What is the usage of getsockopt in Linux?

`getsockopt()` in Linux is a system call function used to retrieve the value of socket options. Its prototype is as follows:

#include <sys/socket.h>
int getsockopt(int sockfd, int level, int optname, void *optval, socklen_t *optlen);

The meanings of the parameters are as follows:

sockfd：套接字文件描述符，指定要获取选项的套接字。
level：选项所属的协议层级，常见的值有 SOL_SOCKET（通用套接字选项）和 IPPROTO_TCP（TCP协议选项）等。
optname：选项的名称，表示您要获取的具体选项。
optval：指向存储选项值的缓冲区的指针。
optlen：指向一个整数的指针，用于指定缓冲区的大小，并返回实际选项数据的长度。

The function `getsockopt()` retrieves the current value of the corresponding option by passing the `sockfd`, `level`, and `optname` parameters, and stores it in the buffer specified by `optval`. Upon successful call, it will return 0; otherwise, it returns -1 and sets the corresponding error code.

Here is a simple example of how to use it:

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
int main() {
    int sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd == -1) {
        perror("socket");
        exit(1);
    }
    int option;
    socklen_t option_len = sizeof(option);
    // 获取套接字的 SO_REUSEADDR 选项值
    if (getsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &option, &option_len) == -1) {
        perror("getsockopt");
        exit(1);
    }
    printf("SO_REUSEADDR: %d\n", option);
    close(sockfd);
    return 0;
}

The sample code here creates a socket and uses the `getsockopt()` function to obtain the current value of the `SO_REUSEADDR` option. The value is then printed out for your reference. Make adjustments based on your specific needs and option names.