C library for MIDI output on Raspberry Pi Model B using UART0 and breakout board with opto-isolators

midi.c

#include <stdio.h>
#include <stdarg.h>
#include <string.h>

#include <errno.h>
#include <fcntl.h>
#include <asm/termios.h>
#include <unistd.h>

typedef unsigned char u8;

// Global variable for holding file descriptor to talk to UART0 for MIDI communications:
int uart0_fd = -1;
// Default UART0 device name on Raspberry Pi Model B:
const char *midi_fname = "/dev/ttyAMA0";

// Open UART0 device for MIDI communications and set baud rate to 31250 per MIDI standard:
int midi_init(void) {
    struct termios2 tio;

    uart0_fd = open(midi_fname, O_RDWR | O_NOCTTY | O_NONBLOCK);
    if (uart0_fd == -1) {
        char err[100];
        sprintf(err, "open('%s')", midi_fname);
        perror(err);
        return -1;
    }

    // Requirements:
    //   * UART0 is enabled
    //   * UART0 is unused by the system for any ttys (remove mention of ttyAMA0 from `/etc/inittab`)
    //   * UART0 clock rate is set to default of 3MHz in /boot/config.txt
    //     * `init_uart_clock=3000000`
    //   * UART0 clock rate for Linux is set to default of 3MHz in /boot/cmdline.txt
    //     * `bcm2708.uart_clock=3000000`
    //
    // NOTE: Any hacks to change the default UART clock speed of the RPi B to adjust the
    // standard baud rates of 38400 to actually be ~31250 DO NOT WORK on latest Raspbian
    // kernels. I'm using `Linux 4.1.18 armv6l` as of 2017-02-20.
    //
    // The only thing that DOES work on latest Raspbian kernels is the code below which uses
    // termios2 to set a custom baud rate of 31250 which is the MIDI standard.
    
    // Set baud rate to 31250 for MIDI:
    ioctl(uart0_fd, TCGETS2, &tio);
    tio.c_cflag &= ~CBAUD;
    tio.c_cflag |= BOTHER;
    tio.c_ispeed = 31250;
    tio.c_ospeed = 31250;
    ioctl(uart0_fd, TCSETS2, &tio);

    return uart0_fd;
}

void midi_send_cmd1_impl(u8 cmd_byte, u8 data1) {
    int count;
    u8 buf[2];
    buf[0] = cmd_byte;
    buf[1] = data1;
    count = write(uart0_fd, buf, 2);
    if (count != 2) {
        perror("Error sending MIDI bytes");
        return;
    }
    printf("MIDI: %02X %02X\n", cmd_byte, data1);
}

void midi_send_cmd2_impl(u8 cmd_byte, u8 data1, u8 data2) {
    int count;
    u8 buf[3];
    buf[0] = cmd_byte;
    buf[1] = data1;
    buf[2] = data2;
    count = write(uart0_fd, buf, 3);
    if (count != 3) {
        perror("Error sending MIDI bytes");
        return;
    }
    printf("MIDI: %02X %02X %02X\n", cmd_byte, data1, data2);
}

// 256 byte buffer for batching up SysEx data to send in one write() call:
u8 sysex[256];
size_t sysex_p = 0;

// Buffer up SysEx data until terminating F7 byte is encountered:
void midi_send_sysex(u8 byte) {
    //printf("MIDI: %02X\n", byte);

    if (sysex_p >= 256) {
        fprintf(stderr, "MIDI SysEx data too large (>= 256 bytes)\n");
        return;
    }

    // Buffer data:
    sysex[sysex_p++] = byte;

    if (byte == 0xF7) {
        size_t i;
        size_t write_count = sysex_p;
        sysex_p = 0;

        printf("MIDI SysEx:");
        for (i = 0; i < write_count; i++) {
            printf(" %02X", sysex[i]);
        }
        printf("\n");

        ssize_t count = write(uart0_fd, sysex, write_count);
        if (count < 0) {
            perror("write in midi_send_sysex");
            return;
        }
        if (count != write_count) {
            fprintf(stderr, "midi_send_sysex write didnt write enough bytes\n");
            return;
        }
    }
}