Epoll_wait() Does Not Recognize Client

Question

I tried to make a simple server-client protocol where the server sends a set of fragmented files to clients, each client gets a fragmented file. I ran into an issue that my epoll_wait() blocks after it accepts a new client. It seems that epoll_wait() did not update the number of active events to be 2 (the server and the first client is active). When I printed the client ip address and port number, it did not match the server port number. Is this something that I should expect that the connection still can be established, even though the client is on a different port. How can I resolve this issue that it did not pick up the client event? I appreciate all help that I can get! Thank you!

Here is my server implementation:

#include <sys/epoll.h>
#include <stdio.h>
#include <stdlib.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <errno.h>

#define MAX_EVENTS 1024
#define LISTEN_BACKLOG 5
#define MAX_BUF_SIZE 256

typedef struct FRAG_FILE
{
    FILE *fp;
    int client_fd;
    int number_bytes;
    int number_bytes_sorted;
    char *line;
    int length;
    int capacity;
    int sorted;
    int assigned; // use it to keep track of the file has been assigned to a client
    int sent;     // use it to keep track of the file has been sent to a client
} frag_file;

FILE *fp_output = NULL;

int initialize_server(int server_port){
    struct sockaddr_in server_addr;
    int server_sock = socket(AF_INET, SOCK_STREAM, 0);
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(server_port);
    server_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    if(bind(server_sock, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0){
        printf("bind error: %s(%d)\n", strerror(errno), errno);
        exit(1);
    }
    if(listen(server_sock, LISTEN_BACKLOG) < 0){
        printf("listen error: %s(%d)\n", strerror(errno), errno);
        exit(1);
    }
    return server_sock;
}

void add_epoll_event(struct epoll_event* event, int fd, int epoll_fd){
    event->events = EPOLLIN | EPOLLET;
    event->data.fd = fd;
    printf("add fd %d to epoll\n", fd);
    if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, fd, event) < 0){
        printf("epoll_ctl error in add_poll_event: %s(%d)\n", strerror(errno), errno);
        exit(1);
    }
}

int handle_level_trigger(int epoll_fd, struct epoll_event event, int input_fragmented_files, struct FRAG_FILE *frag_file_array){
    int fd = event.data.fd;
    if (event.events & EPOLLIN){
        int i;
        int num_file_sorted = 0;
        for (i = 0; i < input_fragmented_files; i++)
        {
            // find which file is sorted and count them
            char buf[MAX_BUF_SIZE];
            if (frag_file_array[i].sorted)
            {
                num_file_sorted++;
                continue;
            }
            if (frag_file_array[i].fp != NULL && frag_file_array[i].sent == 0)
            {
                // read in every line of the fragment file
                while (fgets(buf, MAX_BUF_SIZE, frag_file_array[i].fp))
                {
                    if (strlen(buf) > 0)
                    {
                        if (buf[strlen(buf) - 1] != '\n')
                        {
                            buf[strlen(buf) - 1] = '\n';
                        }
                        frag_file_array[i].number_bytes += strlen(buf);
                        int ret_write = write(fd, buf, strlen(buf));
                        if (ret_write == -1)
                        {
                            printf("write error %s\n", strerror(errno));
                            exit(1);
                        }
                    }
                }
                // when it is done sending the data, send a closing delimiter
                if (sprintf(buf, "%s", "-1\n") < 0)
                {
                    printf("sprintf error %s\n", strerror(errno));
                    exit(1);
                }
                if (write(frag_file_array[i].client_fd, buf, strlen(buf)) == -1)
                {
                    printf("write error %s\n", strerror(errno));
                    exit(1);
                }
                frag_file_array[i].sent = 1;
            }
        }
        printf("%d sorted files out of %d\n", num_file_sorted, input_fragmented_files);
        if (num_file_sorted == input_fragmented_files)
        {
            // write to the output file and put into the tree???? I am not sure if this is the right way to do it
            fclose(fp_output);
            fp_output = NULL;
            printf("All files have been merged\n");
            // flag = 1; // all files have been merged
            // break;
        }
    }
    if (epoll_ctl(epoll_fd, EPOLL_CTL_DEL, fd, NULL) < 0){
        printf("epoll_ctl error in edge trigger: %s(%d)\n", strerror(errno), errno);
        exit(1);
    }
    close(fd);
    return 0;
}

void handle_client(int server_sock, struct epoll_event *event, int epoll_fd){
    struct sockaddr_in client_addr;
    socklen_t client_addr_len = sizeof(client_addr);
    int client_sock = accept(server_sock, (struct sockaddr *)&client_addr, &client_addr_len);
    printf("accept a new client: %s:%d\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));
    printf("client sock: %d\n", client_sock);
    if(client_sock < 0){
        printf("accept error: %s(%d)\n", strerror(errno), errno);
        exit(1);
    }
    add_epoll_event(event, client_sock, epoll_fd); // fd = 9
}

void clean_up(frag_file *file, int num_fragmented_files)
{
    int i;
    for (i = 0; i < num_fragmented_files; i++)
    {
        if (file[i].line != NULL)
        {
            free(file[i].line);
        }
        if (file[i].fp != NULL)
        {
            fclose(file[i].fp);
        }
        if (file[i].client_fd >= 0)
        {
            close(file[i].client_fd);
        }
    }
}

int read_server_file(char *filename)
{
    // read in the file from the argv[1]. Then get the number of fragmented files.
    FILE *fp = fopen(filename, "r");
    if (fp == NULL)
    {
        printf("Error: Cannot open file %s because %s\n", filename, strerror(errno));
        exit(EXIT_FAILURE);
    }
    int line_num = 0, input_fragmented_files = 0;
    while (!feof(fp))
    {
        char buffer[1024];
        if (fscanf(fp, "%s\n", buffer) < 1)
        {
            printf("Fail to read file %s because %s\n", filename, strerror(errno));
            exit(EXIT_FAILURE);
        }
        // first line is the name of the output file
        if (line_num == 0)
        {
            if ((fp_output = fopen(buffer, "w")) == NULL)
            {
                printf("Fail to open file %s because %s\n", buffer, strerror(errno));
                exit(EXIT_FAILURE);
            }
        }
        // second line is the names of files that contain numbered lines for fragments of the main file
        else
        {
            FILE *fp_fragment = NULL;
            if ((fp_fragment = fopen(buffer, "r")) == NULL)
            {
                printf("Fail to open file %s because %s\n", buffer, strerror(errno));
                exit(EXIT_FAILURE);
            }
            input_fragmented_files++;
        }
        line_num++;
    }
    fclose(fp);
    if (input_fragmented_files <= 0)
    {
        printf("Error: Failed to read the server file to get the fragmented files\n");
        exit(1);
    }
    return input_fragmented_files;
}

void initialize_frag_file(frag_file *frag_file_array, int num_fragmented_files)
{
    int i;
    for (i = 0; i < num_fragmented_files; i++)
    {
        frag_file_array[i].fp = NULL;
        frag_file_array[i].client_fd = -1; // -1 means invalid
        frag_file_array[i].number_bytes = 0;
        frag_file_array[i].line = NULL;
        frag_file_array[i].length = 0;
        frag_file_array[i].capacity = 0;
        frag_file_array[i].sorted = 0;
        frag_file_array[i].assigned = 0;
        frag_file_array[i].sent = 0;
    }
}

void open_each_file(frag_file *frag_file_array, int num_fragmented_files, char *filename)
{
    // could do better here by combining the fuunction read_server_file and open_each_file into one function
    FILE *fp = fopen(filename, "r");
    int line_num = 0;
    while (!feof(fp))
    {
        char buffer[1024];
        if (fscanf(fp, "%s\n", buffer) < 1)
        {
            printf("Fail to read file %s because %s\n", filename, strerror(errno));
            exit(EXIT_FAILURE);
        }
        // first line is the name of the output file
        if (line_num == 0) continue;
        // second line is the names of files that contain numbered lines for fragments of the main file
        else
        {
            FILE *fp_fragment = NULL;
            if ((fp_fragment = fopen(buffer, "r")) == NULL)
            {
                printf("Fail to open file %s because %s\n", buffer, strerror(errno));
                exit(EXIT_FAILURE);
            }
            frag_file_array[line_num - 1].fp = fp_fragment;
        }
        line_num++;
    }
    fclose(fp);
}

int main(int argc, char *argv[]){
    struct epoll_event events[MAX_EVENTS];
    struct epoll_event event;

    // checking the arguments
    if (argc != 3)
    {
        printf("Incorrect number of arguments.\nUsage: lab3_server <filename> <port>\n");
        exit(1);
    }
    int server_port = atoi(argv[2]);
    char *filename = argv[1];
    
    // getting the file from the server
    int input_fragmented_files = read_server_file(filename);                                      // step 2
    frag_file *frag_file_array = (frag_file *)malloc(input_fragmented_files * sizeof(frag_file)); // step 3
    if (frag_file_array == NULL)
    {
        printf("Error: Failed to allocate memory for frag_file_array\n");
        exit(1);
    }
    initialize_frag_file(frag_file_array, input_fragmented_files); // step 3
    open_each_file(frag_file_array, input_fragmented_files, filename);

    int epoll_fd = epoll_create1(0);
    if(epoll_fd == -1){
        printf("epoll_create1 error: %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    }
    int server_sock = initialize_server(server_port);
    add_epoll_event(&event, server_sock, epoll_fd); // fd = 8
    int number_fds = 0;
    while(1){
        number_fds = epoll_wait(epoll_fd, events, MAX_EVENTS, -1);
        printf("number_fds: %d\n", number_fds);
        if(number_fds == -1){
            printf("epoll_wait error: %s\n", strerror(errno));
            exit(EXIT_FAILURE);
        }
        int i;
        for(i = 0; i < number_fds; i++){
            if(events[i].data.fd == server_sock){
                printf("Server port %d is ready to accept\n", server_port);
                handle_client(server_sock, &event, epoll_fd);
            }
            else{
                handle_level_trigger(epoll_fd, events[i], input_fragmented_files, frag_file_array);
            }
        }
    }
    close(server_sock);
    close(epoll_fd);
    void clean_up(frag_file * file, int input_fragmented_files);
    return 0;
}

Here is my client implementation:

#define GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <sys/epoll.h>

#define MAX_EVENTS 1
#define MAX_BUF_SIZE 1024

// create a client side
int create_client(char *ip, int port)
{
    int cfd;
    char hostname[100];
    char ip_addr[100];
    char port_num[100];
    struct sockaddr_in my_client_addr;
    cfd = socket(AF_INET, SOCK_STREAM, 0);
    if (cfd == -1)
    {
        perror("socket");
        exit(1);
    }
    memset(&my_client_addr, 0, sizeof(my_client_addr));
    my_client_addr.sin_family = AF_INET;
    my_client_addr.sin_port = htons(port);
    my_client_addr.sin_addr.s_addr = inet_addr(ip);
    gethostname(hostname, 100);
    printf("hostname: %s\n", hostname);
    int con_ret = connect(cfd, (const struct sockaddr *)&my_client_addr, sizeof(my_client_addr));
    printf("connect ret: %d\n", con_ret);
    // get name of the server
    getnameinfo((const struct sockaddr *)&my_client_addr, sizeof(my_client_addr), hostname, 100, NULL, 0, NI_NUMERICHOST | NI_NUMERICSERV);
    printf("server name: %s\n", hostname);
    // get ip address
    getnameinfo((const struct sockaddr *)&my_client_addr, sizeof(my_client_addr), ip_addr, 100, NULL, 0, NI_NUMERICHOST);
    printf("server ip address: %s\n", ip_addr);
    // get port number
    getnameinfo((const struct sockaddr *)&my_client_addr, sizeof(my_client_addr), NULL, 0, port_num, 100, NI_NUMERICSERV);
    printf("server port: %s\n", port_num);
    if (con_ret < 0)
    {
        printf("Error: Failed to connect to server: %s\n", strerror(errno));
        exit(1);
    }
    printf("Success: Connected to server\n");
    return cfd;
}

int main(int argc, char *argv[])
{
    char *line = NULL;
    if (argc != 3)
    {
        printf("Usage: %s <internet address> <port>\n", argv[0]);
        exit(1);
    }

    // read in the ip and the port number
    char *ip = argv[1];
    int port = atoi(argv[2]);
    if (port < 0 || port > 65535)
    {
        printf("Invalid port number\n");
        exit(1);
    }
    int cfd = create_client(ip, port);
    int epfd = epoll_create1(0);
    if (epfd < 0)
    {
        printf("Error: Failed to create epoll file descriptor: %s\n", strerror(errno));
        exit(1);
    }
    struct epoll_event ev, events[MAX_EVENTS];
    ev.events = EPOLLIN | EPOLLRDHUP;
    ev.data.fd = cfd;
    if (epoll_ctl(epfd, EPOLL_CTL_ADD, cfd, &ev) < 0)
    {
        printf("Error: Failed to add file descriptor to epoll: %s\n", strerror(errno));
        exit(1);
    }
    char buf[MAX_BUF_SIZE];
    int nfds;
    int length = 0;
    int capacity = 0;
    // connect to the server
    while (1)
    {
        nfds = epoll_wait(epfd, events, MAX_EVENTS, -1);
        printf("nfds: %d\n", nfds);
        if (nfds < 0)
        {
            printf("Error: Failed to wait on epoll: %s\n", strerror(errno));
            exit(1);
        }
        else if (nfds > 0)
        {
            int i;
            for (i = 0; i < nfds ; i++){
                printf("EPOLLIN: %s\n", events[i].events & EPOLLIN ? "true" : "false");
                if (events[i].events & EPOLLRDHUP)
                { // since there is only one connection, index it into the 0 index
                    int ret_ctl = epoll_ctl(epfd, EPOLL_CTL_DEL, cfd, NULL);
                    if (ret_ctl < 0)
                    {
                        printf("Error: Failed to delete file descriptor from epoll: %s\n", strerror(errno));
                        exit(1);
                    }
                }
                else if (events[i].events & EPOLLIN)
                {
                    int avail_bytes = capacity - length;
                    ssize_t num_bytes = read(cfd, buf, strlen(buf));
                    printf("read %ld bytes\n", num_bytes);
                    printf("Read in: %s\n", buf);
                    if (num_bytes < 0)
                    {
                        printf("Error: Failed to read from socket: %s\n", strerror(errno));
                    }
                    int i;
                    for (i = 0; i < num_bytes; i++)
                    {
                        int line_num;
                        char c = buf[i];
                        // create a dynamic array to store the lines
                        if (line == NULL)
                        {
                            line = (char *)malloc(MAX_BUF_SIZE);
                            capacity = MAX_BUF_SIZE;
                            length = 0;
                            avail_bytes = MAX_BUF_SIZE;
                        }
                        // save character into the array
                        // check for the reallocation of the dynamic memory
                        if (avail_bytes == 0)
                        {
                            line = (char *)realloc(line, capacity * 2);
                            capacity *= 2;
                            avail_bytes += MAX_BUF_SIZE; // reset the bytes count
                        }
                        line[length++] = c;
                        avail_bytes--;
                        if (c == '\n')
                        {
                            // end of a line
                            line[length++] = '\0';
                            avail_bytes--;
                            sscanf(line, "%d", &line_num); // the next line is the line number
                            if (line_num == -1)
                            {
                                // when there is no new line, get it from the data structuresend it back to the server
                                // right now I just have nothing here so it does not break
                                printf("%s\n", line);
                            }
                            else
                            {
                                // when there is a line, add it to the data structures
                                // right now I just have nothing here so it does not break
                                printf("%s\n", line);
                            }
                            avail_bytes += length;
                            length = 0; // reset the line length
                        }
                    }
                }
            }
        }
    }
    close(cfd);
    close(epfd);
    return 0;
}

Here is the output of the server when I ran it. The epoll_wait() call blocked the execution of the server and did not recognize the client that has been connected.

add fd 8 to epoll
number_fds: 1
Server port 36085 is ready to accept
accept a new client: 128.252.167.161:56486
client sock: 9
add fd 9 to epoll

Here is the output of the client side:

hostname: DESKTOP-34H42IU
connect ret: 0
server name: 127.0.0.1
server ip address: 127.0.0.1
server port: 36085
Success: Connected to server

Answer 1

• After you connect to the server on the client side, you should send a request to the server, the server can receive the EPOLLIN event, or

• add an EPOLLOUT event after accept on the server side, and send the content to the client when it can be written.