bad file descriptor: error on accept() (socket)-CodePudding

I am trying to write a simple chat application using TCP protocol in Linux, in which server receives strings from the client and it sends strings size to the client. When I run client, the server throws a "Bad file descriptor" error and exits. Nevertheless if I send input strings by client it receives correct sizes of strings, but as I said before server goes down. Here is the code :

SERVER

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>
#define MAX_LINE 1024

int main(int argc, char const *argv[]) {
    int listen_fd, connect_fd, logging, str_lenght;
    struct sockaddr_in serv_addr, client;
    char buffer_str[MAX_LINE], buffer[MAX_LINE];
    pid_t pid;
    socklen_t len;
    logging = 1;
    if ((listen_fd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        perror("socket");
        exit(-1);
    }
    bzero(&serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    serv_addr.sin_port = htons(1024);
    if (bind(listen_fd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {
        perror("bind");
        exit(-1);
    }
    if (listen(listen_fd, 1024) < 0) {
        perror("listen");
        exit(-1);
    }
    for (;;) {
        len = sizeof(client);
        if ((connect_fd = accept(listen_fd, (struct sockaddr *)&client, &len)) < 0) {
            perror("accept");
            exit(-1);
        }
        if ((pid = fork()) < 0) {
            perror("fork");
            exit(-1);
        }
        if (pid == 0) {
            close(listen_fd);
            for (;;) {
                bzero(buffer_str, MAX_LINE);
                if (read(connect_fd, buffer_str, sizeof(buffer_str)) < 0) {
                    perror("read");
                    exit(-1);
                }
                if (strcmp(buffer_str, "exit") == 0) break;
                str_lenght = (int)strlen(buffer_str) - 1;
                bzero(buffer_str, MAX_LINE);
                snprintf(buffer_str, sizeof(buffer_str), "%d\n", str_lenght);
                if (write(connect_fd, buffer_str, sizeof(buffer_str)) < 0) {
                    perror("write");
                    exit(-1);
                }
                if (logging) {
                    inet_ntop(AF_INET, &client.sin_addr, buffer, sizeof(buffer));
                    printf("Request from host %s, port %d\n", buffer, ntohs(client.sin_port));
                }
            }
            close(connect_fd);
            exit(0);
        } else close(listen_fd);
    }
    exit(0);
}

CLIENT

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>
#define MAX_LINE 1024

void clientEcho(int socket_fd) {
    char recv_buffer[MAX_LINE], send_buffer[MAX_LINE];
    for (;;) {
        bzero(recv_buffer, MAX_LINE);
        bzero(send_buffer, MAX_LINE);
        printf("insert string: ");
        if (fgets(send_buffer, sizeof(send_buffer), stdin) == NULL) {
            perror("fgets");
            return;
        }
        if (write(socket_fd, send_buffer, sizeof(send_buffer)) < 0) {
            perror("write");
            return;
        }
        if (strcmp(send_buffer, "exit") == 0) break;
        if (read(socket_fd, recv_buffer, sizeof(recv_buffer)) < 0) {
            perror("read");
            return;
        }
        printf("string size: ");
        if (fputs(recv_buffer, stdout) < 0) {
            perror("fputs");
            return;
        }
    }
}

int main(int argc, char const *argv[]) {
    int socket_fd;
    struct sockaddr_in serv_addr;
    if (argc != 2) {
        perror("usage: <IPaddress>");
        exit(-1);
    }
    if ((socket_fd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        perror("socket");
        exit(-1);
    }
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(1024);
    if (inet_pton(AF_INET, argv[1], &serv_addr.sin_addr) <= 0) {
        perror("inet_pton");
        exit(-1);
    }
    if (connect(socket_fd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {
        perror("connect");
        exit(-1);
    }
    clientEcho(socket_fd);
    close(socket_fd);
    exit(0);
}

CodePudding user response：

In addition to what the other answer says: you're never sending the "exit" string, but "exit\n" (i.e. followed by a newfile -- and 1019 zero bytes), and the strcmp(buffer_str, "exit") == 0 in the server will never be true.

BTW, this piece is code is broken:

                bzero(buffer_str, MAX_LINE);
                if (read(connect_fd, buffer_str, sizeof(buffer_str)) < 0) {
                    perror("read");
                    exit(-1);
                }
                if (strcmp(buffer_str, "exit") == 0) break;
                str_lenght = (int)strlen(buffer_str) - 1;

If the read returns exactly sizeof(buffer_str) bytes, the buffer may not be zero-terminated, and you could not use strcmp on it. And I don't see the point of setting str_lenght (sic) to one less than the actual length of the string.

CodePudding user response：

As per the comment. Your server code is essentially the following...

if (listen(listen_fd, 1024) < 0) {
    perror("listen");
    exit(-1);
}
for (;;) {
    len = sizeof(client);
    if ((connect_fd = accept(listen_fd, (struct sockaddr *)&client, &len)) < 0) {
        perror("accept");
        exit(-1);
    }
    if ((pid = fork()) < 0) {
        perror("fork");
        exit(-1);
    }
    if (pid == 0) {
        ...
        exit(0);
    } else {
        close(listen_fd); /* @Here */
    }
}
exit(0);

Ate the line marked @Here you close the listening file descriptor listen_fd meaning the next...

connect_fd = accept(listen_fd, (struct sockaddr *)&client, &len)

call in the for (;;) loop will fail due to listen_fd being invalid.

Your subsequent comments suggest that might not be the only issue but I'm pretty sure it's the one causing the bad file descriptor: error on accept() error message.

CodePudding user response：

Thank you to every one. This code run:

SERVER

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>
#define MAX_LINE 1024

int main(int argc, char const *argv[]) {
    int listen_fd, connect_fd, logging, str_lenght;
    struct sockaddr_in serv_addr, client;
    char buffer_str[MAX_LINE], buffer[MAX_LINE];
    pid_t pid;
    socklen_t len;
    logging = 1;
    if ((listen_fd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        perror("socket");
        exit(-1);
    }
    bzero(&serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    serv_addr.sin_port = htons(1024);
    if (bind(listen_fd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {
        perror("bind");
        exit(-1);
    }
    if (listen(listen_fd, 1024) < 0) {
        perror("listen");
        exit(-1);
    }
    for (;;) {
        len = sizeof(client);
        if ((connect_fd = accept(listen_fd, (struct sockaddr *)&client, &len)) < 0) {
            perror("accept");
            exit(-1);
        }
        if ((pid = fork()) < 0) {
            perror("fork");
            exit(-1);
        }
        if (pid == 0) {
            for (;;) {
                bzero(buffer_str, MAX_LINE);
                if (read(connect_fd, buffer_str, sizeof(buffer_str)) < 0) {
                    perror("read");
                    exit(-1);
                }
                if (strcmp(buffer_str, "exit\n") == 0) break;
                str_lenght = (int)strlen(buffer_str) - 1;
                bzero(buffer_str, MAX_LINE);
                snprintf(buffer_str, sizeof(buffer_str), "%d\n", str_lenght);
                if (write(connect_fd, buffer_str, sizeof(buffer_str)) < 0) {
                    perror("write");
                    exit(-1);
                }
                if (logging) {
                    inet_ntop(AF_INET, &client.sin_addr, buffer, sizeof(buffer));
                    printf("Request from host %s, port %d\n", buffer, ntohs(client.sin_port));
                }
            }
            close(connect_fd);
            exit(0);
        } else close(connect_fd);
    }
    exit(0);
}

CLIENT

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>
#define MAX_LINE 1024

void clientEcho(int socket_fd) {
    char recv_buffer[MAX_LINE], send_buffer[MAX_LINE];
    for (;;) {
        bzero(recv_buffer, MAX_LINE);
        bzero(send_buffer, MAX_LINE);
        printf("insert string: ");
        if (fgets(send_buffer, sizeof(send_buffer), stdin) == NULL) {
            perror("fgets");
            return;
        }
        if (write(socket_fd, send_buffer, sizeof(send_buffer)) < 0) {
            perror("write");
            return;
        }
        if (strcmp(send_buffer, "exit\n") == 0) break;
        if (read(socket_fd, recv_buffer, sizeof(recv_buffer)) < 0) {
            perror("read");
            return;
        }
        printf("string size: ");
        if (fputs(recv_buffer, stdout) < 0) {
            perror("fputs");
            return;
        }
    }
}

int main(int argc, char const *argv[]) {
    int socket_fd;
    struct sockaddr_in serv_addr;
    if (argc != 2) {
        perror("usage: <IPaddress>");
        exit(-1);
    }
    if ((socket_fd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        perror("socket");
        exit(-1);
    }
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(1024);
    if (inet_pton(AF_INET, argv[1], &serv_addr.sin_addr) <= 0) {
        perror("inet_pton");
        exit(-1);
    }
    if (connect(socket_fd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {
        perror("connect");
        exit(-1);
    }
    clientEcho(socket_fd);
    close(socket_fd);
    exit(0);
}