网络编程

1、不相关线程传送文件描述符使用unix套接字的CMSG的辅助数据传输，其中CMSG_DATA宏返回cmsg结构中辅助数据的指针，而这个CMSG_DATA宏仅仅是根据其参数提供的cmsg指针来返回其中的辅助数据成员，没有什么特殊之处。而内核实际是根据套接字选项，传送进程描述表中指向文件表的指针。

根据linux中 /usr/src/linux-headers-5.11.0-41-generic/include/linux/socket.h 文件中的定义

#define CMSG_DATA(cmsg) \
     ((void *)(cmsg) + sizeof(struct cmsghdr))

struct msghdr {
 51     void        *msg_name;  /* ptr to socket address structure */
 52     int     msg_namelen;    /* size of socket address structure */
 53     struct iov_iter msg_iter;   /* data */
 54     
 55     /*
 56      * Ancillary data. msg_control_user is the user buffer used for the
 57      * recv* side when msg_control_is_user is set, msg_control is the kernel
 58      * buffer used for all other cases.
 59      */
 60     union {
 61         void        *msg_control;
 62         void __user *msg_control_user;
 63     };
 64     bool        msg_control_is_user : 1;
 65     __kernel_size_t msg_controllen; /* ancillary data buffer length */
 66     unsigned int    msg_flags;  /* flags on received message */
 67     struct kiocb    *msg_iocb;  /* ptr to iocb for async requests */
 68 };