A Sample Program with Two Threads

Below is a program that uses some of eCos' system calls. It creates two threads, each of which goes into an infinite loop in which it sleeps for a while (using cyg_thread_delay()). This code is found in the file twothreads.c in the examples directory.

eCos two-threaded program listing

#include <cyg/kernel/kapi.h>
#include <stdio.h>
#include <math.h>
#include <stdlib.h>

/* now declare (and allocate space for) some kernel objects,
  like the two threads we will use */
cyg_thread thread_s[2];	/* space for two thread objects */

char stack[2][4096];	/* space for two 4K stacks */

/* now the handles for the threads */
cyg_handle_t simple_threadA, simple_threadB;

/* and now variables for the procedure which is the thread */
cyg_thread_entry_t simple_program;

/* and now a mutex to protect calls to the C library */
cyg_mutex_t cliblock;

/* we install our own startup routine which sets up threads */
void cyg_user_start(void)
{
  printf("Entering twothreads' cyg_user_start() function\n");

  cyg_mutex_init(&cliblock);

  cyg_thread_create(4, simple_program, (cyg_addrword_t) 0,
  "Thread A", (void *) stack[0], 4096,
  &simple_threadA, &thread_s[0]);
  cyg_thread_create(4, simple_program, (cyg_addrword_t) 1,
  "Thread B", (void *) stack[1], 4096,
  &simple_threadB, &thread_s[1]);

  cyg_thread_resume(simple_threadA);
  cyg_thread_resume(simple_threadB);
}

/* this is a simple program which runs in a thread */
void simple_program(cyg_addrword_t data)
{
  int message = (int) data;
  int delay;

  printf("Beginning execution; thread data is %d\n", message);

  cyg_thread_delay(200);

  for (;;) {
    delay = 200 + (rand() % 50);

    /* note: printf() must be protected by a
    call to cyg_mutex_lock() */
    cyg_mutex_lock(&cliblock);
    printf("Thread %d: and now a delay of %d clock ticks\n", message, delay);
    cyg_mutex_unlock(&cliblock);
    cyg_thread_delay(delay);
  }
}

When you run the program (by typing continue at the (gdb) prompt) the output should look like this:

Starting program: <BASE_DIR>/examples/twothreads.exe
Entering twothreads' cyg_user_start()
function
Beginning execution; thread data is 0
Beginning execution; thread data is 1
Thread 0: and now a delay of 240 clock ticks
Thread 1: and now a delay of 225 clock ticks
Thread 1: and now a delay of 234 clock ticks
Thread 0: and now a delay of 231 clock ticks
Thread 1: and now a delay of 224 clock ticks
Thread 0: and now a delay of 249 clock ticks
Thread 1: and now a delay of 202 clock ticks
Thread 0: and now a delay of 235 clock ticks

Note: When running in a simulator the delays might be quite long. On a hardware board (where the clock speed is 100 ticks/second) the delays should average to about 2.25 seconds. In simulation, the delay will depend on the speed of the host processor and will almost always be much slower than the actual board. You might want to reduce the delay parameter when running in simulation.

Figure 9-1 shows how this multitasking program executes. Note that apart from the thread creation system calls, this program also creates and uses a mutex for synchronization between the printf() calls in the two threads. This is because the C library standard I/O (by default) is configured not to be thread-safe, which means that if more than one thread is using standard I/O they might corrupt each other. This is fixed by a mutual exclusion (or mutex) lockout mechanism: the threads do not call printf() until cyg_mutex_lock() has returned, which only happens when the other thread calls cyg_mutex_unlock().

You could avoid using the mutex by configuring the C library to be thread-safe (by selecting the component CYGSEM_LIBC_STDIO_THREAD_SAFE_STREAMS).

Figure 9-1. Two threads with simple print statements after random delays

2017-02-09
Documentation license for this page: Open Publication License