Unix domain socket non-blocking operation failed at listen, in C

Question

This program will calculate RTT for non blocking communication for simple IPC program using UNIX domain socket.

I'm having a problem with socket programming where at server-side my socket exit with error at listen(). The blocking version of the code works fine; I tried to modify it to non-blocking with SOCK_NONBLOCK at socket(), but it seems to fail at listening:

hafizul96@ubuntu:~/asyntest$ ./Server [SERVER PROCESS] Socket Created, Process ID : 7334 [SERVER PROCESS] Socket Listening.. [SERVER PROCESS] Error Accepting **

Client:

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/unistd.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <time.h>

#define SERV_SOCK_PATH "ipc_unix_sock.server"
#define CLIN_SOCK_PATH "ipc_unix_sock.client"

void Array_Init(int *, int);
void Write_to_File(FILE *, int *, int);

int main(int argc, char *argv[])
{
    struct sockaddr_un serv_addr, cli_addr;
    int client_socket, add_len, rc, array_len, send_bytes, recv_bytes;
    pid_t process_id = getpid();
    clock_t start, end;
    double elapsed = 0.0;
    FILE *cli_arr_recv;
    FILE *cli_arr_send;
    char arr_len[10];
    int *Array_Buff;
    array_len = atoi(argv[1]);
    sprintf(arr_len, "%d", array_len);


    Array_Buff = malloc(sizeof(int)*array_len);
    cli_arr_send = fopen("cli_arr_send.txt","w+");
    cli_arr_recv = fopen("cli_arr_recv.txt","w+");

    memset(&serv_addr, 0, sizeof(struct sockaddr_un));
    memset(&cli_addr, 0, sizeof(struct sockaddr_un));
    memset(Array_Buff, 0, array_len);
    Array_Init(Array_Buff, array_len);
    //Copy arrays to file
    Write_to_File(cli_arr_send, Array_Buff, array_len);

//NON-BLOCKING SOCKET//
    if((client_socket = socket(AF_UNIX, SOCK_STREAM | SOCK_NONBLOCK, 0))<0)
    {
        printf("[CLIENT PROCESS] Socket Creation Error\n");
        fclose(cli_arr_recv);
        fclose(cli_arr_send);
        close(client_socket);
        exit(EXIT_FAILURE);
    }
    else
    {
        printf("[CLIENT PROCESS] Socket Created, Process ID :%d\n", process_id);
        printf("[CLIENT PROCESS] %lubytes Allocated\n", sizeof(int)*array_len);
    }

    cli_addr.sun_family = AF_UNIX;
    strcpy(cli_addr.sun_path, CLIN_SOCK_PATH);
    add_len = sizeof(cli_addr);

    unlink(CLIN_SOCK_PATH);
    if((rc = bind(client_socket, (struct sockaddr*)&cli_addr, add_len ))<0)
    {
        printf("[CLIENT PROCESS] Bind error \n");
        fclose(cli_arr_recv);
        fclose(cli_arr_send);
        close(client_socket);
        exit(EXIT_FAILURE);
    }

    serv_addr.sun_family = AF_UNIX;
    strcpy(serv_addr.sun_path, SERV_SOCK_PATH);
    if((rc = connect(client_socket, (struct sockaddr*)&serv_addr, add_len))<0)
    {
        printf("[CLIENT PROCESS] Failed to Connect to Server Process\n");
        fclose(cli_arr_recv);
        fclose(cli_arr_send);
        close(client_socket);
        exit(EXIT_FAILURE);
    }


    start = clock();
    if((rc = send(client_socket, arr_len, sizeof(arr_len), 0))<0)
    {
        printf("[CLIENT PROCESS] Failed to Send Array Len to Server Process");
        fclose(cli_arr_recv);
        fclose(cli_arr_send);
        close(client_socket);
        exit(EXIT_FAILURE);
    }

    if((send_bytes = send(client_socket, Array_Buff, sizeof(int)*array_len, 0))<0)
    {
        printf("[CLIENT PROCESS] Failed to Send Array Elements to Server Process");
        fclose(cli_arr_recv);
        fclose(cli_arr_send);
        close(client_socket);
        exit(EXIT_FAILURE);
    }
    else
    {
        printf("[CLIENT PROCESS] Total Bytes Sent = %dbytes\n",send_bytes);
        memset(Array_Buff, 0, array_len); //reset buffer
        if((recv_bytes = recv(client_socket, Array_Buff, sizeof(int)*array_len, MSG_WAITALL))<0)
        {
            fclose(cli_arr_recv);
            fclose(cli_arr_send);
            close(client_socket);
            exit(EXIT_FAILURE);
        }
        else
        {
            //Finishes RTT
            end = clock();
            printf("[CLIENT PROCESS] Total Bytes Received Back = %dbytes\n",recv_bytes);
            Write_to_File(cli_arr_recv, Array_Buff, array_len);
        }

    }
    elapsed = (end - start)/(double)CLOCKS_PER_SEC;
    printf("[CLIENT PROCESS] RTT(Round Trip Time) is =  %fs\n", elapsed);
    printf("[CLIENT PROCESS] Program Completed.\n");

    free(Array_Buff);
    fclose(cli_arr_recv);
    fclose(cli_arr_send);
    close(client_socket);
    exit(EXIT_SUCCESS);
}
void Array_Init(int *Array_Buff, int array_len)
{
    unsigned int seed = 184611;
    srand(seed);
    for(int i = 0; i< array_len; i++)
    {
        *(Array_Buff++) = rand() % 999999;
    }
}
void Write_to_File(FILE *outfile, int *Array_Buff, int array_len)
{
    for(int i = 0; i<array_len; i++)
    {
        fprintf(outfile, " %d ", *(Array_Buff++));
    }
    printf("[CLIENT PROCESS] File Write Completed.. \n");
}

Server:

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>

#define SERV_SOCK_PATH "ipc_unix_sock.server"

void Write_to_File(FILE *, int *, int);

int main(int argc, char *argv[])
{
    struct sockaddr_un serv_addr, cli_addr;
    int server_sock, client_sock, add_len, rc, array_len;
    int backlog = 2, recv_bytes = 0, send_bytes = 0;
    pid_t process_id = getpid();
    FILE *serv_file;
    int *Array_Buff;
    char arr_len[10];

    serv_file = fopen("serv_recv_arr.txt","w+");

    /*Making sure that address struct set to zero*/
    memset(&serv_addr, 0, sizeof(struct sockaddr_un));
    memset(&cli_addr, 0, sizeof(struct sockaddr_un));
    memset(arr_len, 0, 10);
//NON-BLOCKING SOCKET TYPE
    if((server_sock = socket(AF_UNIX, SOCK_STREAM | SOCK_NONBLOCK, 0)) < 0 )
    {
        printf("[SERVER PROCESS] Error Socket\n");
        exit(EXIT_FAILURE);
    }
    else
    {
        printf("[SERVER PROCESS] Socket Created, Process ID : %d\n", process_id);
    }

    serv_addr.sun_family = AF_UNIX;
    strcpy(serv_addr.sun_path, SERV_SOCK_PATH);
    add_len = sizeof(serv_addr);

    unlink(SERV_SOCK_PATH);
    if((rc = bind(server_sock, (struct sockaddr*)&serv_addr, add_len))<0)
    {
    printf("[SERVER PROCESS] Error Binding\n");
    close(server_sock);
    exit(EXIT_FAILURE);
    }

    if((rc = listen(server_sock, backlog)) <0 )
    {
        printf("[SERVER PROCESS] Error Listening\n");
        close(server_sock);
        fclose(serv_file);
        exit(EXIT_FAILURE);
    }

    printf("[SERVER PROCESS] Socket Listening..\n");

    if((client_sock = accept(server_sock, (struct sockaddr*)&client_sock, &add_len))<0)
    {
        printf("[SERVER PROCESS] Error Accepting\n");
        close(server_sock);
        close(client_sock);
        fclose(serv_file);
        exit(EXIT_FAILURE);
    }

    add_len = sizeof(cli_addr);
    if((rc = getpeername(client_sock, (struct sockaddr*)&cli_addr, &add_len))<0)
    {
        printf("[SERVER PROCESS] Error Getting Peername\n");
        close(server_sock);
        close(client_sock);
        fclose(serv_file);
        exit(EXIT_FAILURE);
    }
    else
    {
        printf("[SERVER PROCESS] Client Socket Filepath = %s\n", cli_addr.sun_path);
    }

    printf("[SERVER PROCESS] Waiting to Recv Msg..\n");


    if((rc =  recv(client_sock, arr_len, sizeof(arr_len), MSG_WAITALL))<0)
    {
        printf("[SERVER PROCESS] Error Receiving Msg..\n");
        close(server_sock);
        close(client_sock);
        fclose(serv_file);
        exit(EXIT_FAILURE);
    }
    printf("[SERVER PROCESS] Array Len Received %s\n", arr_len);
    array_len = atoi(arr_len);
    Array_Buff = malloc(sizeof(int)*array_len);
    printf("[SERVER PROCESS] %lubytes Allocated\n", sizeof(int)*array_len);
    memset(Array_Buff, 0, array_len);

    if((recv_bytes = recv(client_sock, Array_Buff, sizeof(int)*array_len, MSG_WAITALL))<0)
    {
        printf("[SERVER PROCESS] Error Receiving Msg..\n");
        close(server_sock);
        close(client_sock);
        fclose(serv_file);
        exit(EXIT_FAILURE);
    }
    else
    {
        printf("[SERVER PROCESS] Total Bytes Recv = %dbytes\n",recv_bytes);
        if((send_bytes = send(client_sock, Array_Buff, sizeof(int)*array_len, 0))<0)
        {
            printf("[SERVER PROCESS] Error Sending Back Array\n");
            close(server_sock);
            close(client_sock);
            fclose(serv_file);
            exit(EXIT_FAILURE);
        }
        else
        {
             printf("[SERVER PROCESS] Total Bytes Sent Back = %dbytes\n", send_bytes);
        }

    }
    Write_to_File(serv_file, Array_Buff, array_len);
    printf("[SERVER PROCESS] Program Completed.\n");

    free(Array_Buff);
    close(server_sock);
    close(client_sock);
    exit(EXIT_SUCCESS);
}

void Write_to_File(FILE *outfile, int *Array_Buff, int array_len)
{
    for(int i = 0; i<array_len; i++)
    {
        fprintf(outfile, " %d ", *(Array_Buff++));
    }
    printf("[SERVER PROCESS] File Write Completed.. \n");
    fclose(outfile);
}

Does anybody know what the problem is and how to fix it?

Answer 1

Because your socket is non-blocking, accept does not wait until a connection happens but it returns EAGAIN or EWOULDBLOCK value. You shall call accept repeatedly until it returns a different value. To avoid consuming 100% of your CPU by those calls, do some sleep between two calls or use select or poll to wait until your socket is readable.