linux网络编程之socket（十六）

5、msg_control：是一个指向cmsghdr 结构体的指针

 struct cmsghdr {

          socklen_t cmsg_len;    /* data byte count, including header */

          int       cmsg_level;  /* originating protocol */

          int       cmsg_type;   /* protocol- specific type */

          /* followed by unsigned char cmsg_data[]; */

      };

6、msg_controllen ：参见下图，即cmsghdr 结构体可能不止一个；

7、flags ： 一般设置为0即可；

为了对齐，可能存在一些填充字节，跟每个系统的实现有关，但我们不必关心，可以通过一些函数宏来获取相关的值， 如下：

#include <sys/socket.h>

      struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *msgh);

      struct cmsghdr *CMSG_NXTHDR(struct msghdr *msgh, struct cmsghdr *cmsg);

      size_t CMSG_ALIGN(size_t length);

      size_t CMSG_SPACE (size_t length);

      size_t CMSG_LEN(size_t length);

      unsigned char *CMSG_DATA(struct cmsghdr *cmsg);

下面通过封装两个函数，send_fd 和 recv_fd 来进一步认识这些函数宏 的作用：

void send_fd(int sock_fd, int send_fd)
{
    int ret;
    struct msghdr msg;
    struct cmsghdr *p_cmsg;
    struct iovec vec;
    char cmsgbuf[CMSG_SPACE(sizeof(send_fd))];
    int *p_fds;
    char sendchar = 0;
    msg.msg_control = cmsgbuf;
    msg.msg_controllen = sizeof(cmsgbuf);
    p_cmsg = CMSG_FIRSTHDR(&msg);
    p_cmsg->cmsg_level = SOL_SOCKET;
    p_cmsg->cmsg_type = SCM_RIGHTS;
    p_cmsg->cmsg_len = CMSG_LEN(sizeof(send_fd));
    p_fds = (int *)CMSG_DATA(p_cmsg);
    *p_fds = send_fd; // 通过传递辅助数据的方式传递文件描述符
    
    msg.msg_name = NULL;
    msg.msg_namelen = 0;
    msg.msg_iov = &vec;
    msg.msg_iovlen = 1; //主要目的不是传递数据，故只传1个字符
    msg.msg_flags = 0;
    
    vec.iov_base = &sendchar;
    vec.iov_len = sizeof(sendchar);
    ret = sendmsg(sock_fd, &msg, 0);
    if (ret != 1)
        ERR_EXIT("sendmsg");
}
    
int recv_fd(const int sock_fd)
{
    int ret;
    struct msghdr msg;
    char recvchar;
    struct iovec vec;
    int recv_fd;
    char cmsgbuf[CMSG_SPACE(sizeof(recv_fd))];
    struct cmsghdr *p_cmsg;
    int *p_fd;
    vec.iov_base = &recvchar;
    vec.iov_len = sizeof(recvchar);
    msg.msg_name = NULL;
    msg.msg_namelen = 0;
    msg.msg_iov = &vec;
    msg.msg_iovlen = 1;
    msg.msg_control = cmsgbuf;
    msg.msg_controllen = sizeof(cmsgbuf);
    msg.msg_flags = 0;
    
    p_fd = (int *)CMSG_DATA(CMSG_FIRSTHDR(&msg));
    *p_fd = -1;
    ret = recvmsg(sock_fd, &msg, 0);
    if (ret != 1)
        ERR_EXIT("recvmsg");
    
    p_cmsg = CMSG_FIRSTHDR(&msg);
    if (p_cmsg == NULL)
        ERR_EXIT("no passed fd");
    
    
    p_fd = (int *)CMSG_DATA(p_cmsg);
    recv_fd = *p_fd;
    if (recv_fd == -1)
        ERR_EXIT("no passed fd");
    
    return recv_fd;
}

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