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><H1
><A
NAME="KERNEL-THREAD-CREATE">Thread creation</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN256"
></A
><H2
>Name</H2
>cyg_thread_create -- Create a new thread</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN259"><H2
>Synopsis</H2
><DIV
CLASS="FUNCSYNOPSIS"
><A
NAME="AEN260"><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_thread_create</CODE
>(cyg_addrword_t sched_info, cyg_thread_entry_t* entry, cyg_addrword_t entry_data, char* name, void* stack_base, cyg_ucount32 stack_size, cyg_handle_t* handle, cyg_thread* thread);</CODE
></P
><P
></P
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="KERNEL-THREAD-CREATE-DESCRIPTION"
></A
><H2
>Description</H2
><P
>The <TT
CLASS="FUNCTION"
>cyg_thread_create</TT
> function allows application
code and eCos packages to create new threads. In many applications
this only happens during system initialization and all required data
is allocated statically. However additional threads can be created at
any time, if necessary. A newly created thread is always in suspended
state and will not start running until it has been resumed via a call
to <TT
CLASS="FUNCTION"
>cyg_thread_resume</TT
>. Also, if threads are
created during system initialization then they will not start running
until the eCos scheduler has been started.
</P
><P
>The <TT
CLASS="PARAMETER"
><I
>name</I
></TT
> argument is used
primarily for debugging purposes, making it easier to keep track of
which <SPAN
CLASS="STRUCTNAME"
>cyg_thread</SPAN
> structure is associated with
which application-level thread. The kernel configuration option
<TT
CLASS="VARNAME"
>CYGVAR_KERNEL_THREADS_NAME</TT
> controls whether or not
this name is actually used.
</P
><P
>On creation each thread is assigned a unique handle, and this will be
stored in the location pointed at by the <TT
CLASS="PARAMETER"
><I
>handle</I
></TT
> argument. Subsequent operations on
this thread including the required
<TT
CLASS="FUNCTION"
>cyg_thread_resume</TT
> should use this handle to
identify the thread.
</P
><P
>The kernel requires a small amount of space for each thread, in the
form of a <SPAN
CLASS="STRUCTNAME"
>cyg_thread</SPAN
> data structure, to hold
information such as the current state of that thread. To avoid any
need for dynamic memory allocation within the kernel this space has to
be provided by higher-level code, typically in the form of a static
variable. The <TT
CLASS="PARAMETER"
><I
>thread</I
></TT
> argument
provides this space.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="KERNEL-THREAD-CREATE-ENTRY"
></A
><H2
>Thread Entry Point</H2
><P
>The entry point for a thread takes the form:
</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>void
thread_entry_function(cyg_addrword_t data)
{
…
}
</PRE
></TD
></TR
></TABLE
><P
>The second argument to <TT
CLASS="FUNCTION"
>cyg_thread_create</TT
> is a
pointer to such a function. The third argument <TT
CLASS="PARAMETER"
><I
>entry_data</I
></TT
> is used to pass additional
data to the function. Typically this takes the form of a pointer to
some static data, or a small integer, or <TT
CLASS="LITERAL"
>0</TT
> if the
thread does not require any additional data.
</P
><P
>If the thread entry function ever returns then this is equivalent to
the thread calling <TT
CLASS="FUNCTION"
>cyg_thread_exit</TT
>. Even though
the thread will no longer run again, it remains registered with the
scheduler. If the application needs to re-use the
<SPAN
CLASS="STRUCTNAME"
>cyg_thread</SPAN
> data structure then a call to
<TT
CLASS="FUNCTION"
>cyg_thread_delete</TT
> is required first.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="KERNEL-THREAD-CREATE-PRIORITIES"
></A
><H2
>Thread Priorities</H2
><P
>The <TT
CLASS="PARAMETER"
><I
>sched_info</I
></TT
> argument
provides additional information to the scheduler. The exact details
depend on the scheduler being used. For the bitmap and mlqueue
schedulers it is a small integer, typically in the range 0 to 31, with
0 being the highest priority. The lowest priority is normally used
only by the system's idle thread. The exact number of priorities is
controlled by the kernel configuration option
<TT
CLASS="VARNAME"
>CYGNUM_KERNEL_SCHED_PRIORITIES</TT
>.
</P
><P
>It is the responsibility of the application developer to be aware of
the various threads in the system, including those created by eCos
packages, and to ensure that all threads run at suitable priorities.
For threads created by other packages the documentation provided by
those packages should indicate any requirements.
</P
><P
>The functions <TT
CLASS="FUNCTION"
>cyg_thread_set_priority</TT
>,
<TT
CLASS="FUNCTION"
>cyg_thread_get_priority</TT
>, and
<TT
CLASS="FUNCTION"
>cyg_thread_get_current_priority</TT
> can be used to
manipulate a thread's priority.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="KERNEL-THREAD-CREATE-STACK"
></A
><H2
>Stacks and Stack Sizes</H2
><P
>Each thread needs its own stack for local variables and to keep track
of function calls and returns. Again it is expected that this stack is
provided by the calling code, usually in the form of static data, so
that the kernel does not need any dynamic memory allocation
facilities. <TT
CLASS="FUNCTION"
>cyg_thread_create</TT
> takes two arguments
related to the stack, a pointer to the base of the stack and the total
size of this stack. On many processors stacks actually descend from the
top down, so the kernel will add the stack size to the base address to
determine the starting location.
</P
><P
>The exact stack size requirements for any given thread depend on a
number of factors. The most important is of course the code that will
be executed in the context of this code: if this involves significant
nesting of function calls, recursion, or large local arrays, then the
stack size needs to be set to a suitably high value. There are some
architectural issues, for example the number of cpu registers and the
calling conventions will have some effect on stack usage. Also,
depending on the configuration, it is possible that some other code
such as interrupt handlers will occasionally run on the current
thread's stack. This depends in part on configuration options such as
<TT
CLASS="VARNAME"
>CYGIMP_HAL_COMMON_INTERRUPTS_USE_INTERRUPT_STACK</TT
>
and <TT
CLASS="VARNAME"
>CYGSEM_HAL_COMMON_INTERRUPTS_ALLOW_NESTING</TT
>.
</P
><P
>Determining an application's actual stack size requirements is the
responsibility of the application developer, since the kernel cannot
know in advance what code a given thread will run. However, the system
does provide some hints about reasonable stack sizes in the form of
two constants: <TT
CLASS="VARNAME"
>CYGNUM_HAL_STACK_SIZE_MINIMUM</TT
> and
<TT
CLASS="VARNAME"
>CYGNUM_HAL_STACK_SIZE_TYPICAL</TT
>. These are defined by
the appropriate HAL package. The <TT
CLASS="VARNAME"
>MINIMUM</TT
> value is
appropriate for a thread that just runs a single function and makes
very simple system calls. Trying to create a thread with a smaller
stack than this is illegal. The <TT
CLASS="VARNAME"
>TYPICAL</TT
> value is
appropriate for applications where application calls are nested no
more than half a dozen or so levels, and there are no large arrays on
the stack.
</P
><P
>If the stack sizes are not estimated correctly and a stack overflow
occurs, the probably result is some form of memory corruption. This
can be very hard to track down. The kernel does contain some code to
help detect stack overflows, controlled by the configuration option
<TT
CLASS="VARNAME"
>CYGFUN_KERNEL_THREADS_STACK_CHECKING</TT
>: a small
amount of space is reserved at the stack limit and filled with a
special signature: every time a thread context switch occurs this
signature is checked, and if invalid that is a good indication (but
not absolute proof) that a stack overflow has occurred. This form of
stack checking is enabled by default when the system is built with
debugging enabled. A related configuration option is
<TT
CLASS="VARNAME"
>CYGFUN_KERNEL_THREADS_STACK_MEASUREMENT</TT
>: enabling
this option means that a thread can call the function
<TT
CLASS="FUNCTION"
>cyg_thread_measure_stack_usage</TT
> to find out the
maximum stack usage to date. Note that this is not necessarily the
true maximum because, for example, it is possible that in the current
run no interrupt occurred at the worst possible moment.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="KERNEL-THREAD-CREATE-CONTEXT"
></A
><H2
>Valid contexts</H2
><P
><TT
CLASS="FUNCTION"
>cyg_thread_create</TT
> may be called during
initialization and from within thread context. It may not be called
from inside a DSR.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="KERNEL-THREAD-CREATE-EXAMPLE"
></A
><H2
>Example</H2
><P
>A simple example of thread creation is shown below. This involves
creating five threads, one producer and four consumers or workers. The
threads are created in the system's
<TT
CLASS="FUNCTION"
>cyg_user_start</TT
>: depending on the configuration it
might be more appropriate to do this elsewhere, for example inside
<TT
CLASS="FUNCTION"
>main</TT
>.
</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>#include <cyg/hal/hal_arch.h>
#include <cyg/kernel/kapi.h>
// These numbers depend entirely on your application
#define NUMBER_OF_WORKERS 4
#define PRODUCER_PRIORITY 10
#define WORKER_PRIORITY 11
#define PRODUCER_STACKSIZE CYGNUM_HAL_STACK_SIZE_TYPICAL
#define WORKER_STACKSIZE (CYGNUM_HAL_STACK_SIZE_MINIMUM + 1024)
static unsigned char producer_stack[PRODUCER_STACKSIZE];
static unsigned char worker_stacks[NUMBER_OF_WORKERS][WORKER_STACKSIZE];
static cyg_handle_t producer_handle, worker_handles[NUMBER_OF_WORKERS];
static cyg_thread_t producer_thread, worker_threads[NUMBER_OF_WORKERS];
static void
producer(cyg_addrword_t data)
{
…
}
static void
worker(cyg_addrword_t data)
{
…
}
void
cyg_user_start(void)
{
int i;
cyg_thread_create(PRODUCER_PRIORITY, &producer, 0, "producer",
producer_stack, PRODUCER_STACKSIZE,
&producer_handle, &producer_thread);
cyg_thread_resume(producer_handle);
for (i = 0; i < NUMBER_OF_WORKERS; i++) {
cyg_thread_create(WORKER_PRIORITY, &worker, i, "worker",
worker_stacks[i], WORKER_STACKSIZE,
&(worker_handles[i]), &(worker_threads[i]));
cyg_thread_resume(worker_handles[i]);
}
}
</PRE
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="KERNEL-THREAD-CREATE-CXX"
></A
><H2
>Thread Entry Points and C++</H2
><P
>For code written in C++ the thread entry function must be either a
static member function of a class or an ordinary function outside any
class. It cannot be a normal member function of a class because such
member functions take an implicit additional argument
<TT
CLASS="VARNAME"
>this</TT
>, and the kernel has no way of knowing what
value to use for this argument. One way around this problem is to make
use of a special static member function, for example:
</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>class fred {
public:
void thread_function();
static void static_thread_aux(cyg_addrword_t);
};
void
fred::static_thread_aux(cyg_addrword_t objptr)
{
fred* object = static_cast<fred*>(objptr);
object->thread_function();
}
static fred instance;
extern "C" void
cyg_start( void )
{
…
cyg_thread_create( …,
&fred::static_thread_aux,
static_cast<cyg_addrword_t>(&instance),
…);
…
}
</PRE
></TD
></TR
></TABLE
><P
>Effectively this uses the <TT
CLASS="PARAMETER"
><I
>entry_data</I
></TT
> argument to
<TT
CLASS="FUNCTION"
>cyg_thread_create</TT
> to hold the
<TT
CLASS="VARNAME"
>this</TT
> pointer. Unfortunately this approach does
require the use of some C++ casts, so some of the type safety that can
be achieved when programming in C++ is lost.
</P
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