No response from C++ server using poll()

Question

I'm studing C++ and sockets from Bgnet guide. I made a simple server which only sends a text for who connects using child processes and select() ways. Now I'm trying to create one using poll() and std::vector to store sockets.

The problem is that it compiles normally, but when I run it and try to connect from browser, unlike the others I have made, this send no response and browser stay forever waiting.

I checked many times my code and found no errors or logic problems. The source's here:

#include <sys/types.h>
#include <sys/socket.h>
#include <poll.h>
#include <vector>
#include <algorithm>
#include <netdb.h>
#include <cstdio>
#include <string.h>
#include <unistd.h>

#define PORT "19456"
#define BACKLOG 10

using namespace std;

int main() {

    sockaddr_storage remote_addr;
    socklen_t remote_addr_size;
    addrinfo settings, *result_info, *info;
    int srv_socket, conn_socket, gai_result, send_status, yes = 1;
    vector <pollfd> fds;
    vector <pollfd>::iterator fd;
    vector <pollfd>::const_iterator last_fd;
    pollfd listener_fd, new_fd;

    memset(&settings, 0, sizeof settings);
    settings.ai_family = AF_INET;
    settings.ai_socktype = SOCK_STREAM;
    settings.ai_flags = AI_PASSIVE;

    if((gai_result = getaddrinfo(NULL, PORT, &settings, &result_info)) != 0) {
            fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(gai_result));
            return 1;
    }

    for(info = result_info; info != NULL; info = info->ai_next) {
            if((srv_socket = socket(info->ai_family, info->ai_socktype, info->ai_protocol)) == -1) {
                    perror("Error on socket()");
                    continue;
            }

            if(setsockopt(srv_socket, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) == -1) {
                    close(srv_socket);
                    perror("Error on setsockopt()");
                    return 1;
            }

            if(bind(srv_socket, info->ai_addr, info->ai_addrlen) == -1) {
                    close(srv_socket);
                    perror("Error on bind()");
                    continue;
            }

            break;
    }

    if(info == NULL) {
            fprintf(stderr, "Failed on all bind()");
            return 1;
    }

    freeaddrinfo(result_info);

    if(listen(srv_socket, BACKLOG) == -1) {
            perror("Error on listen()");
            return 1;
    }

    listener_fd.fd = srv_socket;
    listener_fd.events = POLLIN;
    fds.push_back(listener_fd);

    while(1) {

            pollfd fds_array[fds.size()];
            copy(fds.begin(), fds.end(), fds_array);

            poll(fds_array, (nfds_t) fds.size(), 0);

            if(listener_fd.revents & POLLIN) {
                    remote_addr_size = sizeof remote_addr;

                    if((conn_socket = accept(srv_socket, (sockaddr *) &remote_addr, &remote_addr_size)) == -1) {
                            close(srv_socket);
                            perror("Error on accept()");
                            return 1;
                    }

                    new_fd.fd = conn_socket;
                    new_fd.events = POLLOUT;
                    fds.push_back(new_fd);
            }

            for(fd = fds.begin() + 1, last_fd = fds.end(); fd != last_fd; ++fd) {
                    if(fd->revents & POLLOUT) {
                            send_status = send(fd->fd, "Test", 4, 0);
                            close(fd->fd);
                            fds.erase(fd);
                            if(send_status == -1) {
                                    perror("Error on send()");
                                    return 1;
                            }

                    }
            }
    }

    return 0;

}

I searched and found no related asks, in all found, the problem was with syntax.

Can someone help me to find the error with this? I will be very grateful.

EDIT

I have changed some things following Ben's answers. Now the only problems are connection reset, which I think be caused by closing socket without read request and other is the server using more than 80% of CPU. With select() or child processes ways this not occurs. The current source:

#include <sys/types.h>
#include <sys/socket.h>
#include <poll.h>
#include <vector>
#include <algorithm>
#include <netdb.h>
#include <cstdio>
#include <string.h>
#include <unistd.h>

#define PORT "19456"
#define BACKLOG 10

using namespace std;

int main() {

    sockaddr_storage remote_addr;
    socklen_t remote_addr_size;
    addrinfo settings, *result_info, *info;
    int srv_socket, conn_socket, gai_result, send_status, fd_index, fds_num, yes = 1;
    vector <pollfd> fds;
    pollfd listener_fd, new_fd;

    memset(&settings, 0, sizeof settings);
    settings.ai_family = AF_INET;
    settings.ai_socktype = SOCK_STREAM;
    settings.ai_flags = AI_PASSIVE;

    if((gai_result = getaddrinfo(NULL, PORT, &settings, &result_info)) != 0) {
            fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(gai_result));
            return 1;
    }

    for(info = result_info; info != NULL; info = info->ai_next) {
            if((srv_socket = socket(info->ai_family, info->ai_socktype, info->ai_protocol)) == -1) {
                    perror("Error on socket()");
                    continue;
            }

            if(setsockopt(srv_socket, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) == -1) {
                    close(srv_socket);
                    perror("Error on setsockopt()");
                    return 1;
            }

            if(bind(srv_socket, info->ai_addr, info->ai_addrlen) == -1) {
                    close(srv_socket);
                    perror("Error on bind()");
                    continue;
            }

            break;
    }

    if(info == NULL) {
            fprintf(stderr, "Failed on all bind()");
            return 1;
    }

    freeaddrinfo(result_info);

    if(listen(srv_socket, BACKLOG) == -1) {
            perror("Error on listen()");
            return 1;
    }

    listener_fd.fd = srv_socket;
    listener_fd.events = POLLIN;
    fds.push_back(listener_fd);

    while(1) {

            pollfd fds_array[fds.size()];
            copy(fds.begin(), fds.end(), fds_array);

            poll(fds_array, (nfds_t) fds.size(), 0);

            if(fds_array[0].revents & POLLIN) {
                    remote_addr_size = sizeof remote_addr;

                    if((conn_socket = accept(srv_socket, (sockaddr *) &remote_addr, &remote_addr_size)) == -1) {
                            close(srv_socket);
                            perror("Error on accept()");
                            return 1;
                    }

                    new_fd.fd = conn_socket;
                    new_fd.events = POLLOUT;
                    fds.push_back(new_fd);
            }

            for(fd_index = 1, fds_num = fds.size(); fd_index < fds_num; fd_index++) {
                    if(fds_array[fd_index].revents & POLLOUT) {
                            send_status = send(fds_array[fd_index].fd, "Test", 4, 0);
                            close(fds_array[fd_index].fd);
                            fds.erase(fds.begin() + fd_index);
                            if(send_status == -1) {
                                    perror("Error on send()");
                                    return 1;
                            }

                    }
            }
    }

    return 0;

}

What should be the problem now?

you may want to look at epoll() – Martin York May 12 '13 at 06:15 — Martin York, May 12 '13 at 06:15

score 1 · Answer 1 · answered May 12 '13 at 04:39

1

Your test for whether something is available to accept uses listener_fd which is not modified by poll(). You have to use the copy of it in fds_array which you passed in to poll(). Similarly in your send loop you need to loop over the values in the array you passed to poll().

answered May 12 '13 at 04:39

Ben Jackson

90,079
9
98
150

Thanks, I don't know how not seen this. I made some changes however now I'm getting segmentation fault, dump do core. Again, I don't know what is causing this. I updated the original post adding the new code. – Tiago.SR May 12 '13 at 05:15
@Tiago.SR `sizeof(array)` is in bytes, not in elements. In this case, `sizeof(pointer)` is the size of the pointer, not the number of elements. Instead save a copy of `fds.size()` before `poll()` – Ben Jackson May 12 '13 at 05:26
Thanks man :D I thought C++'s sizeof works equals PHP's sizeof. I assigned `fds.size()` for `fds_num` and segmentation fault stopped. However, now I'm getting connection interrupted/redefined in most connect attempts in Chromium. I think this is because I'm not reading the request and will fix this later. Other problem is that server is using more than 80% of CPU, what not occurs using select() or child processes. This I don't know the reason. – Tiago.SR May 12 '13 at 05:55

score 0 · Accepted Answer · answered May 12 '13 at 10:00

0

Regarding the 80% CPU load, I suspect that you're not setting the timeout parameter quite right. From the manual:

Specifying a timeout of zero causes poll() to return immediately, even if no file descriptors are ready.

If your server is working fine otherwise, there's no reason not to put a fairly large number in as the timeout. I use 60s (so 60000ms) on my poll / epoll / etc based systems.

I also don't see the reason that you are copying the array each time. With a std::vector, you should be able to get rid of the fds_array entirely, and just do:

poll( &fds[0], (nfds_t) fds.size(), 60000 );

(Along with corresponding changes to the rest of the code.)

answered May 12 '13 at 10:00

AnthonyFoiani

504
5
7

Thanks for reply. About the `timeout` parameter: setting it for 60s will not freeze the server in `poll()` for 60s until the function returns? This will not prevent server from accepting new requests for 60s? About the `&fds[0]` instead of `copy()`: this only sets a pointer for first element. Isn't guaranteed that all `std::vector` elements are in consecutive memory, so using this isn't guaranteed to work all the time. At least, it's what ISO 14882 says. This was changed? – Tiago.SR May 12 '13 at 23:48
@Tiago.SR -- `timeout` is the *maximum* time the call will wait before returning *if* nothing else happens. If something happens before the time runs out, it will return immediately. And std::vector uses contiguous storage, so it *is* guaranteed to work (until the vector is modified, at least). See: http://stackoverflow.com/questions/1733143/converting-between-c-stdvector-and-c-array-without-copying – AnthonyFoiani May 13 '13 at 04:21
@Tiago.SR -- glad to hear that it's working. If the replies were helpful, please upvote or select as answer. Happy hacking! – AnthonyFoiani May 14 '13 at 00:44

No response from C++ server using poll()

2 Answers2