c语言 ioctl

C语言ioctl函数的作用是什么

C语言中的ioctl函数是一个非常重要的系统调用，它主要用于设备驱动程序中，用于实现对设备的控制和配置，ioctl函数的主要作用是通过设备文件描述符来操作设备，实现对设备的读写、设置参数等操作，本文将详细介绍ioctl函数的定义、用法以及相关的技术概念。

ioctl函数的定义

在Linux系统中，ioctl函数的原型如下：

include <sys/ioctl.h>
int ioctl(int fd, unsigned int request, ...);

fd是设备文件描述符，request是要执行的操作命令，后面的省略号表示可变参数列表，用于传递操作所需的数据。

ioctl函数的用法

1、读取设备数据

要从设备中读取数据，可以使用FIONREAD命令，首先需要打开设备文件，然后使用ioctl函数发送FIONREAD命令，并将结果存储在变量中，示例代码如下：

include <fcntl.h>
include <unistd.h>
include <sys/ioctl.h>
int main() {
    int fd = open("/dev/mydevice", O_RDONLY);
    if (fd == -1) {
        perror("open");
        return 1;
    }
    struct stat st;
    if (fstat(fd, &st) == -1) {
        perror("fstat");
        close(fd);
        return 1;
    }
    char buffer[st.st_size];
    ssize_t bytesRead = read(fd, buffer, sizeof(buffer));
    if (bytesRead == -1) {
        perror("read");
        close(fd);
        return 1;
    } else if (bytesRead == 0) {
        printf("Device is empty
");
    } else {
        printf("Read %zd bytes from device: ", bytesRead);
        for (size_t i = 0; i < bytesRead; i++) {
            printf("%02x ", buffer[i]);
        }
        printf("
");
    }
    close(fd);
    return 0;
}

2、向设备写入数据

要向设备写入数据，可以使用FIOCLEX命令释放设备的文件锁，然后使用write函数将数据写入设备，示例代码如下：

include <fcntl.h>
include <unistd.h>
include <sys/ioctl.h>
include <sys/types.h>
include <sys/stat.h>
include <fcntl.h> // for O_WRONLY and O_NONBLOCK flags
include <unistd.h> // for write() function call to send data to the device file descriptor 'fd'. The number of bytes written is returned through the argument 'bytesWritten'. If an error occurs while writing the data to the device file, a negative value is returned instead of the number of bytes that were successfully written. In this case, we will print an error message and exit the program with a non-zero exit status value. Otherwise, we will print the number of bytes successfully written to the device file and exit the program with a zero exit status value. If 'bytesWritten' is less than the number of bytes you want to write to the device file, then you need to repeat the process until all of the data has been written to the device file. You can do this by calling write() again with additional data or by using another method to send more data to the device file. Note that you should not use any other functions or methods to write or read data from the device file after releasing the lock with FIOCLEX because this will cause an error message to be printed to standard output and the program will be terminated with a non-zero exit status value. Also note that if you are using a blocking mode when opening the device file, then you must wait for the device driver to acknowledge receipt of the data before continuing with your program execution. This can take some time depending on the size of the data being written and the speed of the device driver. Therefore, it is recommended that you use non-blocking mode when opening the device file to avoid waiting for the device driver to acknowledge receipt of the data. However, this also means that you cannot guarantee that all of the data will be written to the device file before exiting your program because there may still be data waiting to be sent to the device driver after you release the lock with FIOCLEX. In this case, you can either wait for all of the data to be written to the device file or use another method to send more data to the device file before exiting your program.