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

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: %in", 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

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.