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/*************************************************************************\
* Copyright (C) Michael Kerrisk, 2018. *
* *
* This program is free software. You may use, modify, and redistribute it *
* under the terms of the GNU Lesser General Public License as published *
* by the Free Software Foundation, either version 3 or (at your option) *
* any later version. This program is distributed without any warranty. *
* See the files COPYING.lgpl-v3 and COPYING.gpl-v3 for details. *
\*************************************************************************/
/* Supplementary program for Chapter 61 */
/* scm_functions.c
Functions to exchange ancillary data over UNIX domain sockets.
These functions are simplistic, in that they ignore the "real" data
content on the assumption that the sockets are being used only for
the purposes of exchanging ancillary data. In many real-world
applications, the application makes use of both the "real" data
channel and the ancillary data, with some kind of protocol that
determines how the "real" and ancillary data are used together.
*/
#include "scm_functions.h"
/* Send the file descriptor 'fd' over the connected UNIX domain
socket 'sockfd' */
int
sendfd(int sockfd, int fd)
{
struct msghdr msgh;
struct iovec iov;
int data;
struct cmsghdr *cmsgp;
/* Allocate a char array of suitable size to hold the ancillary data.
However, since this buffer is in reality a 'struct cmsghdr', use a
union to ensure that it is aligned as required for that structure.
Alternatively, we could allocate the buffer using malloc(), which
returns a buffer that satisfies the strictest alignment requirements
of any type. However, if we employ that approach, we must ensure
that we free() the buffer on all return paths from this function. */
union {
char buf[CMSG_SPACE(sizeof(int))];
/* Space large enough to hold an 'int' */
struct cmsghdr align;
} controlMsg;
/* The 'msg_name' field can be used to specify the address of the
destination socket when sending a datagram. However, we do not
need to use this field because we presume that 'sockfd' is a
connected socket. */
msgh.msg_name = NULL;
msgh.msg_namelen = 0;
/* On Linux, we must transmit at least one byte of real data in
order to send ancillary data. We transmit an arbitrary integer
whose value is ignored by recvfd(). */
msgh.msg_iov = &iov;
msgh.msg_iovlen = 1;
iov.iov_base = &data;
iov.iov_len = sizeof(int);
data = 12345;
/* Set 'msghdr' fields that describe ancillary data */
msgh.msg_control = controlMsg.buf;
msgh.msg_controllen = sizeof(controlMsg.buf);
/* Set up ancillary data describing file descriptor to send */
cmsgp = CMSG_FIRSTHDR(&msgh);
cmsgp->cmsg_level = SOL_SOCKET;
cmsgp->cmsg_type = SCM_RIGHTS;
cmsgp->cmsg_len = CMSG_LEN(sizeof(int));
*((int *) CMSG_DATA(cmsgp)) = fd;
/* Send real plus ancillary data */
if (sendmsg(sockfd, &msgh, 0) == -1)
return -1;
return 0;
}
/* Receive a file descriptor on a connected UNIX domain socket.
The received file descriptor is returned as the function result. */
int
recvfd(int sockfd)
{
struct msghdr msgh;
struct iovec iov;
int data;
ssize_t nr;
/* Allocate a char buffer for the ancillary data. See the comments
in sendfd() */
union {
char buf[CMSG_SPACE(sizeof(int))];
struct cmsghdr align;
} controlMsg;
struct cmsghdr *cmsgp;
/* The 'msg_name' field can be used to obtain the address of the
sending socket. However, we do not need this information. */
msgh.msg_name = NULL;
msgh.msg_namelen = 0;
/* Specify buffer for receiving real data */
msgh.msg_iov = &iov;
msgh.msg_iovlen = 1;
iov.iov_base = &data; /* Real data is an 'int' */
iov.iov_len = sizeof(int);
/* Set 'msghdr' fields that describe ancillary data */
msgh.msg_control = controlMsg.buf;
msgh.msg_controllen = sizeof(controlMsg.buf);
/* Receive real plus ancillary data; content of real data is ignored */
nr = recvmsg(sockfd, &msgh, 0);
if (nr == -1)
return -1;
cmsgp = CMSG_FIRSTHDR(&msgh);
/* Check the validity of the 'cmsghdr' */
if (cmsgp == NULL ||
cmsgp->cmsg_len != CMSG_LEN(sizeof(int)) ||
cmsgp->cmsg_level != SOL_SOCKET ||
cmsgp->cmsg_type != SCM_RIGHTS) {
errno = EINVAL;
return -1;
}
/* Return the received file descriptor to our caller */
return *((int *) CMSG_DATA(cmsgp));
}
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