write_fd函数

#include    <stdio.h>

#include    <stddef.h>
#include    <sys/socket.h>

void err_sys(const char *, ...);

ssize_t write_fd(int fd, void *ptr, size_t nbytes, int sendfd)
{
    struct msghdr    msg;
    struct iovec    iov[1];

    union {
      struct cmsghdr    cm;
      char   control[CMSG_SPACE(sizeof(int))];
    } control_un;
    struct cmsghdr    *cmptr;

    msg.msg_control = control_un.control;
    msg.msg_controllen = sizeof(control_un.control);

    cmptr = CMSG_FIRSTHDR(&msg);
    cmptr->cmsg_len = CMSG_LEN(sizeof(int));
    cmptr->cmsg_level = SOL_SOCKET;
    cmptr->cmsg_type = SCM_RIGHTS;
    *((int *) CMSG_DATA(cmptr)) = sendfd;

    msg.msg_name = NULL;
    msg.msg_namelen = 0;

    iov[0].iov_base = ptr;
    iov[0].iov_len = nbytes;
    msg.msg_iov = iov;
    msg.msg_iovlen = 1;

    return (sendmsg(fd, &msg, 0));
}

ssize_t Write_fd(int fd, void *ptr, size_t nbytes, int sendfd)
{
    ssize_t        n;

    if ( (n = write_fd(fd, ptr, nbytes, sendfd)) < 0) {
        err_sys("write_fd error");
    }

    return (n);
}