tn_sem.h File Reference

Detailed Description

A semaphore: an object to provide signaling mechanism.

There is a lot of confusion about differences between semaphores and mutexes, so, it's quite recommended to read small article by Michael Barr: Mutexes and Semaphores Demystified.

Very short:

While mutex is seemingly similar to a semaphore with maximum count of 1 (the so-called binary semaphore), their usage is very different: the purpose of mutex is to protect shared resource. A locked mutex is "owned" by the task that locked it, and only the same task may unlock it. This ownership allows to implement algorithms to prevent priority inversion. So, mutex is a locking mechanism.

Semaphore, on the other hand, is signaling mechanism. It's quite legal and encouraged for semaphore to be waited for in the task A, and then signaled from task B or even from ISR. It may be used in situations like "producer and consumer", etc.

In addition to the article mentioned above, you may want to look at the related question on stackoverflow.com.

Definition in file tn_sem.h.

Go to the source code of this file.

Data Structures
struct	TN_Sem
	Semaphore. More...

Functions
enum TN_RCode	tn_sem_create (struct TN_Sem *sem, int start_count, int max_count)
	Construct the semaphore. More...
enum TN_RCode	tn_sem_delete (struct TN_Sem *sem)
	Destruct the semaphore. More...
enum TN_RCode	tn_sem_signal (struct TN_Sem *sem)
	Signal the semaphore. More...
enum TN_RCode	tn_sem_isignal (struct TN_Sem *sem)
	The same as tn_sem_signal() but for using in the ISR. More...
enum TN_RCode	tn_sem_wait (struct TN_Sem *sem, TN_TickCnt timeout)
	Wait for the semaphore. More...
enum TN_RCode	tn_sem_wait_polling (struct TN_Sem *sem)
	The same as tn_sem_wait() with zero timeout. More...
enum TN_RCode	tn_sem_iwait_polling (struct TN_Sem *sem)
	The same as tn_sem_wait() with zero timeout, but for using in the ISR. More...

Function Documentation

tn_sem_create()

enum TN_RCode tn_sem_create	(	struct TN_Sem *	sem,
		int	start_count,
		int	max_count
	)

Construct the semaphore.

id_sem field should not contain TN_ID_SEMAPHORE, otherwise, TN_RC_WPARAM is returned.

(refer to Legend for details)

Parameters

sem	Pointer to already allocated struct TN_Sem
start_count	Initial counter value, typically it is equal to max_count
max_count	Maximum counter value.

Returns

• TN_RC_OK if semaphore was successfully created;
• If TN_CHECK_PARAM is non-zero, additional return code is available: TN_RC_WPARAM.

tn_sem_delete()

enum TN_RCode tn_sem_delete

(

struct TN_Sem *

sem

)

Destruct the semaphore.

All tasks that wait for the semaphore become runnable with TN_RC_DELETED code returned.

(refer to Legend for details)

Parameters

sem	semaphore to destruct

Returns

• TN_RC_OK if semaphore was successfully deleted;
• TN_RC_WCONTEXT if called from wrong context;
• If TN_CHECK_PARAM is non-zero, additional return codes are available: TN_RC_WPARAM and TN_RC_INVALID_OBJ.

tn_sem_signal()

enum TN_RCode tn_sem_signal

(

struct TN_Sem *

sem

)

Signal the semaphore.

If current semaphore counter (count) is less than max_count, counter is incremented by one, and first task (if any) that waits for the semaphore becomes runnable with TN_RC_OK returned from tn_sem_wait().

if semaphore counter is already has its max value, no action performed and TN_RC_OVERFLOW is returned

(refer to Legend for details)

Parameters

sem	semaphore to signal

Returns

• TN_RC_OK if successful
• TN_RC_WCONTEXT if called from wrong context;
• TN_RC_OVERFLOW if count is already at maximum value (max_count)
• If TN_CHECK_PARAM is non-zero, additional return codes are available: TN_RC_WPARAM and TN_RC_INVALID_OBJ.

tn_sem_isignal()

enum TN_RCode tn_sem_isignal

(

struct TN_Sem *

sem

)

The same as tn_sem_signal() but for using in the ISR.

(refer to Legend for details)

tn_sem_wait()

enum TN_RCode tn_sem_wait	(	struct TN_Sem *	sem,
		TN_TickCnt	timeout
	)

Wait for the semaphore.

If the current semaphore counter (count) is non-zero, it is decremented and TN_RC_OK is returned. Otherwise, behavior depends on timeout value: task might switch to WAIT state until someone signaled the semaphore or until the timeout expired. refer to TN_TickCnt.

(refer to Legend for details)

Parameters

sem	semaphore to wait for
timeout	refer to TN_TickCnt

Returns

• TN_RC_OK if waiting was successfull
• Other possible return codes depend on timeout value, refer to TN_TickCnt
• If TN_CHECK_PARAM is non-zero, additional return codes are available: TN_RC_WPARAM and TN_RC_INVALID_OBJ.

tn_sem_wait_polling()

enum TN_RCode tn_sem_wait_polling

(

struct TN_Sem *

sem

)

The same as tn_sem_wait() with zero timeout.

(refer to Legend for details)

tn_sem_iwait_polling()

enum TN_RCode tn_sem_iwait_polling

(

struct TN_Sem *

sem

)

The same as tn_sem_wait() with zero timeout, but for using in the ISR.

(refer to Legend for details)