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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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>Thread control</TITLE
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>eCos Reference Manual</TH
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><H1
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><A
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NAME="KERNEL-THREAD-CONTROL">Thread control</H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN482"
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></A
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><H2
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>Name</H2
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>cyg_thread_yield, cyg_thread_delay, cyg_thread_suspend, cyg_thread_resume, cyg_thread_release&nbsp;--&nbsp;Control whether or not a thread is running</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN489"><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="AEN490"><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_thread_yield</CODE
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>(void);</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_thread_delay</CODE
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>(cyg_tick_count_t delay);</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_thread_suspend</CODE
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>(cyg_handle_t thread);</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_thread_resume</CODE
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>(cyg_handle_t thread);</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_thread_release</CODE
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>(cyg_handle_t thread);</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="AEN516"
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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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>These functions provide some control over whether or not a particular
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thread can run. Apart from the required use of
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<TT
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CLASS="FUNCTION"
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>cyg_thread_resume</TT
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> to start a newly-created
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thread, application code should normally use proper synchronization
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primitives such as condition variables or mail boxes.
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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="AEN520"
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></A
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><H2
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>Yield</H2
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><P
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><TT
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CLASS="FUNCTION"
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>cyg_thread_yield</TT
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> allows a thread to relinquish
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control of the processor to some other runnable thread which has the
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same priority. This can have no effect on any higher-priority thread
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since, if such a thread were runnable, the current thread would have
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been preempted in its favour. Similarly it can have no effect on any
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lower-priority thread because the current thread will always be run in
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preference to those. As a consequence this function is only useful
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in configurations with a scheduler that allows multiple threads to run
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at the same priority, for example the mlqueue scheduler. If instead
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the bitmap scheduler was being used then
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<TT
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CLASS="FUNCTION"
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>cyg_thread_yield()</TT
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> would serve no purpose.
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      </P
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><P
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>Even if a suitable scheduler such as the mlqueue scheduler has been
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configured, <TT
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CLASS="FUNCTION"
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>cyg_thread_yield</TT
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> will still rarely
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prove useful: instead timeslicing will be used to ensure that all
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threads of a given priority get a fair slice of the available
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processor time. However it is possible to disable timeslicing via the
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configuration option <TT
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CLASS="VARNAME"
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>CYGSEM_KERNEL_SCHED_TIMESLICE</TT
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>,
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in which case <TT
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CLASS="FUNCTION"
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>cyg_thread_yield</TT
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> can be used to
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implement a form of cooperative multitasking.
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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="AEN529"
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></A
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><H2
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>Delay</H2
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><P
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><TT
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CLASS="FUNCTION"
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>cyg_thread_delay</TT
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> allows a thread to suspend until
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the specified number of clock ticks have occurred. For example, if a
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value of 1 is used and the system clock runs at a frequency of 100Hz
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then the thread will sleep for up to 10 milliseconds. This
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functionality depends on the presence of a real-time system clock, as
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controlled by the configuration option
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<TT
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CLASS="VARNAME"
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>CYGVAR_KERNEL_COUNTERS_CLOCK</TT
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>.
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      </P
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><P
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>If the application requires delays measured in milliseconds or similar
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units rather than in clock ticks, some calculations are needed to
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convert between these units as described in <A
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HREF="kernel-clocks.html"
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>Clocks</A
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>. Usually these calculations can be done by
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the application developer, or at compile-time. Performing such
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calculations prior to every call to
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<TT
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CLASS="FUNCTION"
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>cyg_thread_delay</TT
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> adds unnecessary overhead to the
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system.
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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="AEN537"
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></A
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><H2
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>Suspend and Resume</H2
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><P
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>Associated with each thread is a suspend counter. When a thread is
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first created this counter is initialized to 1.
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<TT
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CLASS="FUNCTION"
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>cyg_thread_suspend</TT
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> can be used to increment the
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suspend counter, and <TT
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CLASS="FUNCTION"
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>cyg_thread_resume</TT
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> decrements
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it. The scheduler will never run a thread with a non-zero suspend
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counter. Therefore a newly created thread will not run until it has
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been resumed.
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      </P
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><P
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>An occasional problem with the use of suspend and resume functionality
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is that a thread gets suspended more times than it is resumed and
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hence never becomes runnable again. This can lead to very confusing
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behaviour. To help with debugging such problems the kernel provides a
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configuration option
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<TT
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CLASS="VARNAME"
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>CYGNUM_KERNEL_MAX_SUSPEND_COUNT_ASSERT</TT
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> which
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imposes an upper bound on the number of suspend calls without matching
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resumes, with a reasonable default value. This functionality depends
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on infrastructure assertions being enabled.
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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="AEN544"
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></A
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><H2
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>Releasing a Blocked Thread</H2
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><P
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>When a thread is blocked on a synchronization primitive such as a
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semaphore or a mutex, or when it is waiting for an alarm to trigger,
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it can be forcibly woken up using
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<TT
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CLASS="FUNCTION"
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>cyg_thread_release</TT
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>. Typically this will call the
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affected synchronization primitive to return false, indicating that
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the operation was not completed successfully. This function has to be
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used with great care, and in particular it should only be used on
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threads that have been designed appropriately and check all return
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codes. If instead it were to be used on, say, an arbitrary thread that
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is attempting to claim a mutex then that thread might not bother to
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check the result of the mutex lock operation - usually there would be
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no reason to do so. Therefore the thread will now continue running in
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the false belief that it has successfully claimed a mutex lock, and
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the resulting behaviour is undefined. If the system has been built
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with assertions enabled then it is possible that an assertion will
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trigger when the thread tries to release the mutex it does not
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actually own.
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      </P
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><P
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>The main use of <TT
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CLASS="FUNCTION"
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>cyg_thread_release</TT
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> is in the
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POSIX compatibility layer, where it is used in the implementation of
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per-thread signals and cancellation handlers.
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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-THREAD-CONTROL-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_thread_yield</TT
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> can only be called from thread
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context, A DSR must always run to completion and cannot yield the
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processor to some thread. <TT
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CLASS="FUNCTION"
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>cyg_thread_suspend</TT
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>,
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<TT
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CLASS="FUNCTION"
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>cyg_thread_resume</TT
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>, and
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<TT
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CLASS="FUNCTION"
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>cyg_thread_release</TT
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> may be called from thread or
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DSR context.
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      </P
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