serial.c

//
//  UNIX Serial Port Interface
//
//  Created by K3A (www.k3a.me)
//  Released under MIT
//

#include "serial.h"

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <strings.h>
#include <termios.h>
#include <unistd.h>

#define error_message(fmt, ...) fprintf(stderr, fmt, __VA_ARGS__)

int serial_fd = 0;

static int set_interface_attribs(int fd, int speed, serial_parity_t parity, int stop_bits)
{
    struct termios tio;

    bzero(&tio, sizeof(tio)); // clear struct for new port settings

    tio.c_cflag = speed | CS8 /* 8 data bits */ | CLOCAL /* Ignore modem control lines */ | CREAD /* Enable receiver */;

    if (stop_bits == 2) {
        tio.c_cflag |= CSTOPB;
    }

    if (parity > 0) {
        tio.c_cflag |= PARENB;
        if (parity & PARITY_ODD) {
            tio.c_cflag |= PARODD;
        }
    }

    // keep input, output and line flags empty
    tio.c_iflag = INPCK; // enable parity check
    tio.c_oflag = 0;
    tio.c_lflag = 0; // will be noncanonical mode (ICANON not set)

    // set blocking
    tio.c_cc[VMIN] = 255; // minimum number of characters for noncanonical read
    tio.c_cc[VTIME] = 20; // 2 second read timeout

    if (tcsetattr(fd, TCSANOW, &tio) != 0) {
        error_message("tcsetattr error %d: %s", errno, strerror(errno));
        return -1;
    }

    return 0;
}

int serial_init(const char *port_path, int speed, serial_parity_t parity, int stop_bits)
{
    int ret;

    serial_fd = open(port_path, O_RDWR | O_NOCTTY);
    if (serial_fd < 0) {
        error_message("error %d opening %s: %s", errno, port_path, strerror(errno));
        return serial_fd;
    }

    // set speed and parity
    if ((ret = set_interface_attribs(serial_fd, speed, parity, stop_bits)) < 0) {
        return ret;
    }

    return 0;
}

// write to serial port size bytes from ptr
int serial_write(const void *ptr, unsigned size) { return write(serial_fd, ptr, size); }

// read size bytes from serial into ptr buffer
int serial_read(void *ptr, unsigned size)
{
    unsigned char *buf = (unsigned char *)ptr;
    unsigned bts_read = 0;

    while (bts_read < size) {
        int ret = read(serial_fd, buf + bts_read, size - bts_read);

        if (ret < 0) {
            return ret;
        } else if (ret == 0) {
            // timeout
            return bts_read;
        }

        bts_read += ret;
    }

    return bts_read;
}