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><H1
><A
NAME="KERNEL-FLAGS">Event Flags</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN1555"
></A
><H2
>Name</H2
>cyg_flag_init, cyg_flag_destroy, cyg_flag_setbits, cyg_flag_maskbits, cyg_flag_wait, cyg_flag_timed_wait, cyg_flag_poll, cyg_flag_peek, cyg_flag_waiting -- Synchronization primitive</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN1566"><H2
>Synopsis</H2
><DIV
CLASS="FUNCSYNOPSIS"
><A
NAME="AEN1567"><P
></P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="FUNCSYNOPSISINFO"
>#include <cyg/kernel/kapi.h>
</PRE
></TD
></TR
></TABLE
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_flag_init</CODE
>(cyg_flag_t* flag);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_flag_destroy</CODE
>(cyg_flag_t* flag);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_flag_setbits</CODE
>(cyg_flag_t* flag, cyg_flag_value_t value);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_flag_maskbits</CODE
>(cyg_flag_t* flag, cyg_flag_value_t value);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>cyg_flag_value_t cyg_flag_wait</CODE
>(cyg_flag_t* flag, cyg_flag_value_t pattern, cyg_flag_mode_t mode);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>cyg_flag_value_t cyg_flag_timed_wait</CODE
>(cyg_flag_t* flag, cyg_flag_value_t pattern, cyg_flag_mode_t mode, cyg_tick_count_t abstime);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>cyg_flag_value_t cyg_flag_poll</CODE
>(cyg_flag_t* flag, cyg_flag_value_t pattern, cyg_flag_mode_t mode);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>cyg_flag_value_t cyg_flag_peek</CODE
>(cyg_flag_t* flag);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>cyg_bool_t cyg_flag_waiting</CODE
>(cyg_flag_t* flag);</CODE
></P
><P
></P
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="KERNEL-FLAGS-DESCRIPTION"
></A
><H2
>Description</H2
><P
>Event flags allow a consumer thread to wait for one of several
different types of event to occur. Alternatively it is possible to
wait for some combination of events. The implementation is relatively
straightforward. Each event flag contains a 32-bit integer.
Application code associates these bits with specific events, so for
example bit 0 could indicate that an I/O operation has completed and
data is available, while bit 1 could indicate that the user has
pressed a start button. A producer thread or a DSR can cause one or
more of the bits to be set, and a consumer thread currently waiting
for these bits will be woken up.
</P
><P
>Unlike semaphores no attempt is made to keep track of event counts. It
does not matter whether a given event occurs once or multiple times
before being consumed, the corresponding bit in the event flag will
change only once. However semaphores cannot easily be used to handle
multiple event sources. Event flags can often be used as an
alternative to condition variables, although they cannot be used for
completely arbitrary conditions and they only support the equivalent
of condition variable broadcasts, not signals.
</P
><P
>Before an event flag can be used it must be initialized by a call to
<TT
CLASS="FUNCTION"
>cyg_flag_init</TT
>. This takes a pointer to a
<SPAN
CLASS="STRUCTNAME"
>cyg_flag_t</SPAN
> data structure, which can be part of a
larger structure. All 32 bits in the event flag will be set to 0,
indicating that no events have yet occurred. If an event flag is no
longer required it can be cleaned up with a call to
<TT
CLASS="FUNCTION"
>cyg_flag_destroy</TT
>, allowing the memory for the
<TT
CLASS="STRUCTFIELD"
><I
>cyg_flag_t</I
></TT
> structure to be re-used.
</P
><P
>A consumer thread can wait for one or more events by calling
<TT
CLASS="FUNCTION"
>cyg_flag_wait</TT
>. This takes three arguments. The
first identifies a particular event flag. The second is some
combination of bits, indicating which events are of interest. The
final argument should be one of the following:
</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="LITERAL"
>CYG_FLAG_WAITMODE_AND</TT
></DT
><DD
><P
>The call to <TT
CLASS="FUNCTION"
>cyg_flag_wait</TT
> will block until all
the specified event bits are set. The event flag is not cleared when
the wait succeeds, in other words all the bits remain set.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>CYG_FLAG_WAITMODE_OR</TT
></DT
><DD
><P
>The call will block until at least one of the specified event bits is
set. The event flag is not cleared on return.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>CYG_FLAG_WAITMODE_AND | CYG_FLAG_WAITMODE_CLR</TT
></DT
><DD
><P
>The call will block until all the specified event bits are set, and
the entire event flag is cleared when the call succeeds. Note that
if this mode of operation is used then a single event flag cannot be
used to store disjoint sets of events, even though enough bits might
be available. Instead each disjoint set of events requires its own
event flag.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>CYG_FLAG_WAITMODE_OR | CYG_FLAG_WAITMODE_CLR</TT
></DT
><DD
><P
>The call will block until at least one of the specified event bits is
set, and the entire flag is cleared when the call succeeds.
</P
></DD
></DL
></DIV
><P
>A call to <TT
CLASS="FUNCTION"
>cyg_flag_wait</TT
> normally blocks until the
required condition is satisfied. It will return the value of the event
flag at the point that the operation succeeded, which may be a
superset of the requested events. If
<TT
CLASS="FUNCTION"
>cyg_thread_release</TT
> is used to unblock a thread
that is currently in a wait operation, the
<TT
CLASS="FUNCTION"
>cyg_flag_wait</TT
> call will instead return 0.
</P
><P
><TT
CLASS="FUNCTION"
>cyg_flag_timed_wait</TT
> is a variant of
<TT
CLASS="FUNCTION"
>cyg_flag_wait</TT
> which adds a timeout: the wait
operation must succeed within the specified number of ticks, or it
will fail with a return value of 0. <TT
CLASS="FUNCTION"
>cyg_flag_poll</TT
>
is a non-blocking variant: if the wait operation can succeed
immediately it acts like <TT
CLASS="FUNCTION"
>cyg_flag_wait</TT
>, otherwise
it returns immediately with a value of 0.
</P
><P
><TT
CLASS="FUNCTION"
>cyg_flag_setbits</TT
> is called by a producer thread
or from inside a DSR when an event occurs. The specified bits are or'd
into the current event flag value. This may cause a waiting thread to
be woken up, if its condition is now satisfied.
</P
><P
><TT
CLASS="FUNCTION"
>cyg_flag_maskbits</TT
> can be used to clear one or
more bits in the event flag. This can be called from a producer when a
particular condition is no longer satisfied, for example when the user
is no longer pressing a particular button. It can also be used by a
consumer thread if <TT
CLASS="LITERAL"
>CYG_FLAG_WAITMODE_CLR</TT
> was not
used as part of the wait operation, to indicate that some but not all
of the active events have been consumed. If there are multiple
consumer threads performing wait operations without using
<TT
CLASS="FUNCTION"
>CYG_FLAG_WAITMODE_CLR</TT
> then typically some
additional synchronization such as a mutex is needed to prevent
multiple threads consuming the same event.
</P
><P
>Two additional functions are provided to query the current state of an
event flag. <TT
CLASS="FUNCTION"
>cyg_flag_peek</TT
> returns the current
value of the event flag, and <TT
CLASS="FUNCTION"
>cyg_flag_waiting</TT
> can
be used to find out whether or not there are any threads currently
blocked on the event flag. Both of these functions must be used with
care because other threads may be operating on the event flag.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="KERNEL-FLAGS-CONTEXT"
></A
><H2
>Valid contexts</H2
><P
><TT
CLASS="FUNCTION"
>cyg_flag_init</TT
> is typically called during system
initialization but may also be called in thread context. The same
applies to <TT
CLASS="FUNCTION"
>cyg_flag_destroy</TT
>.
<TT
CLASS="FUNCTION"
>cyg_flag_wait</TT
> and
<TT
CLASS="FUNCTION"
>cyg_flag_timed_wait</TT
> may only be called from
thread context. The remaining functions may be called from thread or
DSR context.
</P
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