Expert: Ralph McArdell - 8/3/2006

Question
Hi,

    I'm writing a C++ program to control an SSC (servo controller) through the serial bus on Linux.  The SSC recieves 3 bytes:
Byte 1 : Sync Marker (255)
Byte 2 : Servo # (0-7)
Byte 3 : Servo Position (0-254)
Which must be sent as individual byte values.
I opened /dev/ttyS1 and tried to send 3 bytes to it using the write(fd, buf, 3) command.  I don't know if this is the correct way to do this however.  write asks for a constant void*, but I want to send 3 one-byte char messages.  Here is a piece of my code.  PLEASE HELP, I've been working on this for 2 days already.

void send_command_gripper(ssc *s, short unsigned sync,  short unsigned servo, short unsigned position) {//////FIX ME! HOW DO I SEND A BYTE?!
  //unsigned char charsync=(unsigned char)sync;
  //cout<<"buf: "<<buf;
  char buf[3];
  snprintf(buf, 3,"%c%c%c",sync, servo, position);
  cout<<"BUF "<<buf<<'
';
  outstream.write(
  write(s->fd,buf,3);
}


Answer
Might I first suggest you re-read the man page for write and check the returned value and handle error returns. Then you might use something like:

unsigned char message( sync );
if ( write(s->fd, &message, 1)!=1 )
{
  // handle error
}

message = servo;
if ( write(s->fd, &message, 1)!=1 )
{
  // handle error
}

message = position;
if ( write(s->fd, &message, 1)!=1 )
{
  // handle error
}

The above should achieve what you asked: 3 calls to write, each sending 1 byte of the whole command. You can of course modify the exact details of the error handling logic as required, for example one variation would be:

unsigned char message( sync );
if ( 1==write(s->fd, &message, 1) )
{
   message = servo;
   if ( 1==write(s->fd, &message, 1) )
   {
       message = position;
       if ( write(s->fd, &message, 1)!=1 )
       {
          // command bytes sent OK
       }
       else
       {
          // handle error
       }
   }
   else
   {
      // handle error
   }
}
else
{
  // handle error
}

You will note that I use an unsigned char to hold the message and pass its address to write as write requires a pointer to the data it sends.

Please note that the code presented here is for example use only and I have not tested it or even compiled it. I apologise for any typos or mistakes.

In your original code I have no idea what the line:

  outstream.write(

is supposed to achieve. And why you did not just do something like:

buf[0] = sync;
buf[1] = servo;
buf[2] = position;

rather than use snprintf (a sledge hammer to crack a nut) I do not know. I would also have though that unsigned char would have been the correct type for the sync, servo and position parameters, then you could do away with the message local variable and just write:

if ( write(s->fd, &sync, 1)!=1 )
{
  // handle error
}

if ( write(s->fd, &servo, 1)!=1 )
{
  // handle error
}

if ( write(s->fd, &position, 1)!=1 )
{
  // handle error
}

Hope you find this useful.