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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 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 file COPYING.gpl-v3 for details. *
\*************************************************************************/
/* Supplementary program for Chapter 61 */
/* scm_rights_send.c
Used in conjunction with scm_rights_recv.c to demonstrate passing of
file descriptors via a UNIX domain socket.
This program sends a file descriptor to a UNIX domain socket.
Usage is as shown in the usageErr() call below.
File descriptors can be exchanged over stream or datagram sockets. This
program uses stream sockets by default; the "-d" command-line option
specifies that datagram sockets should be used instead.
This program is Linux-specific.
See also scm_multi_recv.c.
*/
#include "scm_rights.h"
int
main(int argc, char *argv[])
{
int data, sfd, opt, fd;
ssize_t ns;
Boolean useDatagramSocket;
struct msghdr msgh;
struct iovec iov;
/* 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. */
union {
char buf[CMSG_SPACE(sizeof(int))];
/* Space large enough to hold an 'int' */
struct cmsghdr align;
} controlMsg;
struct cmsghdr *cmsgp; /* Pointer used to iterate through
headers in ancillary data */
/* Parse command-line options */
useDatagramSocket = FALSE;
while ((opt = getopt(argc, argv, "d")) != -1) {
switch (opt) {
case 'd':
useDatagramSocket = TRUE;
break;
default:
usageErr("%s [-d] file\n"
" -d use datagram socket\n", argv[0]);
}
}
if (argc != optind + 1)
usageErr("%s [-d] file\n", argv[0]);
/* Open the file named on the command line */
fd = open(argv[optind], O_RDONLY);
if (fd == -1)
errExit("open");
/* 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 use connect() below, which sets
a default outgoing address for datagrams. */
msgh.msg_name = NULL;
msgh.msg_namelen = 0;
/* On Linux, we must transmit at least 1 byte of real data in
order to send ancillary data */
msgh.msg_iov = &iov;
msgh.msg_iovlen = 1;
iov.iov_base = &data;
iov.iov_len = sizeof(int);
data = 12345;
fprintf(stderr, "Sending data = %d\n", data);
/* Set 'msgh' fields to describe the ancillary data buffer */
msgh.msg_control = controlMsg.buf;
msgh.msg_controllen = sizeof(controlMsg.buf);
/* The control message buffer must be zero-initialized in order
for the CMSG_NXTHDR() macro to work correctly. Although we
don't need to use CMSG_NXTHDR() in this example (because
there is only one block of ancillary data), we show this
step to demonstrate best practice */
memset(controlMsg.buf, 0, sizeof(controlMsg.buf));
/* Set message header to describe the ancillary data that
we want to send */
cmsgp = CMSG_FIRSTHDR(&msgh);
cmsgp->cmsg_len = CMSG_LEN(sizeof(int));
cmsgp->cmsg_level = SOL_SOCKET;
cmsgp->cmsg_type = SCM_RIGHTS;
*((int *) CMSG_DATA(cmsgp)) = fd;
/* Connect to the peer socket */
sfd = unixConnect(SOCK_PATH, useDatagramSocket ? SOCK_DGRAM : SOCK_STREAM);
if (sfd == -1)
errExit("unixConnect");
fprintf(stderr, "Sending FD %d\n", fd);
/* Send real plus ancillary data */
ns = sendmsg(sfd, &msgh, 0);
if (ns == -1)
errExit("sendmsg");
fprintf(stderr, "sendmsg() returned %ld\n", (long) ns);
exit(EXIT_SUCCESS);
}
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