#include <stropts.h>int fattach(int filedes, const char *path);
//成功返回0，错误返回-1。

#include <stropts.h>int fdetach(const cha *path);
//成功返回0，错误返回-1。

#include <sys/socket.h>int socketpair(int domain, int type, int protocol, int sockfd[2]);
//成功返回0，错误返回-1。

#include "apue.h"int send_fd(int fd, int fd_to_send);
int send_err(int fd, int status, const char *errmsg);
//成功返回0，错误返回-1。int recv_fd(int fd, ssize_t (*userfunc)(int, const void *, size_t));
//成功返回文件描述符，错误返回负值。

#include <sys/socket.h>unsigned char *CMSG_DATA(struct cmsghdr *cp);
//返回和cmsghdr结构体相关的数据的指针。struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *mp);
//返回和msghdr结构体相关联的第一个cmsghdr结构体的指针，或者没有一个存在时返回NULL。struct cmsghdr *CMSG_NXTHDR(struct msghdr *mp, struct cmsghdr *cp);
//给定当前cmsghdr结构体，返回和msghdr结构体相关联的下一个cmsghdr结构体的指针，或我们已经在最后一个上时返回NULL

unsigned int CMSG_LEN(unsigned int nbytes);
//返回为nbytes大的数据对象分配的尺寸。

struct msghdr {void         *msg_name;              /* optional address */socklen_t    msg_namelen;            /* address size in bytes */struct       iovec  *msg_iov;        /* array of I/O buffers */int          msg_iovlen;             /* number of elements in array */void         *msg_control;           /* ancillary data */socklen_t    msg_controllen;         /* number of ancillary bytes */int          msg_flags;              /* flags for received message */};struct cmsghdr {socklen_t  cmsg_len;     /* data byte count, including header */int        cmsg_level;   /* originating protocol */int        cmsg_type;    /* protocol-specific type *//* followed by the actual control message data */
};