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Non-canonical mode on OS X?

I'm trying to read a Mac's built-in modem in raw mode (after some help from this lists months ago, then a long period of downtime) and I'm running into something annoying. When data shows up, it comes through as a small burst (100 characters or so) followed by a brief pause (.02 seconds, say) and then another small burst (32 characters).

The difficulty I'm having is that the read() I eventually issue returns before the second burst shows up. Here's the setup routine.

static int set_local_mode(int inFileDescriptor)
{
  int theResult = 0;
  struct termios theOptions = {};

if(tcgetattr(inFileDescriptor, &theOptions) == -1) {theResult = errno;}
cfmakeraw(&theOptions);
//theOptions.c_lflag &= ~(ICANON | ECHO); /* Clear ICANON and ECHO. */
theOptions.c_cc[VMIN] = 1;
theOptions.c_cc[VTIME] = 5;
theOptions.c_cflag |= (CS8 | CLOCAL);
if(tcsetattr(inFileDescriptor, TCSANOW, &theOptions) == -1) {theResult = errno;}
return theResult;
}

My understanding is that if this completes successfully, the read() should return after once there has been a half-second interval since the last character received. The buffer passed to read() is much larger than the returned data, and the commented line doesn't have any impact (c_lflag is already zero by the time we get there). Any advice?
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