Thread: Help with pointers

Hi,

I'm trying to write a function which can modify the values of an array passed to it, and the compiler keeps telling me, "invalid operands to binary *"

Here's the function as I'm trying it:

Code:

void TryLocationCalculation(int& cricket_dist[3], double& location[3])
{

  location[0] = cricket_dist[0]*cricket_dist[0];
}

What pointers?
I see references, but no pointers.

Pointers look like: int* num;
But like matsp said, just take out the '&' symbols and you should be fine since arrays automatically degrade to pointers.

I took out the &'s and the code compiled. Now I'm having some difficulty with the threads in my code. I have three threads that I want to run simultaneously. They essentially listen to three different serial ports. The problem is the first thread loops far faster (more often?) than the other two for some reason. Is there a way I can easily synchronize the three threads so that they execute the same fast? Without wasting cpu time?

Here is the code. I added the printfs so I could see what was going on.

Code:

void *serial_thread_1()
{
   for(;;)
   {
       printf("thread1");
       calling_to_beacon = 2;
       RequestMsg[5] = '2';
       TxBytes = write(comfd1, &RequestMsg, 7)
       cricket_dist1[0] = GetCricketDist(RxData1, comfd1);
       TryLocationCalculation(cricket_dist1, location1);
       calling_to_beacon = 4;
       RequestMsg[5] = '4';
       TxBytes = write(comfd1, &RequestMsg, 7)
       cricket_dist1[0] = GetCricketDist(RxData1, comfd1);
       TryLocationCalculation(cricket_dist1, location1);
       calling_to_beacon = 3;
       RequestMsg[5] = '3';
       TxBytes = write(comfd1, &RequestMsg, 7)
       cricket_dist1[0] = GetCricketDist(RxData1, comfd1);

   }
return 0;
}
void *serial_thread_2()
{
   while(calling_to_beacon !=0)
   {
         printf("thread2");
         if(calling_to_beacon ==2)
         cricket_dist2[0] = GetCricketDist(RxData2,comfd2);
         if(calling_to_beacon ==4)
         cricket_dist2[0] = GetCricketDist(RxData2,comfd2);
         if(calling_to_beacon ==3)
         cricket_dist2[0] = GetCricketDist(RxData2,comfd2);
   }
}
void *serial_thread_3()
{
   while(calling_to_beacon !=0)
   {
         printf("thread3");
         if(calling_to_beacon ==2)
         cricket_dist2[0] = GetCricketDist(RxData2,comfd2);
         if(calling_to_beacon ==4)
         cricket_dist2[0] = GetCricketDist(RxData2,comfd2);
         if(calling_to_beacon ==3)
         cricket_dist2[0] = GetCricketDist(RxData2,comfd2);
   }
}

Of course, though the better solutions that I have to offer are all platform specific. So what say ye? Windows or Linux or Mac OS? Or if you are feeling adventurous in C++, you could check out the qthead library.

This is an application for embedded linux, running on Gumstix. I don't know the details of the kernel and whatnot, and don't understand such things very well at all anyway. However, I think it wouldn't be too difficult to switch to C++. Is it necessary? A messy solution will do for now, as this project is "due" in two days... I can refine it afterwards

I just assumed you were a C++ fan since you posted code that has C++ specific features that do not exist in C. Be that as it may, the qthread library is actually very clean and easy to impliment. There are other options such as POSIX which should be boxed with your compiler (hell, mine came with qthreads too). Is the C++ thing an issue for your skill level or your project's guidelines? If so, POSIX is fine.

Does GetCricketDist() block on read? If so, then you should manually alternate between threads, i think.
EDIT: ah, sorry, i thought 2nd thread is more often than 3rd (not 1st more than 2nd & 3rd).

I have no idea what sched_yield() does or would do, and a little "wasting" can be tolerated...

I didn't know that my code used functions that were C++ specific... this is concerning as I thought I was using a C compiler. Hmm...

I don't mind using the qthread libraries if they help make things simpler, however I think what I am using now is the POSIX stuff?

You are, I take it, aware that the code in thread 2 and thread 3 in your posted code use the same comfd2 and RxData2 to access the comport?

Yeah, I'm sorry. It's supposed to be comfd3 and RxData3. I copied it down wrong.

The funciton GetCricketDist blocks (waits/listens?) for 0.4 seconds on read, which is what I intended. The hardware is waiting for an ultrasonic signal to arrive and if it hasn't arrived in 0.4 seconds, its not coming. I set this up in my options struct by setting VTIME = 4 and VMIN = 0

Just add two join statements to the end of each thread. Then when one thread finishes it will wait for the other to to finish before starting the loop again.