Issues writing to serial port on MAC OSX using unistd.h in c

Question

I am trying to write to a bluetooth device on MAC OSX using the unistd.h Linux functions in c. I am connecting fine and writing the first few bytes with success. When I try to write other commands to it (there are bytes added to the write buffer every 15ms), I don't see any results even though the write() function returns 1 (write success).

If you start a write and it doesn't finish by the time you try to start another write (since it is non-blocking), could that possibly screw up the initial write? (If so, is there any way to check if a write has completed?) That is the only thing I can think of since the writes are occurring fairly frequently and the first two are successfully sent.

qwbyte() simply adds a byte to the output array and increments its length

The open port function:

BAMid = -1;
struct termios options;

struct termios originalTTYAttrs;

// Open the serial port read/write, nonblocking, with no controlling terminal, and don't wait for a connection.
BAMid = open(strPath, O_RDWR | O_NOCTTY | O_NONBLOCK);
if (BAMid == -1)
{
    printf("Error opening serial port %s - %s(%d).\n",
           strPath, strerror(errno), errno);
    goto error;
}

// Issue TIOCEXCL ioctl to prevent additional opens except by root-owned processes.
if (ioctl(BAMid, TIOCEXCL) == -1)
{
    printf("Error setting TIOCEXCL on %s - %s(%d).\n",
  strPath, strerror(errno), errno);
    goto error;
}

// Get the current options and save them so we can restore the default settings later.
if (tcgetattr(BAMid, &originalTTYAttrs) == -1)
{
    printf("Error getting tty attributes %s - %s(%d).\n",
  strPath, strerror(errno), errno);
    goto error;
}

// The serial port attributes such as timeouts and baud rate are set by modifying the termios
// structure and then calling tcsetattr() to cause the changes to take effect. Note that the
// changes will not become effective without the tcsetattr() call.

options = originalTTYAttrs;

// Set raw input (non-canonical) mode, with reads blocking until either a single character 
// has been received or a one second timeout expires. [should be moot since we are leaving it as nonblocking]

cfmakeraw(&options);

options.c_cc[VMIN] = 1; options.c_cc[VTIME] = 10; cfsetspeed(&options, B57600); // Set 57600 baud
options.c_cflag |= CS8; // Use 8 bit words

// Cause the new options to take effect immediately.
if (tcsetattr(BAMid, TCSANOW, &options) == -1)
{
    printf("Error setting tty attributes %s - %s(%d).\n",
  strPath, strerror(errno), errno);
    goto error;
}

//flush old transmissions if (tcflush(BAMid,TCIOFLUSH) == -1) { printf("Error flushing BAM serial port - %s(%d).\n", strerror(errno), errno); }

oBufLength = 0;

// Ask it to start if (! qwbyte(CmdStart) ) { goto error; }

if (! qwbyte(CmdFull) ) { goto error; } //this transmit works txbytes();

printf("success opening port!"); return -1; // Failure path error: if (BAMid != -1) { close(BAMid); } printf("returning an error--%d",errno); return errno; }

The write function (txbytes):

int i, bufSize, numBytes;

if(oBufLength != 0) { //if the output array isn't empty

        //duplicating the output array and its size so it can 
        //be overwritten while this write is occuring
        printf("about to transmit: ");
  for(i = 0; i < oBufLength; i++) {
   printf(" %u",oBuf[i]);
   tempBuf[i] = oBuf[i];
  }
  printf("\n");

bufSize = oBufLength; oBufLength = 0;

numBytes = write(BAMid, &tempBuf, bufSize);

printf("bytes written = %d\n",numBytes);

if (numBytes == -1) { printf("Error writing to port - %s(%d).\n", strerror(errno), errno); }

return (numBytes > 0); } else { return 0; }

Answer 1

Non-blocking writes don't work how you're expecting them to.

If the write can't be completed immediately, write() returns to your code - but it doesn't keep trying to send the data in the background. All it does is say "I couldn't write at the moment - try again later". It does this by returning -1, with errno set to EAGAIN.

Also, remember that when successful, write() returns the number of bytes successfully written. So if you're getting a return value of 1 when you asked for 2 bytes to be written, that means it was only partially successful, and you need to call write() again some time for the second byte.

Basically, if you're using non-blocking IO, you want your txbytes() function to call write() in a loop until either your buffer is empty, or it returns -1. If it returned -1 you need to check errno - if it's EAGAIN, you'll have to call write() again some other time; anything else is probably a real error. Something like this:

ssize_t written = 0;

while (oBufLength > 0 && written > -1)
{
    size_t i;

    printf("about to transmit %d bytes: ", oBufLength);
    for(i = 0; i < oBufLength; i++) {
            printf(" %u",oBuf[i]);
    }
    printf("\n");

    written = write(BAMid, oBuf, oBufLength);

    printf("Write returned %d\n", written);

    if (written > 0)
    {
        /* The first "written" number of bytes in the buffer have now been sent, so
         * we discard them and move up the remaining bytes (if any) to the start of 
         * the buffer */

        oBufLength -= written;
        memmove(oBuf, oBuf + written, oBufLength);

        printf("Now have %d bytes left to send.\n", oBufLength);
     }
}

if (written > -1 || errno == EAGAIN)
{
    /* No fatal errors... */
    return 0;
} else
    /* error left in errno for caller to inspect */
    return -1;
}

Note that there's no need to duplicate the buffer - since write() doesn't do anything in parallel with your code. If it says its written the bytes, it doesn't need your buffer any more. Hope that helps!