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><H1
><A
NAME="FILEIO-SELECT">Chapter 27. Select</H1
><P
>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.</P
><P
>The main part of the mechanism is the <TT
CLASS="FUNCTION"
>select()</TT
>
API call. This processes its arguments and calls the
<TT
CLASS="FUNCTION"
>fo_select()</TT
> 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
<TT
CLASS="FUNCTION"
>fo_select()</TT
> function will be called separately
for each appearance.</P
><P
>The <TT
CLASS="PARAMETER"
><I
>which</I
></TT
> argument of the
<TT
CLASS="FUNCTION"
>fo_select()</TT
> function will either be
<TT
CLASS="LITERAL"
>CYG_FREAD</TT
> to test for read conditions,
<TT
CLASS="LITERAL"
>CYG_FWRITE</TT
> 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
<TT
CLASS="FUNCTION"
>cyg_selrecord()</TT
> with the
<TT
CLASS="PARAMETER"
><I
>info</I
></TT
> argument that was passed to the function
and a pointer to a <SPAN
CLASS="STRUCTNAME"
>cyg_selinfo</SPAN
> structure.</P
><P
>The <SPAN
CLASS="STRUCTNAME"
>cyg_selinfo</SPAN
> 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 <SPAN
CLASS="STRUCTNAME"
>cyg_selinfo</SPAN
>
structures should be kept for each of the options that the object can
select on - read, write or exception.</P
><P
>If none of the file objects report that the select condition is
satisfied, then the <TT
CLASS="FUNCTION"
>select()</TT
> API function puts
the calling thread to sleep waiting either for a condition to become
satisfied, or for the optional timeout to expire.</P
><P
>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 <TT
CLASS="FUNCTION"
>cyg_selwakeup()</TT
> with a pointer to
the appropriate <SPAN
CLASS="STRUCTNAME"
>cyg_selinfo</SPAN
> 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
<TT
CLASS="FUNCTION"
>select()</TT
> API call will return.</P
><P
>Note that <TT
CLASS="FUNCTION"
>select()</TT
> 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.</P
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