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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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<HTML
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><HEAD
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><TITLE
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>A Sample Program with Two Threads</TITLE
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TITLE="Building and Running Sample Applications"
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TITLE="Building and Running Sample Applications"
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TITLE="More Features &#8212; Clocks and Alarm
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>eCos User Guide</TH
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><A
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HREF="building-and-running-sample-appliations.html"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 13. Building and Running Sample Applications</TD
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><TD
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WIDTH="10%"
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VALIGN="bottom"
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><DIV
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="SAMPLE-TWOTHREADS">A Sample Program with Two Threads</H1
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><P
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>Below is a program that uses some of <SPAN
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CLASS="PRODUCTNAME"
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>eCos</SPAN
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>' system calls. It
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creates two threads, each of which goes into an infinite loop in which
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it sleeps for a while (using cyg_thread_delay()).  This code is found
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in the file <TT
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CLASS="FILENAME"
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>twothreads.c</TT
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>
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in the examples directory.</P
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="AEN871"><SPAN
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CLASS="PRODUCTNAME"
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>eCos</SPAN
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> two-threaded program listing</H2
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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="PROGRAMLISTING"
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>#include &lt;cyg/kernel/kapi.h&#62;
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#include &lt;stdio.h&#62;
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#include &lt;math.h&#62;
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#include &lt;stdlib.h&#62;
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/* now declare (and allocate space for) some kernel objects,
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  like the two threads we will use */
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cyg_thread thread_s[2]; /* space for two thread objects */
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char stack[2][4096];    /* space for two 4K stacks */
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/* now the handles for the threads */
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cyg_handle_t simple_threadA, simple_threadB;
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/* and now variables for the procedure which is the thread */
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cyg_thread_entry_t simple_program;
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/* and now a mutex to protect calls to the C library */
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cyg_mutex_t cliblock;
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/* we install our own startup routine which sets up threads */
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void cyg_user_start(void)
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{
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 printf("Entering twothreads' cyg_user_start() function\n");
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 cyg_mutex_init(&amp;cliblock);
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 cyg_thread_create(4, simple_program, (cyg_addrword_t) 0,
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        "Thread A", (void *) stack[0], 4096,
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        &amp;simple_threadA, &amp;thread_s[0]);
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 cyg_thread_create(4, simple_program, (cyg_addrword_t) 1,
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        "Thread B", (void *) stack[1], 4096,
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        &amp;simple_threadB, &amp;thread_s[1]);
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 cyg_thread_resume(simple_threadA);
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 cyg_thread_resume(simple_threadB);
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}
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/* this is a simple program which runs in a thread */
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void simple_program(cyg_addrword_t data)
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{
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 int message = (int) data;
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 int delay;
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 printf("Beginning execution; thread data is %d\n", message);
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 cyg_thread_delay(200);
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 for (;;) {
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 delay = 200 + (rand() % 50);
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 /* note: printf() must be protected by a
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 call to cyg_mutex_lock() */
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 cyg_mutex_lock(&amp;cliblock); {
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 printf("Thread %d: and now a delay of %d clock ticks\n",
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        message, delay);
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 }
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 cyg_mutex_unlock(&amp;cliblock);
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 cyg_thread_delay(delay);
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 }
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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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>When you run the program (by typing <B
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CLASS="COMMAND"
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>continue</B
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> at
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the (<SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>gdb</I
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></SPAN
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>) prompt) the output should look like
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this:</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="PROGRAMLISTING"
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>Starting program: <TT
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CLASS="REPLACEABLE"
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><I
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>BASE_DIR</I
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></TT
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>/examples/twothreads.exe
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Entering twothreads' cyg_user_start()
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function
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Beginning execution; thread data is 0
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Beginning execution; thread data is 1
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Thread 0: and now a delay of 240 clock ticks
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Thread 1: and now a delay of 225 clock ticks
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Thread 1: and now a delay of 234 clock ticks
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Thread 0: and now a delay of 231 clock ticks
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Thread 1: and now a delay of 224 clock ticks
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Thread 0: and now a delay of 249 clock ticks
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Thread 1: and now a delay of 202 clock ticks
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Thread 0: and now a delay of 235 clock ticks</PRE
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></TD
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></TR
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></TABLE
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><DIV
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CLASS="NOTE"
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><BLOCKQUOTE
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CLASS="NOTE"
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><P
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><B
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>Note: </B
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>When running in a simulator the
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delays might be quite long. On a hardware board (where the clock
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speed is 100 ticks/second) the delays should average to
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about 2.25 seconds. In simulation, the delay will depend on the
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speed of the host processor and will almost always be much slower than
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the actual board. You might want to reduce the delay parameter when running
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in simulation.</P
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></BLOCKQUOTE
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></DIV
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><P
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><A
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HREF="sample-twothreads.html#FIGURE-TWOTHREADS-WITH-SIMPLE-PRINTS"
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>Figure 13-1</A
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> shows how this
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multitasking program executes.  Note that apart from the thread
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creation system calls, this program also creates and uses a
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<SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>mutex</I
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></SPAN
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> for synchronization
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between the <TT
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CLASS="FUNCTION"
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>printf()</TT
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> calls in the two
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threads. This is because the C library standard I/O (by default) is
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configured not to be thread-safe, which means that if more than one
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thread is using standard I/O they might corrupt each other. This is
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fixed by a mutual exclusion (or <SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>mutex</I
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></SPAN
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>) lockout
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mechanism: the threads do not call <TT
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CLASS="FUNCTION"
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>printf()</TT
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> until
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<TT
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CLASS="FUNCTION"
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>cyg_mutex_lock()</TT
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> has returned, which only happens
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when the other thread calls
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<TT
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CLASS="FUNCTION"
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>cyg_mutex_unlock()</TT
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>.</P
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><P
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>You could avoid using the mutex by configuring the C library to
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be thread-safe (by selecting the component
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<TT
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CLASS="LITERAL"
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>CYGSEM_LIBC_STDIO_THREAD_SAFE_STREAMS</TT
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>).</P
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><DIV
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CLASS="FIGURE"
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><A
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NAME="FIGURE-TWOTHREADS-WITH-SIMPLE-PRINTS"><P
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><B
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>Figure 13-1. Two
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threads with simple print statements after random delays</B
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></P
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><P
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><IMG
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SRC="pix/twothreads2.png"></P
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