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>Mail boxes</TITLE
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><HR
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><H1
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><A
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NAME="KERNEL-MAIL-BOXES">Mail boxes</H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN1425"
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></A
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><H2
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>Name</H2
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>cyg_mbox_create, cyg_mbox_delete, cyg_mbox_get, cyg_mbox_timed_get, cyg_mbox_tryget, cyg_mbox_peek_item, cyg_mbox_put, cyg_mbox_timed_put, cyg_mbox_tryput, cyg_mbox_peek, cyg_mbox_waiting_to_get, cyg_mbox_waiting_to_put&nbsp;--&nbsp;Synchronization primitive</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN1439"><H2
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>Synopsis</H2
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><DIV
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CLASS="FUNCSYNOPSIS"
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><A
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NAME="AEN1440"><P
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></P
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><TABLE
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BORDER="5"
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
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><TD
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><PRE
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CLASS="FUNCSYNOPSISINFO"
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>#include &lt;cyg/kernel/kapi.h&gt;
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        </PRE
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></TD
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></TR
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></TABLE
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void cyg_mbox_create</CODE
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>(cyg_handle_t* handle, cyg_mbox* mbox);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void cyg_mbox_delete</CODE
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>(cyg_handle_t mbox);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void* cyg_mbox_get</CODE
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>(cyg_handle_t mbox);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void* cyg_mbox_timed_get</CODE
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>(cyg_handle_t mbox, cyg_tick_count_t abstime);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void* cyg_mbox_tryget</CODE
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>(cyg_handle_t mbox);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>cyg_count32 cyg_mbox_peek</CODE
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>(cyg_handle_t mbox);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void* cyg_mbox_peek_item</CODE
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>(cyg_handle_t mbox);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>cyg_bool_t cyg_mbox_put</CODE
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>(cyg_handle_t mbox, void* item);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>cyg_bool_t cyg_mbox_timed_put</CODE
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>(cyg_handle_t mbox, void* item, cyg_tick_count_t abstime);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>cyg_bool_t cyg_mbox_tryput</CODE
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>(cyg_handle_t mbox, void* item);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>cyg_bool_t cyg_mbox_waiting_to_get</CODE
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>(cyg_handle_t mbox);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>cyg_bool_t cyg_mbox_waiting_to_put</CODE
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>(cyg_handle_t mbox);</CODE
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></P
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><P
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></P
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></DIV
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="KERNEL-MAIL-BOXES-DESCRIPTION"
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></A
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><H2
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>Description</H2
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><P
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>Mail boxes are a synchronization primitive. Like semaphores they
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can be used by a consumer thread to wait until a certain event has
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occurred, but the producer also has the ability to transmit some data
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along with each event. This data, the message, is normally a pointer
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to some data structure. It is stored in the mail box itself, so the
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producer thread that generates the event and provides the data usually
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does not have to block until some consumer thread is ready to receive
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the event. However a mail box will only have a finite capacity,
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typically ten slots. Even if the system is balanced and events are
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typically consumed at least as fast as they are generated, a burst of
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events can cause the mail box to fill up and the generating thread
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will block until space is available again. This behaviour is very
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different from semaphores, where it is only necessary to maintain a
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counter and hence an overflow is unlikely.
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      </P
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><P
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>Before a mail box can be used it must be created with a call to
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<TT
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CLASS="FUNCTION"
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>cyg_mbox_create</TT
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>. Each mail box has a unique
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handle which will be returned via the first argument and which should
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be used for subsequent operations.
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<TT
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CLASS="FUNCTION"
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>cyg_mbox_create</TT
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> also requires an area of memory
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for the kernel structure, which is provided by the
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<SPAN
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CLASS="STRUCTNAME"
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>cyg_mbox</SPAN
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> second argument. If a mail box is
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no longer required then <TT
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CLASS="FUNCTION"
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>cyg_mbox_delete</TT
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> can be
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used. This will simply discard any messages that remain posted.
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      </P
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><P
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>The main function for waiting on a mail box is
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<TT
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CLASS="FUNCTION"
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>cyg_mbox_get</TT
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>. If there is a pending message
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because of a call to <TT
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CLASS="FUNCTION"
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>cyg_mbox_put</TT
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> then
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<TT
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CLASS="FUNCTION"
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>cyg_mbox_get</TT
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> will return immediately with the
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message that was put into the mail box. Otherwise this function
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will block until there is a put operation. Exceptionally the thread
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can instead be unblocked by a call to
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<TT
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CLASS="FUNCTION"
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>cyg_thread_release</TT
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>, in which case
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<TT
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CLASS="FUNCTION"
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>cyg_mbox_get</TT
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> will return a null pointer. It is
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assumed that there will never be a call to
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<TT
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CLASS="FUNCTION"
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>cyg_mbox_put</TT
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> with a null pointer, because it
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would not be possible to distinguish between that and a release
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operation. Messages are always retrieved in the order in which they
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were put into the mail box, and there is no support for messages
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with different priorities.
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      </P
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><P
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>There are two variants of <TT
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CLASS="FUNCTION"
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>cyg_mbox_get</TT
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>. The
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first, <TT
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CLASS="FUNCTION"
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>cyg_mbox_timed_get</TT
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> will wait until either
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a message is available or until a number of clock ticks have occurred.
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If no message is posted within the timeout then a null pointer will be
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returned. <TT
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CLASS="FUNCTION"
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>cyg_mbox_tryget</TT
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> is a non-blocking
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operation which will either return a message if one is available or a
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null pointer.
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      </P
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><P
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>New messages are placed in the mail box by calling
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<TT
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CLASS="FUNCTION"
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>cyg_mbox_put</TT
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> or one of its variants. The main put
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function takes two arguments, a handle to the mail box and a
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pointer for the message itself. If there is a spare slot in the
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mail box then the new message can be placed there immediately, and
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if there is a waiting thread it will be woken up so that it can
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receive the message. If the mail box is currently full then
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<TT
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CLASS="FUNCTION"
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>cyg_mbox_put</TT
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> will block until there has been a
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get operation and a slot is available. The
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<TT
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CLASS="FUNCTION"
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>cyg_mbox_timed_put</TT
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> variant imposes a time limit
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on the put operation, returning false if the operation cannot be
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completed within the specified number of clock ticks. The
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<TT
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CLASS="FUNCTION"
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>cyg_mbox_tryput</TT
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> variant is non-blocking,
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returning false if there are no free slots available and the message
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cannot be posted without blocking.
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      </P
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><P
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>There are a further four functions available for examining the current
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state of a mailbox. The results of these functions must be used with
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care because usually the state can change at any time as a result of
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activity within other threads, but they may prove occasionally useful
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during debugging or in special situations.
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<TT
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CLASS="FUNCTION"
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>cyg_mbox_peek</TT
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> returns a count of the number of
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messages currently stored in the mail box.
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<TT
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CLASS="FUNCTION"
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>cyg_mbox_peek_item</TT
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> retrieves the first message,
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but it remains in the mail box until a get operation is performed.
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<TT
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CLASS="FUNCTION"
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>cyg_mbox_waiting_to_get</TT
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> and
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<TT
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CLASS="FUNCTION"
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>cyg_mbox_waiting_to_put</TT
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> indicate whether or not
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there are currently threads blocked in a get or a put operation on a
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given mail box.
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      </P
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><P
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>The number of slots in each mail box is controlled by a
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configuration option
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<TT
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CLASS="VARNAME"
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>CYGNUM_KERNEL_SYNCH_MBOX_QUEUE_SIZE</TT
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>, with a default
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value of 10. All mail boxes are the same size.
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      </P
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="KERNEL-MAIL-BOXES-CONTEXT"
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></A
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><H2
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>Valid contexts</H2
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><P
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><TT
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CLASS="FUNCTION"
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>cyg_mbox_create</TT
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> is typically called during
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system initialization but may also be called in thread context.
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The remaining functions are normally called only during thread
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context. Of special note is <TT
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CLASS="FUNCTION"
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>cyg_mbox_put</TT
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> which
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can be a blocking operation when the mail box is full, and which
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therefore must never be called from DSR context. It is permitted to
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call <TT
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CLASS="FUNCTION"
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>cyg_mbox_tryput</TT
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>,
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<TT
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CLASS="FUNCTION"
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>cyg_mbox_tryget</TT
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>, and the information functions
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from DSR context but this is rarely useful.
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      </P
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