如果不是用minicom打开一次,就不能从串行端口读取数据

我已经在C中实现了一个从串行端口的阻塞读取。我的目标是有一个在新数据到达之前阻塞的读取。

下面是我如何实现串行伪对象(为了让代码更清晰,我去掉了多线程保护)。

typedef struct
{
    int fd;
    se_serial_speed_t speed;
    se_serial_parity_t parity;
    bool flow_control;
}se_serial_t;

int se_serial_constructor(se_serial_t** self, char* serial_port)
{
    int fd;

    if(NULL != *self)
    {
        return ERR_NNULL_PTR;
    }
    else if(0 != access(serial_port, F_OK))
    {
        ERRNO("Serial port is not available");
        return ERR_ILLEGAL_PARAM;
    }
    else
    {
        if(-1 == (fd = open(serial_port, O_RDWR | O_NOCTTY)))
        {
            ERRNO("Error opening %s in rw mode", serial_port);
            return ERR_OFILE_FAIL;
        }
        else if(NULL == (*self = malloc(sizeof(se_serial_t))))
        {
            ERROR("Error allocating memory for Serial");
            return ERR_MALLOC_FAIL;
        }
        (*self)->fd = fd;
    }
    return ERR_OK;
}

int se_serial_configure_interface(se_serial_t* self, se_serial_speed_t speed, se_serial_parity_t parity, bool flow_control)
{
    struct termios options;

    if(NULL == self)
    {
        return ERR_NULL_PTR;
    }
    else
    {
        if(0 != tcgetattr(self->fd,  &options))
        {
            ERRNO("Unable to get serial port current configuration");
        }
        if(0 != cfsetospeed(&options, speed))
        {
            ERRNO("Unable to set serial port output speed");
        }
        if(0 != cfsetispeed(&options, speed))
        {
            ERRNO("Unable to set serial port input speed");
        }

        switch(parity)
        {
        case SE_SERIAL_PARITY_8N1:
            options.c_cflag &= ~PARENB;
            options.c_cflag &= ~CSTOPB;
            options.c_cflag &= ~CSIZE;
            options.c_cflag |= CS8;
            break;
        case SE_SERIAL_PARITY_7E1:
            options.c_cflag |= PARENB;
            options.c_cflag &= ~PARODD;
            options.c_cflag &= ~CSTOPB;
            options.c_cflag &= ~CSIZE;
            options.c_cflag |= CS7;
            break;
        case SE_SERIAL_PARITY_7O1:
            options.c_cflag |= PARENB;
            options.c_cflag |= PARODD;
            options.c_cflag &= ~CSTOPB;
            options.c_cflag &= ~CSIZE;
            options.c_cflag |= CS7;
            break;
        case SE_SERIAL_PARITY_7S1:
            options.c_cflag &= ~PARENB;
            options.c_cflag &= ~CSTOPB;
            options.c_cflag &= ~CSIZE;
            options.c_cflag |= CS8;
            break;
        default:
            WARNING("Unable to set serial port parity");
            break;
        }

        if(flow_control)
            options.c_cflag |= CRTSCTS;
        else
            options.c_cflag &= ~CRTSCTS;

        options.c_cc[VMIN] = 1;
        options.c_cc[VTIME] = 0;

        if(0 != tcsetattr(self->fd, TCSANOW, &options))
        {
            ERRNO("Error configuring serial port");
            return ERR_SERIAL_CONF_FAIL;
        }
        self->speed = speed;
        self->parity = parity;
        self->flow_control = flow_control;
    }
    return ERR_OK;
}

int se_serial_read(se_serial_t* self, uint8_t* buffer, int size)
{
    int bytes_read = 0;
    int ret;

    if(NULL == self)
    {
        return ERR_NULL_PTR;
    }
    else
    {
        while(bytes_read < size)
        {
            if(0 > (ret = read(self->fd, &(buffer[bytes_read]), size - bytes_read)))
            {
                ERROR("Error reading from %s : %d\n", self->serial_port, ret);
                return ERR_RFILE_FAIL;
            }
            bytes_read += ret;
        }
        size = bytes_read;
    }

    return size;
}

我正在与之通信的设备在启动后每秒发送一个11字节的帧。

所以在初始化串口后,我在无限循环中接收帧,然后打印它们。

se_serial_t* serial = NULL;
uint8_t buffer[1024] = {0};
int ret = 0;
int i;

if(0 > (ret = se_serial_constructor(&serial, "/dev/ttyUSB0")))
{
    ERROR("Error creating serial : %d", ret);
    return ERR_SERIAL_CREATION_FAIL;
}
else if(0 > (ret = se_serial_configure_interface(serial, SE_SERIAL_SPEED_B115200, SE_SERIAL_PARITY_8N1, false)))
{
    ERROR("Error configuring serial interface : %d", ret);
    return ERR_SERIAL_CONFIG_FAIL;
}

while(1)
{
    if(0 > (ret = se_serial_read(serial, buffer, 11)))
    {
        ERROR("Error reading from serial : %d", ret);
        return ret;
    }
    else
    {
        for(i=0;i<ret;i++)
        {
            printf("%02x ", buffer[i]);
        }
        printf("\n");
    }
}

奇怪的是,即使我知道设备正在发送帧,读取也会永远阻塞。

但是,如果我用另一个程序(如minicom)打开端口,我可以接收其中的帧。一旦用minicom打开了端口,并且我退出了它,我的程序就可以正常工作,直到下次重新启动我的计算机。

如果我重新启动设备,代码会阻塞,直到它开始发送帧并很好地接收帧。

我也试过树莓皮3,以确保它不是一个配置问题,我的笔记本电脑,但我得到了同样的结果。