Reading and writing to serial port in C on Linux

I'm trying to send/receive data over an USB Port using FTDI, so I need to handle serial communication using C/C++. I'm working on Linux (Ubuntu). Basically, I am connected to a device which is listening for incoming commands. I need to send those commands and read device's response. Both commands and response are ASCII characters. Everything works fine using GtkTerm but, when I switch to C programming, I encounter problems. Here's my code: ``` #include <stdio.h> // standard input / output functions #include <stdlib.h> #include <string.h> // string function definitions #include <unistd.h> // UNIX standard function definitions #include <fcntl.h> // File control definitions #include <errno.h> // Error number definitions #include <termios.h> // POSIX terminal control definitions /* Open File Descriptor */ int USB = open( "/dev/ttyUSB0", O_RDWR| O_NONBLOCK | O_NDELAY ); /* Error Handling */ if ( USB < 0 ) { cout << "Error " << errno << " opening " << "/dev/ttyUSB0" << ": " << strerror (errno) << endl; } /* *** Configure Port *** */ struct termios tty; memset (&tty, 0, sizeof tty); /* Error Handling */ if ( tcgetattr ( USB, &tty ) != 0 ) { cout << "Error " << errno << " from tcgetattr: " << strerror(errno) << endl; } /* Set Baud Rate */ cfsetospeed (&tty, B9600); cfsetispeed (&tty, B9600); /* Setting other Port Stuff */ tty.c_cflag &= ~PARENB; // Make 8n1 tty.c_cflag &= ~CSTOPB; tty.c_cflag &= ~CSIZE; tty.c_cflag |= CS8; tty.c_cflag &= ~CRTSCTS; // no flow control tty.c_lflag = 0; // no signaling chars, no echo, no canonical processing tty.c_oflag = 0; // no remapping, no delays tty.c_cc[VMIN] = 0; // read doesn't block tty.c_cc[VTIME] = 5; // 0.5 seconds read timeout tty.c_cflag |= CREAD | CLOCAL; // turn on READ & ignore ctrl lines tty.c_iflag &= ~(IXON | IXOFF | IXANY);// turn off s/w flow ctrl tty.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG); // make raw tty.c_oflag &= ~OPOST; // make raw /* Flush Port, then applies attributes */ tcflush( USB, TCIFLUSH ); if ( tcsetattr ( USB, TCSANOW, &tty ) != 0) { cout << "Error " << errno << " from tcsetattr" << endl; } /* *** WRITE *** */ unsigned char cmd[] = {'I', 'N', 'I', 'T', ' ', '\r', '\0'}; int n_written = write( USB, cmd, sizeof(cmd) -1 ); /* Allocate memory for read buffer */ char buf [256]; memset (&buf, '\0', sizeof buf); /* *** READ *** */ int n = read( USB, &buf , sizeof buf ); /* Error Handling */ if (n < 0) { cout << "Error reading: " << strerror(errno) << endl; } /* Print what I read... */ cout << "Read: " << buf << endl; close(USB); ``` What happens is that `read()` returns 0 (no bytes read at all) or block until timeout (`VTIME`). I'm assuming this happens because `write()` does not send anything. In that case, device wouldn't receive command and I cannot receive response. In fact, turning off the device while my program is blocked on reading actually succeded in getting a response (device sends something while shutting down). Strange thing is that adding this ``` cout << "I've written: " << n_written << "bytes" << endl; ``` right after `write()` call, I receive: ``` I've written 6 bytes ``` which is exactly what I expect. Only my program doesn't work as it should, like my device cannot receive what I'm actually writing on port. I've tried different things and solution, also regarding data types (I've tried using std::string, such as `cmd = "INIT \r"` or `const char`) but nothing really worked. Can someone tell me where I'm wrong? Thank you in advance. EDIT: Previously version of this code used `unsigned char cmd[] = "INIT \n"` and also `cmd[] = "INIT \r\n"`. I changed it because command sintax for my device is reported as `<command><SPACE><CR>`. I've also tried avoiding the `O_NONBLOCK` flag on reading, but then I only block until forever. I've tried using `select()` but nothing happens. Just for a try, I've created a waiting loop until data is avaliable, but my code never exit the loop. Btw, waiting or `usleep()` is something I need to avoid. Reported one is only an excerpt of my code. Complete code needs to work in a real-time environment (specifically OROCOS) so I don't really want sleep-like function.