C Socket programming

Question

I am new to c programming. I tried a basic socket programming. It is working good but my problem is that I created a linked list to store the sent and received messages. When I print the linked list only sent messages(ptr3->request) is displayed and my received message (ptr3->response) is empty.

I think the passing of the arguments is where I am making mistake. But, in both case I am passing the address only. I cannot really figure out the mistake.

Code snippet:

#include<stdio.h>
#include<string.h>   
#include<sys/socket.h>
#include<arpa/inet.h> 
#include<time.h>
#include<errno.h>
#include<stdlib.h>

void create(char*,char*);
void insert(char*,char*);
void send_receive(char*,int);
void display();

struct packet_details
{
char *request;
char *response;
struct packet_details *next;
};

typedef struct packet_details details;
details *head = NULL ,*temp = NULL;

void create(char *send,char *receive)
{
    details *new_node;
    new_node= (details*)malloc(sizeof( details));

    if(NULL == new_node)
    {
        printf("Cannot allocate requested memory %d %s %s",__LINE__,__FILE__,__FUNCTION__);
    }

    else
    {
        new_node->request= send;
        new_node->response = receive;
        new_node->next = NULL;

        head = temp = new_node;
    }
}

void insert(char *send,char *receive)
{
    details *new_node;
    if(NULL == head)
    {
        create(send,receive);
        return;
    }
    new_node= (details*)malloc(sizeof( details));

        new_node->request = send;
        new_node->response = receive;
        new_node->next = NULL;

       temp->next = new_node;
       temp = new_node;
}

void display()
{
    details *ptr3;
    ptr3 = head;
    while(NULL != ptr3)
    {
        printf("\n**********************************\n");
        printf("\n**********************************\n");

        printf("\n%s",ptr3->request);
        printf("\n%s",ptr3->response);

        printf("\n***********************************\n");
        printf("\n***********************************\n");
        ptr3=ptr3->next;
    }
}

int main()
{
    int socket_desc,a=0;
    struct sockaddr_in server;
    char *message;

    //Create socket
    socket_desc = socket(AF_INET , SOCK_STREAM , 0);
    if (socket_desc == -1)
    {
        printf("Could not create socket! %s\n",strerror(errno));
    }

    server.sin_addr.s_addr = inet_addr("127.0.0.1");
    server.sin_family = AF_INET;
    server.sin_port = htons(7887);

    //Connect to server
    if (connect(socket_desc , (struct sockaddr *)&server , sizeof(server)) < 0)
    {
        printf("connect error ! %s\n",strerror(errno));
        return 1;
    }

    puts("Connected\n");

    while(a < 5 )
    {
    printf("\nCLIENT:");
    message =  "Hello Server\n";
    printf("%s",message);
    send_receive(message,socket_desc);
    a++;

    sleep(10);

    printf("\nCLIENT:");
    message =  "How are you?\n";
    printf("%s",message);
    send_receive(message,socket_desc);
    a++;

    sleep(10);

    printf("\nCLIENT:");
    message =  "Are you busy?\n";
    printf("%s",message);
    send_receive(message,socket_desc);
    a++;

    sleep(10);

    printf("\nCLIENT:");
    message =  "Process my request\n";
    printf("%s",message);
    send_receive(message,socket_desc);
    a++;

    sleep(10);

    printf("\nCLIENT:");
    message =  "Bye\n";
    printf("%s",message);
    send_receive(message,socket_desc);
    a++;
    }

    close(socket_desc);
    return 0;
}


 void send_receive(char *msg,int socket_desc)
    {
    char server_reply[2000];
    char *reply;
    if(write(socket_desc,msg,strlen(msg))<0)
    {
    printf("\n data could not be sent");
    }
    read(socket_desc,server_reply,sizeof(server_reply));
    printf("server:%s",server_reply);
    reply = &server_reply[0];
    insert(msg,reply);  /* passing of two addresses as arguments */
}

Could anybody help me please?

char server_reply[2000] is local. the value in it will be gone once it gets out of the scope. you need to malloc a chunk of memory every time you receive a message. and reply = &server_reply[0]; is unnecessary too. — Jason Hu
– Jason Hu, Commented Jul 7, 2015 at 16:52
@HuStmpHrrr Thank you . I understand that reply = &server_reply[0]; is unnecessary. But can you ellaborate on using that malloc. I am sorry , i am new to programming.. — SRA
– SRA, Commented Jul 7, 2015 at 16:59
@SRA the answer down there is correct. strdup calls malloc in the background. — Jason Hu
– Jason Hu, Commented Jul 7, 2015 at 17:04

dbush · Accepted Answer · 2015-07-07 16:54:41Z

3

Every time you call insert, you're passing in the address of a buffer that was allocated on the stack, i.e. server_reply, and saving that in your list. Then, when send_receive exist, that memory points to garbage.

Instead of doing this:

new_node->request= send;
new_node->response = receive;

Do this:

new_node->request= strdup(send);
new_node->response = strdup(receive);

That makes a copy of the sent/recieved data in a newly allocated buffer. Just be sure to free() all of that memory when you clean up.

answered Jul 7, 2015 at 16:54

dbush

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