I'm thinking

int fd_usb = open("/dev/ttyS0", O_RDONLY);

fcntl(fd_usb, F_SETFL, 0);
char data_in;

while(read(fd_usb, &data_in, sizeof(char))>0)
{
printf("%c\n",data_in);
//do other stuff here
}
What I don't want is for printf to print the same character over and over when there is no new character to be read. There is only one character that is interesting, the letter 'p'. The data rate will be serial port rate.

I'm not sure that what I want is conceptually possible - to have the read function block until new data is ready.

1. 'read' returns the number of bytes, store it in a variable
2. if you don't want 'read' to block, use non-blocking mode
3. if you want to read the input per-character (and not per-line) use 'termios' to set non-canonical mode (Example: http://dtelnet.sf.net/shkeys.c)

I want read to block. But I need to restart my thinking. The command I am using in my OP for opening a file descriptor for a USB device is not working. How do I open a file descriptor for a USB device that connects on COM4? My platform is both Cygwin on Windows 7 and Ubuntu.

/dev/ttyUSB0 is a possible device name, but you have to found out it yourself