Message boxes in eCos

[Altera_Forum]

Hi all, 

 cyg_mbox_peek (), is always returning the count 1 less than the actual number of messages in the message box. When the message box is empty, then count is returned as 0. Why this is so?  I am attaching the code below for reference.  With regards, Sanjay   /* **** Include standard header files **** */# include <cyg/kernel/kapi.h># include <cyg/infra/diag.h>  /* **** Constants **** */# define TRUE 1 /* Size of a task&#39;s stack */# define STACKSIZE (CYGNUM_HAL_STACK_SIZE_TYPICAL + 4096)# define NTHREADS 2 /* No. of threads */  /* **** User-defined data types **** */ typedef char stack_t; /* Each stack entry is 8-bit wide */  /* **** Function prototypes **** */ void task_a (cyg_addrword_t); void task_b (cyg_addrword_t);  /* **** Gobal variables **** */ cyg_thread task_housekeeping_info[NTHREADS]; stack_t task_stack[NTHREADS+1][STACKSIZE]; cyg_handle_t mbox_handle; cyg_mbox mbox; int msg1=12, msg2=24;  /* Startup function */ void cyg_user_start () {  cyg_handle_t task_handle[NTHREADS];   cyg_mbox_create (&mbox_handle, &mbox);   cyg_thread_create (0, task_a, (cyg_addrword_t) 0, "task_a",   (void *) task_stack[0], STACKSIZE, &task_handle[0],   &task_housekeeping_info[0]);  cyg_thread_create (0, task_b, (cyg_addrword_t) 1, "task_b",   (void *) task_stack[1], STACKSIZE, &task_handle[1],   &task_housekeeping_info[1]);   cyg_thread_resume (task_handle[0]);  cyg_thread_resume (task_handle[1]);  cyg_scheduler_start (); }  void task_a (cyg_addrword_t data) {  cyg_bool_t stat;  long count;    // Run this thread forever.  while (TRUE)  {  // Delay for 1000 ticks.  cyg_thread_delay (1000);   count = cyg_mbox_peek (mbox_handle);  diag_printf ("Count = %ld\n", count);   // Send a message to Thread B.  cyg_mbox_put (mbox_handle, &msg1);  cyg_mbox_put (mbox_handle, &msg1);   count = cyg_mbox_peek (mbox_handle);  diag_printf ("Count = %ld\n", count);  } }  void task_b (cyg_addrword_t data) {  void *message;    // Run this thread forever.  while (TRUE)  {  // Wait for the message.  message = cyg_mbox_get (mbox_handle);  // Make sure we received the message before attempting to process it.  if (message != NULL)  {  // Process the message.  diag_printf ("Message: %d\n", *((int*)message));  }  } }

[Altera_Forum]

--- Quote Start ---  originally posted by sanjay822@Dec 22 2005, 08:45 AM hi all,  cyg_mbox_peek (), is always returning the count 1 less than the actual number of messages in the message box. when the message box is empty, then count is returned as 0. why this is so?  i am attaching the code below for reference.  with regards, sanjay   /* **** include standard header files **** */# include <cyg/kernel/kapi.h># include <cyg/infra/diag.h>  /* **** constants **** */# define true                          1 /* size of a task&#39;s stack */# define stacksize              (cygnum_hal_stack_size_typical + 4096)# define nthreads                  2 /* no. of threads */  /* **** user-defined data types **** */ typedef char stack_t; /* each stack entry is 8-bit wide */  /* **** function prototypes **** */ void        task_a          (cyg_addrword_t); void        task_b          (cyg_addrword_t);  /* **** gobal variables **** */ cyg_thread task_housekeeping_info[nthreads]; stack_t task_stack[nthreads+1][stacksize]; cyg_handle_t mbox_handle; cyg_mbox mbox; int msg1=12, msg2=24;  /* startup function */ void cyg_user_start () {   cyg_handle_t task_handle[nthreads];    cyg_mbox_create (&mbox_handle, &mbox);    cyg_thread_create (0, task_a, (cyg_addrword_t) 0, "task_a",                    (void *) task_stack[0], stacksize, &task_handle[0],                    &task_housekeeping_info[0]);   cyg_thread_create (0, task_b, (cyg_addrword_t) 1, "task_b",                    (void *) task_stack[1], stacksize, &task_handle[1],                    &task_housekeeping_info[1]);    cyg_thread_resume (task_handle[0]);   cyg_thread_resume (task_handle[1]);   cyg_scheduler_start (); }  void task_a (cyg_addrword_t data) {   cyg_bool_t stat;   long count;      // run this thread forever.   while (true)   {     // delay for 1000 ticks.     cyg_thread_delay (1000);      count = cyg_mbox_peek (mbox_handle);     diag_printf ("count = %ld\n", count);      // send a message to thread b.     cyg_mbox_put (mbox_handle, &msg1);     cyg_mbox_put (mbox_handle, &msg1);      count = cyg_mbox_peek (mbox_handle);     diag_printf ("count = %ld\n", count);   } }  void task_b (cyg_addrword_t data) {   void *message;      // run this thread forever.   while (true)   {     // wait for the message.     message = cyg_mbox_get (mbox_handle);     // make sure we received the message before attempting to process it.     if (message != null)     {       // process the message.       diag_printf ("message: %d\n", *((int*)message));     }   } } <div align='right'><{post_snapback}> (index.php?act=findpost&pid=11736)  --- quote end ---    --- Quote End ---     Sanjay, have a look on page 75/76 in the "eCos Reference Manual": cyg_mbox_get () is a blocking function. The thread is woken up immediately after the 1. call of cyg_mbox_put () and empties the mailbox. the 2. cyg_mbox_put () increments the counter again to "1".  I am using a "timed put " to send and a "try get" to read for not blocking the threads. Also an additional acknowledge flag is usful to synchronize the threads.  Regards, Sepp

[Altera_Forum]

--- Quote Start ---     --- Quote Start ---    Hi,  Thanks Sepp for your suggestion.  The program is giving correct results now, using cyg_mbox_tryget () instead of cyg_mbox_get (). But still I have one doubt. The reason that you had specified for cyg_mbox_get () is okay, but then using that concept cyg_mbox_peek () would always had returned the count as 0?  With regards, Sanjay

[Altera_Forum]

Hi, 

 Thanks Sepp for your suggestion.  The program is giving correct results now, using cyg_mbox_tryget () instead of cyg_mbox_get (). But still I have one doubt. The reason that you had specified for cyg_mbox_get () is okay, but then using that concept cyg_mbox_peek () would always had returned the count as 0?  With regards, Sanjay   Sanjay, I am not using the function cyg_mbox_peek () so I have no idea. What I am doing is something like this and it works fine:  sending thread: while(1) { ... // when a message should be sent:  if(!cyg_mbox_timed_put(mbox_hdl,(void *)psMsg1,1000))  {  psMsg1 = NULL;  // Do something to resend the message  }  else  {  // wait for acknowledge  ret = cyg_flag_wait(&FlgAckMsg,FLG_MBOX_ACK,CYG_FLAG_WAITMODE_AND | CYG_FLAG_WAITMODE_CLR);  }   ... }    receiving thread: while(1) { ... // allways have a look if there is a new message  pMsg= cyg_mbox_tryget(mbox_hdl);  if(pMsg2 != NULL)  { save message:  // Saving the message data into a local struct acknowledge:  cyg_flag_setbits(&FlgAckMsg,FLG_MBOX_ACK); work with received data:  // Decoding the received message and do some action with it  }   ... }  Regards, Sepp