SR cpu_sr = 0u;
#endif
#if OS_ARG_CHK_EN > 0u
if (pevent == (OS_EVENT *)0) { /* Validate 'pevent' */
return (0u);
}
#endif
if (pevent->OSEventType != OS_EVENT_TYPE_SEM) { /* Validate event block type */
return (0u);
}
OS_ENTER_CRITICAL();
cnt = pevent->OSEventCnt;
if (cnt > 0u) { /* See if resource is available */
pevent->OSEventCnt--; /* Yes, decrement semaphore and notify caller */
}
OS_EXIT_CRITICAL();
return (cnt); /* Return semaphore count */
}
#endif
6.1.7 查询信号量的当前状态 - OSSemQuery() 函数
查询信号量的信息。确切地说，需要提前申请一个 OS_SEM_DATA 类型结构体的内存，然后将 ECB 中某几个域的信息复制到这个结构体中。
/*
*********************************************************************************************************
* QUERY A SEMAPHORE
*
* Description: This function obtains information about a semaphore
*
* Arguments : pevent is a pointer to the event control block associated with the desired
* semaphore
*
* p_sem_data is a pointer to a structure that will contain information about the
* semaphore.
*
* Returns : OS_ERR_NONE The call was successful and the message was sent
* OS_ERR_EVENT_TYPE If you are attempting to obtain data from a non semaphore.
* OS_ERR_PEVENT_NULL If 'pevent' is a NULL pointer.
* OS_ERR_PDATA_NULL If 'p_sem_data' is a NULL pointer
*********************************************************************************************************
*/
#if OS_SEM_QUERY_EN > 0u
INT8U OSSemQuery (OS_EVENT *pevent,
OS_SEM_DATA *p_sem_data)
{
INT8U i;
OS_PRIO *psrc;
OS_PRIO *pdest;
#if OS_CRITICAL_METHOD == 3u /* Allocate storage for CPU status register */
OS_CPU_SR cpu_sr = 0u;
#endif
#if OS_ARG_CHK_EN > 0u
if (pevent == (OS_EVENT *)0) { /* Validate 'pevent' */
return (OS_ERR_PEVENT_NULL);
}
if (p_sem_data == (OS_SEM_DATA *)0) { /* Validate 'p_sem_data' */
return (OS_ERR_PDATA_NULL);
}
#endif
if (pevent->OSEventType != OS_EVENT_TYPE_SEM) { /* Validate event block type */
return (OS_ERR_EVENT_TYPE);
}
OS_ENTER_CRITICAL();
p_sem_data->OSEventGrp = pevent->OSEventGrp; /* Copy message mailbox wait list */
psrc = &pevent->OSEventTbl[0];
pdest = &p_sem_data->OSEventTbl[0];
for (i = 0u; i < OS_EVENT_TBL_SIZE; i++) {
*pdest++ = *psrc++;
}
p_sem_data->OSCnt = pevent->OSEventCnt; /* Get semaphore count */
OS_EXIT_CRITICAL();
return (OS_ERR_NONE);
}
#endif
Summary
总体上这一块还是比较简单的，各个函数的功能也都简洁明了。需要注意的是，所有会将自己挂起的函数，在中断中不能被调用，只能在任务中被调用。