I have a simple client-server program implemented in C where a client can send integers to a server and the server replies with their sums. However, there is a troubling Segmentation fault (core dumped) shown on the server side whenever the client disconnects suddenly.
The Client:

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <arpa/inet.h>

#define PORT 5010

int main(int argc, char **argv) {
    char buf[BUFSIZ], buf2[BUFSIZ], message[BUFSIZ], serverReply[BUFSIZ];
    int SOCKET;
    struct sockaddr_in server;
    SOCKET = socket(AF_INET, SOCK_STREAM, 0);
    if (SOCKET < 0) {
        perror("Could not create socket");
        return -1;
    }
    printf("Socket created\n");
    server.sin_addr.s_addr = inet_addr("127.0.0.1");
    server.sin_family = AF_INET;
    server.sin_port = htons(PORT);
    if (connect(SOCKET, (struct sockaddr *) &server, sizeof(struct sockaddr_in)) < 0) {
        perror("Could not connect");
        return -1;
    }
    memset(&serverReply, 0, sizeof(serverReply));
    printf("Connected to server.\nEnter first number: ");
    scanf("%s", buf);
    fflush(stdin);
    printf("Enter second number: ");
    scanf("%s", buf2);
    strcat(buf, " ");
    strcat(buf, buf2);
    strcpy(message, buf);
    if (send(SOCKET, message, strlen(message), 0) < 0) {
        perror("Failed to send message");
        return -1;
    }
    if (recv(SOCKET, serverReply, sizeof(serverReply), 0) < 0) {
        perror("Could not receive message");
        return -1;
    }
    printf("Server: %s", serverReply);
    close(SOCKET);
}

The Server:

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

#define PORT 5010

int main(int argc, char *argv[]) {
    char msg[BUFSIZ], reply[BUFSIZ];

    struct sockaddr_in server, client;
    int SOCKET, ACCEPT, READ, sockSize, num1, num2, option = 1, maxClients = 30,
            h, clientSocket[maxClients], maxsd, sd, SELECT;
    fd_set readfds;
    for (h = 0; h < maxClients; h  ) {
        clientSocket[h] = 0;
    }
    SOCKET = socket(AF_INET, SOCK_STREAM, 0);
    if (SOCKET == -1) {
        perror("Could not create socket");
        return -1;
    }
    if (setsockopt(SOCKET, SOL_SOCKET, SO_REUSEADDR, &option, sizeof(option)) == -1) {
        perror("Could not set OPTNAME");
        return -1;
    }
    server.sin_family = AF_INET;
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons(PORT);
    printf("Created socket.\n");
    if (bind(SOCKET, (struct sockaddr *) &server, sizeof(server)) < 0) {
        perror("Could not bind");
        return -1;
    }
    if (listen(SOCKET, 1) < 0) {
        perror("Listen failed");
        return -1;
    }
    printf("Server is listening.\n");
    sockSize = sizeof(struct sockaddr_in);

    while (1) {
        FD_ZERO(&readfds);
        FD_SET(SOCKET, &readfds);
        maxsd = SOCKET;
        for (h = 0; h < maxClients; h  ) {
            sd = clientSocket[h];
            if (sd > 0) { FD_SET(sd, &readfds); }
            if (sd > maxsd) { maxsd = sd; }
        }
        SELECT = select(maxsd   1, &readfds, NULL, NULL, NULL);
        if ((SELECT < 0) && (errno != EINTR)) {
            perror("select error");
        }
        if (FD_ISSET(SOCKET, &readfds)) {

            ACCEPT = accept(SOCKET, (struct sockaddr *) &server, (socklen_t *) &sockSize);
            if (ACCEPT < 0) {
                perror("Could not accept client");
                return -1;
            }
            for (h = 0; h < maxClients; h  ) {
                if (clientSocket[h] == 0) {
                    clientSocket[h] = ACCEPT;
                    break;
                }
            }
            printf("Client has joined the server.\n");
        }
        for (h = 0; h < maxClients; h  ) {
            sd = clientSocket[h];
            if (FD_ISSET(sd, &readfds)) {
                READ = read(sd, msg, sizeof(msg));
                if (READ == -1) {
                    perror("Could not receive message");
                    return -1;
                }
                if (READ == 0) {
                    printf("Client disconnected\n");
                    fflush(stdout);
                    clientSocket[h]=0;
                }
                int e = 0;
                char *p = strtok(msg, " ");
                char *arr[2];
                while (p != NULL) {
                    arr[e  ] = p;
                    p = strtok(NULL, " ");
                }
                num1 = atoi(arr[0]);
                num2 = atoi(arr[1]);

                if ((strcmp(arr[0], "0") != 0 && num1 != 0) && (strcmp(arr[1], "0") != 0 && num2 != 0)) {
                    printf("Client: %d, %d\n", num1, num2);
                    sprintf(reply, "%d\n", num1   num2);
                    if (write(sd, reply, strlen(reply)) < 0) {
                        perror("Could not send message");
                        return -1;
                    }
                    memset(&reply, 0, sizeof(reply));

                } else {
                    printf("Conversion error");
                    strcpy(reply, "Conversion error.");
                    if (write(sd, reply, strlen(reply)) < 0) {
                        perror("Could not send message");
                        return -1;
                    }
                }
            }
        }
    }
}

How can the segfault be solved? How else can the codes be improved?

CodePudding user response：

The segfault comes from the atoi function since NULL is received. Checking for NULL seems to do the trick...