How to Access and Communicate with Linux Serial Ports Using C

Serial Port Device Files

Linux exposes peripheral devices as files under /dev. The traditional COM ports correspond to /dev/ttyS0 (COM1), /dev/ttyS1 (COM2), while USB‑to‑serial adapters appear as /dev/ttyUSB0, /dev/ttyUSB1, etc. The command ls /dev/ttyS* lists available serial devices.

Method 1 – Polling

Open the device with open(port, O_RDWR | O_NOCTTY | O_NDELAY), configure it using tcgetattr and tcsetattr with a 115200 baud rate, 8‑N‑1 settings, and enable CLOCAL and CREAD. The relevant flag meanings are listed.

fd = open(port, O_RDWR | O_NOCTTY | O_NDELAY);
if (fd == -1) { perror("open_port: Unable to open serial port"); return -1; }
tcgetattr(fd, &options);
cfsetispeed(&options, B115200);
cfsetospeed(&options, B115200);
options.c_cflag |= (CLOCAL | CREAD);
options.c_cflag &= ~PARENB;
options.c_cflag &= ~CSTOPB;
options.c_cflag &= ~CSIZE;
options.c_cflag |= CS8;
options.c_cflag &= ~CRTSCTS;
tcsetattr(fd, TCSANOW, &options);

Read and write with read(fd, buf, sizeof(buf)) and write(fd, buf, strlen(buf)), then close with close(fd). A full example program is provided.

Method 2 – Select (Interrupt‑Driven)

Using select to wait for the file descriptor to become readable avoids busy‑waiting. The example opens /dev/ttyUSB0, sets the same 9600‑baud 8‑N‑1 parameters, then loops:

FD_ZERO(&rfds);
FD_SET(fd, &rfds);
select(fd+1, &rfds, NULL, NULL, NULL);
int n = read(fd, buf, sizeof(buf));
if (n > 0) printf("Received data: %.*s
", n, buf);

The descriptor must be put into non‑blocking mode with fcntl(fd, F_SETFL, flags | O_NONBLOCK).

Method 3 – Signal (SIGIO) Notification

Configure the descriptor for asynchronous I/O with fcntl(fd, F_SETOWN, getpid()) and fcntl(fd, F_SETFL, flags | O_ASYNC). Register a sigio_handler via sigaction(SIGIO, &sa, NULL) that reads the data when the signal arrives.

void sigio_handler(int sig) {
    char buf[256];
    int n = read(fd, buf, sizeof(buf));
    if (n > 0) printf("Received data: %.*s
", n, buf);
}

Method 4 – Threaded Reading

Spawn a POSIX thread that continuously calls read(fd, buf, sizeof(buf)). The main thread can perform other work while the reader thread processes incoming bytes.

void *read_thread(void *arg) {
    int fd = *(int *)arg;
    char buf[256];
    while (1) {
        int n = read(fd, buf, sizeof(buf));
        if (n > 0) printf("Received data: %.*s
", n, buf);
    }
    return NULL;
}

All four approaches demonstrate how to open, configure, read from, and close a Linux serial port using standard POSIX APIs, allowing developers to choose between simple polling, event‑driven select, signal‑based notification, or multithreaded handling based on performance and design requirements.