Read data off USB at 31250 baud rate

I've got an Arduino board and want to read the data it's spitting off using USB at a custom baud rate. Hacking up some of the code Arduino suggest, I get this C code:

int serialport_开发者_JAVA百科init(const char* serialport, int baud)
{
    struct termios toptions;
    int fd;

    printf("init_serialport: opening port %s @ %d bps\n", serialport,baud);

    fd = open(serialport, O_RDWR | O_NOCTTY | O_NDELAY);
    serialPortPointer = fd;

    if (fd == -1)
    {
        printf("Unable to open port when initialising hardware'n");
        return -1;
    }

    if (tcgetattr(fd, &toptions) < 0)
    {
        printf("Couldn't get term attributes when initialising hardware\n");
        return -1;
    }
    speed_t brate = baud; // let you override switch below if needed
    switch(baud) {
        case 4800:   brate=B4800;   break;
        case 9600:   brate=B9600;   break;
        case 14400:  brate=B14400;  break;
        case 19200:  brate=B19200;  break;
        case 28800:  brate=B28800;  break;
        case 38400:  brate=B38400;  break;
        case 57600:  brate=B57600;  break;
        case 115200: brate=B115200; break;
    }
    cfsetispeed(&toptions, EXTA);
    cfsetospeed(&toptions, EXTA);

    // 8N1
    toptions.c_cflag &= ~PARENB;
    toptions.c_cflag &= ~CSTOPB;
    toptions.c_cflag &= ~CSIZE;
    toptions.c_cflag |= CS8;
    // no flow control
    toptions.c_cflag &= ~CRTSCTS;

    toptions.c_cflag |= CREAD | CLOCAL;  // turn on READ & ignore ctrl lines
    toptions.c_iflag &= ~(IXON | IXOFF | IXANY); // turn off s/w flow ctrl

    toptions.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG); // make raw
    toptions.c_oflag &= ~OPOST; // make raw

    // see: http://unixwiz.net/techtips/termios-vmin-vtime.html
    toptions.c_cc[VMIN]  = 0;
    toptions.c_cc[VTIME] = 20;

    if(tcsetattr(fd, TCSANOW, &toptions) < 0)
    {
        printf("Couldn't set term attributes when initialising hardware\n");
        return -1;
    }

    return fd;
}

The problem is that the termios.h file doesn't support 31250 (MIDI) baud rate... If I try entering 31250 as the baud rate, this function returns -1 and says "Couldn't set term attributes when initialising hardware" (it fails right at the end).

So - how can I write a program, in C, or any other language, that reads the data off at the baud rate I want? Does termios.h support custom baud rates?

I literally just want to read the data on the serial port - nothing else.

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This is a library that enables MIDI I/O communications on the Arduino serial ports. You need an Arduino capable of minimum 2 serial ports (like this one). One serial will be used for communication to MIDI devices (31250bps), and the other one to PC (for example 115200bps). If you have only one serial port on your Arduino board then you can also experiment with a software serial library like this.

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The termios.h API simply does not have a way to express user-defined baud rates, though depending on your operating system there may be an extension to do so. Can you try setting up the Arduino to use the more standard 38400 instead?