C write() doesn't send data until close(fd) is called.

Question

So I have this test code to send "HELLO" over a USB serial port:

int fd;
struct termios tty;

if((fd = open("/dev/ttyUSB0", O_WRONLY|O_NONBLOCK|O_NOCTTY)) == -1){
err(1, "Cannot open write on /dev/ttyUSB0");
}

tcgetattr(fd, &tty);
tty.c_iflag = 0;
tty.c_oflag = 0;
tty.c_lflag = 0;
tty.c_cflag = 0;
tty.c_cc[VMIN] = 0;
tty.c_cc[VTIME] = 0;
cfsetospeed(&tty, B19200);
cfsetispeed(&tty, B19200);
tty.c_cflag |= CREAD|CRTSCTS|HUPCL|CS8;
tcsetattr(fd, TCSANOW, &tty);

printf("Write: %i\n", write(fd, "HELLO", 5));

sleep(5);

if(close(fd) != 0){
warn("Could not close write fd");
}

The program executes fine and "HELLO" is sent but there is one problem. "HELLO" doesn't seem to be sent when the write() function is called, but rather when the file descriptor is closed. I added the sleep(5) line above to test out this theory and sure enough, "HELLO" is sent ~5 seconds after the program is executed. How can I get "HELLO" to be sent immediately after the write() command instead of on close()?

Answer 1

the buffer isn't flushed. fflush.

Answer 2

From the man page of write():

A successful return from write() does not make any guarantee that data has been committed to disk. In fact, on some buggy implementations, it does not even guarantee that space has successfully been reserved for the data. The only way to be sure is to call fsync(2) after you are done writing all your data.

You need to call fsync() on your file descriptor to ensure the data is actually committed.

Answer 3

Please see this question. BAsically you need to flush the file in order for IO to take place when you want to.

Answer 4

Try doing a

fflush( NULL );

after the write(). Maybe there is some internal buffer which is not flushed.

Answer 5

Output ports are often buffered, so that there's a greater or lesser gap between you writing to an output stream, and the content actually being sent to the disk, line, or whatever. This is generally for efficiency.

See fflush(3) to force the buffer to be committed to the output.

You might also be able to open the output descriptor in a way which makes it non-buffered, but using fflush to say 'that's it, I'm done', is probably better.

Answer 6

Change this line:

tty.c_cc[VMIN] = 0;

to this:

tty.c_cc[VMIN] = 1;

Answer 7

The device is a tty device, so fsync isn't going to help, maybe not fflush either.

By default the device is working in canonical mode which means that data is packaged up into units of lines. You'll probably find that adding a cr/lf pair to your data will cause it to be sent.

You need to make sure canonical mode is off. Also, R's answer will be of use.

http://en.wikibooks.org/wiki/Serial_Programming/termios