C++ Serial Communication

Opening a Port

1
fd_ = ::open(port.c_str(), O_RDWR | O_NOCTTY | O_NONBLOCK);

Flags:

• O_RDWR: open the device file for both reading and writing.
• O_NOCTTY: prevents the device from becoming the process’s controlling terminal.
  • Without this flag, if the device sends a signal byte like 0x03 (Ctrl-C), it could deliver SIGINT to your program.
  • Especially important for background processes and daemons.
• O_NONBLOCK: opens the device without blocking. Normally, opening a serial port can block until the device is ready.

DTR (Data Terminal Ready): Many USB serial devices (e.g., Arduino, motor controllers) use DTR, a control signal in RS-232 serial communication from Data Terminal Equipment (DTE, such as a computer) to Data Communications Equipment (DCE).

Port Configuration: termios

termios is a POSIX struct for configuring serial port behavior: baud rate, parity, stop bits, raw vs. canonical mode, echo, signals, etc.

Reading from a Serial Port

Read count bytes from a file descriptor with a timeout:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
// Use select() to block at most `timeout_ms` waiting for data,
// rather than spinning or blocking indefinitely.
size_t received = 0;
while (received < count) {
    fd_set fds;
    FD_ZERO(&fds);
    FD_SET(fd_, &fds);

    struct timeval tv;
    tv.tv_sec  = timeout_ms / 1000;
    tv.tv_usec = (timeout_ms % 1000) * 1000;

    int sel = ::select(fd_ + 1, &fds, nullptr, nullptr, &tv);
    if (sel <= 0) {
        return false;   // timeout (sel == 0) or error (sel < 0)
    }

    ssize_t n = ::read(fd_, buf + received, count - received);
    if (n <= 0) {
        return false;
    }
    received += static_cast<size_t>(n);
}

Key functions:

• select(): blocks until the file descriptor is ready or the timeout expires.
  • Return values: sel > 0 — data ready; sel == 0 — timeout; sel < 0 — error.
  • No busy-waiting: the process sleeps in the kernel, freeing the CPU for other work.
  • Can monitor multiple file descriptors simultaneously (sockets, serial ports, pipes, stdin, etc.).
• read(fd_, buf + received, count - received): reads up to count - received bytes into buf at offset received.

• Previous
  Bash Scripting
• Next
  Point Cloud Processing