+++ /dev/null
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-<HTML
-><HEAD
-><TITLE
->Select</TITLE
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-TITLE="eCos Reference Manual"
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-REL="UP"
-TITLE="File System Support Infrastructure"
-HREF="fileio.html"><LINK
-REL="PREVIOUS"
-TITLE="Sockets"
-HREF="fileio-sockets.html"><LINK
-REL="NEXT"
-TITLE="Devices"
-HREF="fileio-devices.html"></HEAD
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-><DIV
-CLASS="CHAPTER"
-><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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->Prev</A
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-WIDTH="33%"
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-VALIGN="top"
->Sockets</TD
-><TD
-WIDTH="34%"
-ALIGN="center"
-VALIGN="top"
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-HREF="fileio.html"
-ACCESSKEY="U"
->Up</A
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-VALIGN="top"
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