eCos Reference Manual
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# Chapter 22. Select

The infrastructure provides support for implementing a select mechanism. This is modeled on the mechanism in the BSD kernel, but has been modified to make it implementation independent.

The main part of the mechanism is the `select()` API call. This processes its arguments and calls the `fo_select()` function pointer on all file objects referenced by the file descriptor sets passed to it. If the same descriptor appears in more than one descriptor set, the `fo_select()` function will be called separately for each appearance.

The `which` argument of the `fo_select()` function will either be `CYG_FREAD` to test for read conditions, `CYG_FWRITE` to test for write conditions or zero to test for exceptions. For each of these options the function should test whether the condition is satisfied and if so return true. If it is not satisfied then it should call `cyg_selrecord()` with the `info` argument that was passed to the function and a pointer to a `cyg_selinfo` structure.

The `cyg_selinfo` structure is used to record information about current select operations. Any object that needs to support select must contain an instance of this structure. Separate `cyg_selinfo` structures should be kept for each of the options that the object can select on - read, write or exception.

If none of the file objects report that the select condition is satisfied, then the `select()` API function puts the calling thread to sleep waiting either for a condition to become satisfied, or for the optional timeout to expire.

A selectable object must have some asynchronous activity that may cause a select condition to become true - either via interrupts or the activities of other threads. Whenever a selectable condition is satisfied, the object should call `cyg_selwakeup()` with a pointer to the appropriate `cyg_selinfo` structure. If the thread is still waiting, this will cause it to wake up and repeat its poll of the file descriptors. This time around, the object that caused the wakeup should indicate that the select condition is satisfied, and the `select()` API call will return.

Note that `select()` does not exhibit real time behaviour: the iterative poll of the descriptors, and the wakeup mechanism mitigate against this. If real time response to device or socket I/O is required then separate threads should be devoted to each device of interest and should use blocking calls to wait for a condition to become ready.

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