Simple utility for logging from serial port

readserial.c

#include <unistd.h>
#include <fcntl.h>
#include <termios.h>
#include <stdio.h>
#include <sys/select.h>
#include <signal.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#include <stdlib.h>

#define STDOUT 1
int fd = -1, logfd = -1;

size_t make_timestamp(char *restrict buf, size_t maxsize) {
    time_t t = time(NULL);
    struct tm tm;
    localtime_r(&t, &tm);
    return strftime(buf, maxsize, "%F %T", &tm);
}

void finish(void) {
    if (fd > 0) {
        fprintf(stderr, "Closing serial port\n");
        close(fd);
    }

    // Write footer info
    {
        char footer[1024];
        char timebuf[1024];

        footer[0] = '\0';
        strcat(footer, "\nLog end: ");
        size_t i = make_timestamp(timebuf, 1024);
        strcat(footer, timebuf);

        strcat(footer, "\n--------------------------------------------------------\n");
        
        write(STDOUT, footer, strlen(footer));
        if (logfd > 0) {
            write(logfd, footer, strlen(footer));
        }
    }

    if (logfd > 0) {
        fprintf(stderr, "Closing log file\n");
        close(logfd);
    }
}

void signal_handler(int signo) {
    fprintf(stderr, "Caught signal %d\n", signo);
    finish();
}

void usage(void) {
    fprintf(stderr, "readserial: usage: readserial [-B baudrate] [-l logfile] device\n");
}

int main(int argc, char **argv) {
    ssize_t len;
    unsigned char buf[8192];
    struct termios tio;

    long baudrate = 115200;
    char *logfile = NULL;
    char *device = NULL;
    int ch;
    char *endptr;
    while ((ch = getopt(argc, argv, "B:l:")) != -1) {
        switch (ch) {
            case 'B':
                baudrate = strtol(optarg, &endptr, 10);
                if (endptr[0] != '\0') {
                    fprintf(stderr, "Invalid baudrate argument `%s'\n", optarg);
                    usage();
                    return -1;
                }
                break;
            case 'l':
                logfile = optarg;
                break;
            case '?':
            default:
                    break;
        }
    }
    argc -= optind;
    argv += optind;

    if (argc == 0) {
        fprintf(stderr, "No serial device specified\n");
        usage();
        return -1;
    }
    device = argv[0];

    fd = open(device, O_RDONLY | O_NOCTTY | O_NONBLOCK);
    if (fd < 0) {
        perror("unable to open file");
        return -1;
    }
    cfmakeraw(&tio);
    cfsetispeed(&tio,baudrate);
    cfsetospeed(&tio,baudrate);
    tcsetattr(fd,TCSANOW,&tio);

    int write_to_logfile = 0;
    if (argc > 2) {
        logfd = open(logfile, O_WRONLY | O_APPEND | O_CREAT | O_NONBLOCK, 0644);
        if (fd < 0) {
            perror("unable to open file for writing");
            return -1;
        } else {
            write_to_logfile = 1;
        }
    }

    // Write header info
    {
        char header[1024];
        char timebuf[1024];

        header[0] = '\0';
        strcat(header, "--------------------------------------------------------\n");
        strcat(header, "Log start: ");

        make_timestamp(timebuf, 1024);
        strcat(header, timebuf);

        strcat(header, " Reading from `");
        strcat(header, device);
        strcat(header, "'\n\n");

        write(STDOUT, header, strlen(header));
        if (write_to_logfile) {
            write(logfd, header, strlen(header));
        }
    }

    signal(SIGINT, &signal_handler);

    for(;;) {
        fd_set read_fds, err_fds;
        FD_SET(fd, &read_fds);
        FD_SET(fd, &err_fds);
        int ready = select(fd + 1, &read_fds, NULL, &err_fds, NULL);

        if (ready == -1) {
            perror("select()");
            return -1;
        } else if (ready == 0) {
            fprintf(stderr, "select () returned 0\n");
            return -1;
        } else {
            if (FD_ISSET(fd, &read_fds)) {
                len = read(fd, &buf[0], 8192);
                if (len > 0) {
                    write(STDOUT, buf, len);
                    if (write_to_logfile) {
                        write(logfd, buf, len);
                    }
                } else {
                    fprintf(stderr, "read 0 bytes\n");
                }
            }
            if (FD_ISSET(fd, &err_fds)) {
                fprintf(stderr, "error with serial port\n");
                break;
            }
        }
    }
    finish();
    return 0;
}