z.c

#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <stdio.h>

int main(int argc, const char *argv[])
{
	int sock_pair[2];
	if (socketpair(AF_UNIX, SOCK_SEQPACKET, 0, sock_pair) < 0) {
		perror("socketpair: ");
		return -1;
	}

	int reader_fd = sock_pair[0];
	int writer_fd = sock_pair[1];

	int enable = 1;
	if (setsockopt(reader_fd, SOL_SOCKET, SO_PASSCRED, &enable, sizeof(enable)) < 0) {
		perror("setsockopt: ");
		return -1;
	}

	char buf[]          = "hello";
	const size_t length = sizeof(buf);
	struct iovec iov    = { buf, length };

	struct msghdr msg = {};

	msg.msg_iov    = &iov;
	msg.msg_iovlen = 1;

	const ssize_t control_len_snd = CMSG_SPACE(sizeof(int));
	char control_buf_snd[control_len_snd];

	msg.msg_control    = control_buf_snd;
	msg.msg_controllen = control_len_snd;

	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);

	cmsg->cmsg_level = SOL_SOCKET;
	cmsg->cmsg_type  = SCM_RIGHTS;
	cmsg->cmsg_len   = CMSG_LEN(sizeof(int));

	memcpy(CMSG_DATA(cmsg), &reader_fd, sizeof(int));

	if (sendmsg(writer_fd, &msg, 0) < 0) {
		perror("sendmesg: ");
		return -1;
	}

	char recv_buf[sizeof(buf)] = {};
	struct iovec iov2          = { recv_buf, length };
	msg.msg_iov                = &iov2;
	msg.msg_iovlen             = 1;

	const ssize_t control_len_rcv = CMSG_SPACE(sizeof(struct ucred)) + CMSG_SPACE(sizeof(int));
	char control_buf_rcv[control_len_rcv];

	msg.msg_control    = control_buf_rcv;
	msg.msg_controllen = control_len_rcv;

	if (recvmsg(reader_fd, &msg, 0) < 0) {
		perror("recvmesg: ");
		return -1;
	}

	int got_fd    = -1;
	pid_t got_pid = -1;

	if (msg.msg_controllen > 0) {
		struct cmsghdr *cmsg; // hides

		for (cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
			const unsigned payload_len = cmsg->cmsg_len - CMSG_LEN(0);

			if (cmsg->cmsg_level != SOL_SOCKET)
				continue;

			switch (cmsg->cmsg_type) {
				case SCM_RIGHTS:
					if (payload_len != sizeof(int)) {
						fprintf(stderr, "expected fd payload sizeof(int)\n");
						return -1;
					}

					got_fd = *((int *)(CMSG_DATA(cmsg)));
					printf("got fd: %d\n", got_fd);

					break;

				case SCM_CREDENTIALS:
					if (payload_len != sizeof(struct ucred)) {
						fprintf(stderr, "expected rights payload sizeof(struct ucred)\n");
						return -1;
					}

					got_pid = ((struct ucred *)(CMSG_DATA(cmsg)))->pid;
					printf("got pid: %d\n", got_pid);

					break;
			}
		}
	}

	printf("got message buffer: %s\n", recv_buf);
	return 0;
}