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Code Block
titleOS_SEM data type
typedef  struct  os_sem  OS_SEM;              (1) 
 
 
struct  os_sem {
    OS_OBJ_TYPE          Type;                (2) 
    CPU_CHAR            *NamePtr;             (3) 
    OS_PEND_LIST         PendList;            (4) 
    OS_SEM_CTR           Ctr;                 (5) 
    CPU_TS               TS;                  (6) 
};


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(1) In µC/OS-III, all structures are given a data type. In fact, all data types start with “OS_” and are all uppercase. When a semaphore is declared, simply use OS_SEM as the data type of the variable used to declare the semaphore.

(2) The structure starts with a “Type” field, which allows it to be recognized by µC/OS-III as a semaphore. In other words, other kernel objects will also have a “Type” as the first member of the structure. If a function is passed a kernel object, µC/OS-III will confirm that it is being passed the proper data type (assuming OS_CFG_OBJ_TYPE_CHK_EN is set to DEF_ENABLED in os_cfg.h). For example, if passing a message queue (OS_Q) to a semaphore service (for example OSSemPend()), µC/OS-III will recognize that an invalid object was passed, and return an error code accordingly.

(3) Each kernel object can be given a name to make them easier to be recognized by debuggers or µC/Probe. This member is simply a pointer to an ASCII string, which is assumed to be NUL terminated.

(4) Since it is possible for multiple tasks to be waiting (or pending) on a semaphore, the semaphore object contains a pend list as described in Pend Lists.

(5) A semaphore contains a counter. As explained above, the counter can be implemented as either an 8-, 16- or 32-bit value, depending on how the data type OS_SEM_CTR is declared in os_type.h. µC/OS-III keeps track of how many times the semaphore is signaled with this counter and this field is typically initialized to zero by OSSemCreate().

(6) A semaphore contains a time stamp, which is used to indicate the last time the semaphore was signaled (or posted to). µC/OS-III assumes the presence of a free-running counter that allows the application to make time measurements. When the semaphore is signaled, the free-running counter is read and the value is placed in this field, which is returned when OSSemPend() is called. This value allows the application to determine either when the signal was performed, or how long it took for the task to get control of the CPU from the signal. In the latter case, you should call OS_TS_GET() to determine the current timestamp and compute the difference.

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