server.cpp

#include <iostream>
#include <memory>

#include <string.h>
#include <errno.h>
#include <unistd.h>

#include <sys/socket.h>
#include <sys/un.h>
#include <sys/mman.h>

using namespace std;

int retrieve_fd(int sock_fd) {
    struct alignas(struct cmsghdr) dummy {
        char buf[CMSG_SPACE(sizeof(int))];
    } u;
    char iobuf[256] = {0};
    struct iovec io = {
        .iov_base = iobuf,
        .iov_len = sizeof(iobuf) - 1
    };
    struct msghdr msg{};
    msg.msg_iov = &io;
    msg.msg_iovlen = 1;
    msg.msg_control = reinterpret_cast<void*>(&u);
    msg.msg_controllen = sizeof(u);
    if (recvmsg(sock_fd, &msg, 0) < 0) {
        cout << "recvmsg failed: " << strerror(errno) << endl;
    } else {
        for (auto cmsg = CMSG_FIRSTHDR(&msg); cmsg != nullptr; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
            if (cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_RIGHTS) {
                int fd;
                memcpy(&fd, CMSG_DATA(cmsg), sizeof(fd));
                close(sock_fd);
                return fd;
            }
        }
    }
    close(sock_fd);
    return -1;
}

int main() {
    int listen_sock = socket(AF_UNIX, SOCK_STREAM, 0);
    if (listen_sock <= 0) {
        cout << "try to listen failed: " << strerror(errno) << endl;
        return 1;
    }
    sockaddr_un listen_addr{AF_UNIX};
    string sock_path = "/tmp/server.sock";
    memcpy(reinterpret_cast<void*>(listen_addr.sun_path),
           reinterpret_cast<const void*>(sock_path.c_str()), sock_path.length() + 1);
    if (bind(listen_sock, reinterpret_cast<sockaddr*>(&listen_addr), sizeof(listen_addr))) {
        cout << "try to bind failed: " << strerror(errno) << endl;
        return 1;
    }
    if (listen(listen_sock, 128)) {
        cout << "try to listen failed: " << strerror(errno) << endl;
        return 1;
    }
    constexpr size_t buf_len = 1024;
    auto buf = make_unique<char[]>(buf_len);
    while (true) {
        int client_sock = accept(listen_sock, nullptr, nullptr);
        if (client_sock == -1) {
            cout << "failed to accept new client: " << strerror(errno) << endl;
            return 1;
        }
        int fd = retrieve_fd(client_sock);
        if (fd < 0) {
            continue;
        }
        auto addr = mmap(nullptr, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
        if (addr == MAP_FAILED) {
            cout << "mmap failed: " << strerror(errno) << endl;
            close(fd);
            continue;
        }
        strcpy(reinterpret_cast<char*>(addr), "some testing content\n");
    }
    return 0;
}