test code

gistfile1.txt

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <strings.h>
#include <errno.h>
#include <unistd.h>
#include <netdb.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <inttypes.h>
#include <stropts.h>
#include <sys/select.h>
#include <sys/un.h>
#include <string.h>


void
main(int argc, char **argv)
{
        int i;
        int enable = 1;
        int len;
        int fds[2];

        if (socketpair(AF_UNIX, SOCK_SEQPACKET, 0, fds) < 0) {
            perror("p: socketpair");
            exit(1);
        }

        if (setsockopt(fds[0], SOL_SOCKET, SO_PASSCRED, &enable,
            sizeof (enable)) < 0) {
            perror("p: setsockopt");
            exit(1);
        }

        if (fork() == 0) {
                char            buf[80];
                struct msghdr   msg;
                struct iovec iov;

                snprintf(buf, sizeof (buf), "child %d", getpid());
                iov.iov_base = buf;
                iov.iov_len = strlen(buf);

                msg.msg_name = NULL;
                msg.msg_namelen = 0;
                msg.msg_iov = &iov;
                msg.msg_iovlen = 1;
                msg.msg_control = NULL;
                msg.msg_controllen = 0;
                msg.msg_flags = 0;

                printf("c1: sendmsg\n");
                if (sendmsg(fds[1], &msg, 0) < 0) {
                        perror("c1: sndmsg");
                        exit(1);
                }
                exit(0);
        }

        if (fork() == 0) {
                char            buf[80];
                struct msghdr   msg;
                struct iovec iov;

                snprintf(buf, sizeof (buf), "child %d", getpid());
                iov.iov_base = buf;
                iov.iov_len = strlen(buf);

                msg.msg_name = NULL;
                msg.msg_namelen = 0;
                msg.msg_iov = &iov;
                msg.msg_iovlen = 1;
                msg.msg_control = NULL;
                msg.msg_controllen = 0;
                msg.msg_flags = 0;

                printf("c2: sendmsg\n");
                if (sendmsg(fds[1], &msg, 0) < 0) {
                        perror("c2: sndmsg");
                        exit(1);
                }
                exit(0);
        }

        /* parent - server */
        for (i = 0; i < 2; i++) {
                char            ctl[256];
                char            buf[80];
                struct msghdr   msg;
                struct iovec iov;

                iov.iov_base = buf;
                iov.iov_len = sizeof (buf);

                msg.msg_name = NULL;
                msg.msg_namelen = 0;
                msg.msg_iov = &iov;
                msg.msg_iovlen = 1;
                msg.msg_control = &ctl;
                msg.msg_controllen = sizeof (ctl);
                msg.msg_flags = 0;

                printf("p: recvmsg\n");
                if ((len = recvmsg(fds[0], &msg, 0)) < 0) {
                        perror("p: Could not read message");
                        exit(1);
                } else {
                        printf("p: len %d, %s\n", len, buf);
                }
        }

        exit(0);
}