无法写入串行设备，但可以从中读取

我使用的是非阻塞IO，我只是尝试对串行端口进行写入和读取。读取串行端口可以正常工作，而写入则不然。

这就是我设置串行线路的方式。正如您所看到的，它被设置为非阻塞和规范。也许有些标志是多余的，但它们大多取自这里的一个示例：串行设置示例

tSerial::tSerial(const char *serialPort, bool echo)
{
// Open the serial port. Change device path as needed (currently set to an standard FTDI USB-UART cable type device)
m_serialPort = open(serialPort, O_RDWR);
// Create new termios struct, we call it 'tty' for convention
struct termios tty;
// Read in existing settings, and handle any error
if (tcgetattr(m_serialPort, &tty) != 0)
{
spdlog::error("[tSerial] error {} from tcgetattr: {}", errno, strerror(errno));
throw std::runtime_error("Failed to get existing serial settings");
}
tty.c_cflag &= ~PARENB;        // Clear parity bit, disabling parity (most common)
tty.c_cflag &= ~CSTOPB;        // Clear stop field, only one stop bit used in communication (most common)
tty.c_cflag &= ~CSIZE;         // Clear all bits that set the data size
tty.c_cflag |= CS8;            // 8 bits per byte (most common)
tty.c_cflag &= ~CRTSCTS;       // Disable RTS/CTS hardware flow control (most common)
tty.c_cflag |= CREAD | CLOCAL; // Turn on READ & ignore ctrl lines (CLOCAL = 1)
// tty.c_lflag &= ~ICANON;
tty.c_cflag |= ICANON;
if (!echo)
{
tty.c_lflag &= ~ECHO; // Disable echo
}
else
{
tty.c_lflag |= ECHO; // Enable echo
}
// tty.c_lflag &= ~ECHOE;                                                       // Disable erasure
// tty.c_lflag &= ~ECHONL;                                                      // Disable new-line echo
// tty.c_lflag &= ~ISIG;                                                        // Disable interpretation of INTR, QUIT and SUSP
tty.c_iflag &= ~(IXON | IXOFF | IXANY);                                      // Turn off s/w flow ctrl
tty.c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR | IGNCR | ICRNL); // Disable any special handling of received bytes
tty.c_oflag &= ~OPOST; // Prevent special interpretation of output bytes (e.g. newline chars)
tty.c_oflag &= ~ONLCR; // Prevent conversion of newline to carriage return/line feed
// Set in/out baud rate to be 115200
cfsetispeed(&tty, B115200);
cfsetospeed(&tty, B115200);
// Save tty settings, also checking for error
if (tcsetattr(m_serialPort, TCSANOW, &tty) != 0)
{
spdlog::error("[tSerial] error {} from tcsetattr: {}", errno, strerror(errno));
throw std::runtime_error("Failed to set new serial settings");
}
// Set non-blocking
int flags;
if ((flags = fcntl(m_serialPort, F_GETFL, 0)) == -1)
{
flags = 0;
}
fcntl(m_serialPort, F_SETFL, flags | O_NONBLOCK);
}

这就是我努力写作的方式。我正在使用select来等待IO资源变为可用，因为尝试直接写入会产生错误Resource temporarily unavailable。但即使等待20秒，该设备仍然不可用。

void tSerial::writeSerial(std::string message)
{
int n;
fd_set rfds;
FD_ZERO(&rfds);
FD_SET(m_serialPort, &rfds);
spdlog::debug("[tSerial] writing command to serial {}", message);
struct timeval tv;
tv.tv_sec = 20;
tv.tv_usec = 0;
int retval = select(1, NULL, &rfds, NULL, &tv);
if (retval == -1)
{
spdlog::error("[tSerial] select error");
}
else if (retval)
{
n = write(m_serialPort, message.c_str(), message.length());
tcflush(m_serialPort, TCIOFLUSH);
if (n < 0)
{
spdlog::error("[tSerial] error writing: {}", strerror(errno));
}
}
else
{
spdlog::error("[tSerial] Resource unavailable after 20 seconds wait");
}
}