example-small.c

Embedded Thread Library

/**
 * This is a very small example that shows how to use
 * protothreads. The program consists of two protothreads that wait
 * for each other to toggle a variable.
 */

/* We must always include pt.h in our protothreads code. */
#include "pt.h"

#include <stdio.h> /* For printf(). */

/* Two flags that the two protothread functions use. */
static int protothread1_flag, protothread2_flag;

/**
 * The first protothread function. A protothread function must always
 * return an integer, but must never explicitly return - returning is
 * performed inside the protothread statements.
 *
 * The protothread function is driven by the main loop further down in
 * the code.
 */
static int
protothread1(struct pt *pt)
{
  /* A protothread function must begin with PT_BEGIN() which takes a
     pointer to a struct pt. */
  PT_BEGIN(pt);

  /* We loop forever here. */
  while(1) {
    /* Wait until the other protothread has set its flag. */
    PT_WAIT_UNTIL(pt, protothread2_flag != 0);
    printf("Protothread 1 running\n");

    /* We then reset the other protothread's flag, and set our own
       flag so that the other protothread can run. */
    protothread2_flag = 0;
    protothread1_flag = 1;

    /* And we loop. */
  }

  /* All protothread functions must end with PT_END() which takes a
     pointer to a struct pt. */
  PT_END(pt);
}

/**
 * The second protothread function. This is almost the same as the
 * first one.
 */
static int
protothread2(struct pt *pt)
{
  PT_BEGIN(pt);

  while(1) {
    /* Let the other protothread run. */
    protothread2_flag = 1;

    /* Wait until the other protothread has set its flag. */
    PT_WAIT_UNTIL(pt, protothread1_flag != 0);
    printf("Protothread 2 running\n");
    
    /* We then reset the other protothread's flag. */
    protothread1_flag = 0;

    /* And we loop. */
  }
  PT_END(pt);
}

/**
 * Finally, we have the main loop. Here is where the protothreads are
 * initialized and scheduled. First, however, we define the
 * protothread state variables pt1 and pt2, which hold the state of
 * the two protothreads.
 */
static struct pt pt1, pt2;
int
main(void)
{
  /* Initialize the protothread state variables with PT_INIT(). */
  PT_INIT(&pt1);
  PT_INIT(&pt2);
  
  /*
   * Then we schedule the two protothreads by repeatedly calling their
   * protothread functions and passing a pointer to the protothread
   * state variables as arguments.
   */
  while(1) {
    protothread1(&pt1);
    protothread2(&pt2);
  }
}