#include "apue.h"#ifdef MACOS#include <sys/uio.h>#endif#include <sys/socket.h>		/* struct msghdr *//* size of control buffer to send/recv one file descriptor */#define	CONTROLLEN	CMSG_LEN(sizeof(int))static struct cmsghdr	*cmptr = NULL;		/* malloc'ed first time *//* * Receive a file descriptor from a server process.  Also, any data * received is passed to (*userfunc)(STDERR_FILENO, buf, nbytes). * We have a 2-byte protocol for receiving the fd from send_fd(). */intrecv_fd(int fd, ssize_t (*userfunc)(int, const void *, size_t)){	int				newfd, nr, status;	char			*ptr;	char			buf[MAXLINE];	struct iovec	iov[1];	struct msghdr	msg;	status = -1;	for ( ; ; ) {		iov[0].iov_base = buf;		iov[0].iov_len  = sizeof(buf);		msg.msg_iov     = iov;		msg.msg_iovlen  = 1;		msg.msg_name    = NULL;		msg.msg_namelen = 0;		if (cmptr == NULL && (cmptr = malloc(CONTROLLEN)) == NULL)			return(-1);		msg.msg_control    = cmptr;		msg.msg_controllen = CONTROLLEN;		if ((nr = recvmsg(fd, &msg, 0)) < 0) {			err_sys("recvmsg error");		} else if (nr == 0) {			err_ret("connection closed by server");			return(-1);		}		/*		 * See if this is the final data with null & status.  Null		 * is next to last byte of buffer; status byte is last byte.		 * Zero status means there is a file descriptor to receive.		 */		for (ptr = buf; ptr < &buf[nr]; ) {			if (*ptr++ == 0) {				if (ptr != &buf[nr-1])					err_dump("message format error"); 				status = *ptr & 0xFF;	/* prevent sign extension */ 				if (status == 0) {					if (msg.msg_controllen != CONTROLLEN)						err_dump("status = 0 but no fd");					newfd = *(int *)CMSG_DATA(cmptr);				} else {					newfd = -status;				}				nr -= 2;			}		}		if (nr > 0 && (*userfunc)(STDERR_FILENO, buf, nr) != nr)			return(-1);		if (status >= 0)	/* final data has arrived */			return(newfd);	/* descriptor, or -status */	}}