Chapter 29. 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.