Control access to shared resources. More...

Macros
#define	osFeature_Semaphore 30
	maximum count for osSemaphoreCreate function More...

#define	osSemaphoreDef(name) const osSemaphoreDef_t os_semaphore_def_##name = { 0 }
	Define a Semaphore object. More...

#define	osSemaphore(name) &os_semaphore_def_##name
	Access a Semaphore definition. More...

Functions
osSemaphoreId	osSemaphoreCreate (const osSemaphoreDef_t *semaphore_def, int32_t count)
	Create and Initialize a Semaphore object used for managing resources. More...

int32_t	osSemaphoreWait (osSemaphoreId semaphore_id, uint32_t millisec)
	Wait until a Semaphore token becomes available. More...

osStatus	osSemaphoreRelease (osSemaphoreId semaphore_id)
	Release a Semaphore token. More...

osStatus	osSemaphoreDelete (osSemaphoreId semaphore_id)
	Delete a Semaphore that was created by osSemaphoreCreate. More...

Description

The Semaphore Management function group is used to manage and protect access to shared resources. For example, with a Semaphore the access to a group of identical peripherals can be managed. The number of available resources is specified as parameter of the osSemaphoreCreate function.

Each time a Semaphore token is obtained with osSemaphoreWait the semaphore count is decremented. When the semaphore count is 0, no Semaphore token can be obtained. Semaphores are released with osSemaphoreRelease; this function increments the semaphore count.

CMSIS-RTOS Semaphore

Macro Definition Documentation

#define osFeature_Semaphore 30

A CMSIS-RTOS implementation may support semaphores. The value osFeature_Semaphore indicates the maximum index count for a semaphore.

#define osSemaphore ( name ) &os_semaphore_def_##name

Access to semaphore object for the functions osSemaphoreCreate.

Parameters

name	name of the semaphore object.

Note: CAN BE CHANGED: The parameter to osSemaphore shall be consistent but the macro body is implementation specific in every CMSIS-RTOS.

#define osSemaphoreDef ( name ) const osSemaphoreDef_t os_semaphore_def_##name = { 0 }

Define a semaphore object that is referenced by osSemaphore.

Parameters

name	name of the semaphore object.

Note: CAN BE CHANGED: The parameter to osSemaphoreDef shall be consistent but the macro body is implementation specific in every CMSIS-RTOS.

Function Documentation

osSemaphoreId osSemaphoreCreate	(	const osSemaphoreDef_t *	semaphore_def,
		int32_t	count
	)

Parameters

[in]	semaphore_def	semaphore definition referenced with osSemaphore.
[in]	count	number of available resources.

Returns: semaphore ID for reference by other functions or NULL in case of error.

Note: MUST REMAIN UNCHANGED: osSemaphoreCreate shall be consistent in every CMSIS-RTOS.

Create and initialize a Semaphore object that is used to manage access to shared resources. The parameter count specifies the number of available resources. The count value 1 creates a binary semaphore.

Example

#include "cmsis_os.h"
   
osThreadId tid_thread1;                          // ID for thread 1
osThreadId tid_thread2;                          // ID for thread 2
  
osSemaphoreId semaphore;                         // Semaphore ID
osSemaphoreDef(semaphore);                       // Semaphore definition
   
/*
     Thread 1 - High Priority - Active every 3ms
 */
void thread1 (void const *argument) {
  int32_t value;
  while (1) {
    osDelay(3);                                  // Pass control to other tasks for 3ms
    val = osSemaphoreWait (semaphore, 1);        // Wait 1ms for the free semaphore
    if (val > 0) {
                                                 // If there was no time-out the semaphore was acquired
      :                                          // OK, the interface is free now, use it.
      osSemaphoreRelease (semaphore);            // Return a token back to a semaphore
    }
  }
}
  
/*
     Thread 2 - Normal Priority - looks for a free semaphore and uses
                                  the resource whenever it is available
 */
void thread2 (void const *argument) {
  while (1) {
    osSemaphoreWait (semaphore, osWaitForever);  // Wait indefinitely for a free semaphore
                                                 // OK, the interface is free now, use it.
    :
    osSemaphoreRelease (semaphore);              // Return a token back to a semaphore.
  }
}
  
/* Thread definitions */
osThreadDef(thread1, osPriorityHigh,   1, 0);
osThreadDef(thread2, osPriorityNormal, 1, 0);
   
void StartApplication (void) {
  semaphore = osSemaphoreCreate(osSemaphore(semaphore), 1);
  tid_thread1 = osThreadCreate(osThread(thread1), NULL);
  tid_thread2 = osThreadCreate(osThread(thread2), NULL);
  :
}

osStatus osSemaphoreDelete ( osSemaphoreId semaphore_id )

Parameters

[in] semaphore_id semaphore object referenced with osSemaphoreCreate.

Returns: status code that indicates the execution status of the function.

Note: MUST REMAIN UNCHANGED: osSemaphoreDelete shall be consistent in every CMSIS-RTOS.

Delete a Semaphore object. The function releases internal memory obtained for Semaphore handling. After this call the semaphore_id is no longer valid and cannot be used. The Semaphore may be created again using the function osSemaphoreCreate.

Status and Error Codes

osOK: the semaphore object has been deleted.
osErrorISR: osSemaphoreDelete cannot be called from interrupt service routines.
osErrorResource: the semaphore object could not be deleted.
osErrorParameter: the parameter semaphore_id is incorrect.

osStatus osSemaphoreRelease ( osSemaphoreId semaphore_id )

Parameters

[in] semaphore_id semaphore object referenced with osSemaphoreCreate.

Returns: status code that indicates the execution status of the function.

Note: MUST REMAIN UNCHANGED: osSemaphoreRelease shall be consistent in every CMSIS-RTOS.

Release a Semaphore token. This increments the count of available semaphore tokens.

Note: osSemaphoreRelease can be called also from interrupt service routines.

Status and Error Codes

osOK: the semaphore has been released.
osErrorResource: all tokens have already been released.
osErrorParameter: the parameter semaphore_id is incorrect.

int32_t osSemaphoreWait	(	osSemaphoreId	semaphore_id,
		uint32_t	millisec
	)

Parameters

[in]	semaphore_id	semaphore object referenced with osSemaphoreCreate.
[in]	millisec	timeout value or 0 in case of no time-out.

Returns: number of available tokens, or -1 in case of incorrect parameters.

Note: MUST REMAIN UNCHANGED: osSemaphoreWait shall be consistent in every CMSIS-RTOS.

Wait until a Semaphore token becomes available. When no Semaphore token is available, the function waits for the time specified with the parameter millisec.

The argument millisec specifies how long the system waits for a Semaphore token to become available. While the system waits the thread that is calling this function is put into the state WAITING. The millisec timeout can have the following values:

when millisec is 0, the function returns instantly.
when millisec is set to osWaitForever the function will wait for an infinite time until the Semaphore token becomes available.
all other values specify a time in millisecond for a timeout.

The return value indicates the number of available tokens (the semaphore count value). If 0 is returned, then no semaphore was available.

Macros

Functions

Description

Macro Definition Documentation

Function Documentation