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Binary semaphores

Posted by Nobody/Anonymous on December 11, 2005
I have few questions on binary semaphore. They're as follows:

1. Is there a way to initialize the initial value in a semaphore?
2. Is it possible to create counting semaphore with FreeRTOS? If Yes,
how do you initialize the counting value?


RE: Binary semaphores

Posted by Richard on December 11, 2005
> 1. Is there a way to initialize the initial value in a semaphore?

With the aim of keeping the source code as small as posible, the semaphore implementation is simply a set of macros that use the existing queue implementation.

If you take a look at semphr.h you will notice that when a semaphore is created, it is immediately "given", so the first call to "take" the semaphore will succeed. If you want to change this behaviour then there are a couple of options.

1) You could change the macro to accept a parameter, and the parameter value is used say whether or not to "give" the semaphore or not.

2) If you want the semaphore to start empty, then simply call xSemaphoreTake() immediately after the call to vSemaphoreCreateBinary() (or at least before the semaphore is used). This is how the demo applications use the semaphore.


> 2. Is it possible to create counting semaphore with FreeRTOS? If Yes,
> how do you initialize the counting value?

The vSemaphoreCreateBinary() macro creates a queue of length 1 for the binary semaphore. If you want a counting semaphore you can create a macro that creates a queue of longer length.

Regards.


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