Thread: How to pass data using Sockets

Guys can anyone help me to How to pass data between two different processes using sockets.

Here are the steps using sockets (correct me if I am wrong)

Process1(server):
1. Create a socket
2. Bind
3. listen
4. Accept
5. Close

Process2(Client):
1. Create a Socket
2. Connect
3. Close

In sockets, we have to use the same name for both the sockets. Can anyone please tell me how to give the same name.

While coming to pass the data between processes, If I wish to pass a struct of data using linked lists...Is it possible.

Can anyone please help me with this and kindly share any sample programs using Sockets if you have any. Thank you.

Code:

SERVER.C
int server (int client_socket)
{
while (1) {
int length;
char* text;
/* First, read the length of the text message from the socket.
read returns zero, the client closed the connection. */
if (read (client_socket, &length, sizeof (length)) == 0)
return 0;
/* Allocate a buffer to hold the text. */
text = (char*) malloc (length);
/* Read the text itself, and print it. */
read (client_socket, text, length);
printf (“%s\n”, text);
/* Free the buffer. */
free (text);
/* If the client sent the message “quit,” we’re all done.
if (!strcmp (text, “quit”))
return 1;
}
}

int main (int argc, char* const argv[])
{
const char* const socket_name = argv[1];
int socket_fd;
struct sockaddr_un name;
int client_sent_quit_message;
/* Create the socket. */
socket_fd = socket (PF_LOCAL, SOCK_STREAM, 0);
/* Indicate that this is a server. */
name.sun_family = AF_LOCAL;
strcpy (name.sun_path, socket_name);
bind (socket_fd, &name, SUN_LEN (&name));
/* Listen for connections. */
listen (socket_fd, 5);
/* Repeatedly accept connections, spinning off one server() to deal
with each client. Continue until a client sends a “quit” message.
do {
struct sockaddr_un client_name;
socklen_t client_name_len;
int client_socket_fd;
*/
/* Accept a connection. */
client_socket_fd = accept (socket_fd, &client_name, &client_name_len);
/* Handle the connection. */
client_sent_quit_message = server (client_socket_fd);
/* Close our end of the connection. */
close (client_socket_fd);
}
while (!client_sent_quit_message);
/* Remove the socket file.
close (socket_fd);
unlink (socket_name);
*/
return 0;
}

Code:

CLIENT.C

/* Write TEXT to the socket given by file descriptor SOCKET_FD.
*/
void write_text (int socket_fd, const char* text)
{
/* Write the number of bytes in the string, including
NUL-termination. */
int length = strlen (text) + 1;
write (socket_fd, &length, sizeof (length));
/* Write the string. */
write (socket_fd, text, length);
}
int main (int argc, char* const argv[])
{
const char* const socket_name = argv[1];
const char* const message = argv[2];
int socket_fd;
struct sockaddr_un name;
/* Create the socket. */
socket_fd = socket (PF_LOCAL, SOCK_STREAM, 0);
/* Store the server’s name in the socket address. */
name.sun_family = AF_LOCAL;
strcpy (name.sun_path, socket_name);
/* Connect the socket. */
connect (socket_fd, &name, SUN_LEN (&name));
/* Write the text on the command line to the socket. */
write_text (socket_fd, message);
close (socket_fd);
return 0;
}

Here for this program we are giving the same name in command prompt for both server & client. What we have to do to give the same program name for the sockets in the program itself (can we use #define and assign a name for program)
How to send a struct of data for employees like name, address, email, etc using linked lists from client to server.
Can anyone please help me with this, I am not getting it. I sincerely appreciate your help. Thank you.

Thank you MK27.

Code:

 const char* const socket_name = argv[1];

Here we are giving the same socket name for server & client using command prompt.
Is there any other way I can include same socket name for both server & client in the program itself.

Thank you MK27.
I have gone through Serialization, it was a good document.

Code:

const char* const message = argv[2];

In the above program, I am sending data from the command prompt.
If I wish to send employee data (name, address, email). If I initialize a structure and array or linked list can I pass this type of data using sockets. Please help me in how to send data from client to sockets and how to read it in server.

Did you come up with a serialization scheme for your data yet? Once you do so, then that will basically tell you how to build your packet. You seem to have the syntax for write/read (based on skimming your code, since I also am not going to spend a lot of time working through unindented code).

You'll have to cast your pointers from sockaddr_un to sockaddr.

The rest indicate some level of bafflement. Read requires a pointer to a piece of memory where you intend to put the incoming information; so you will need to provide it with such a thing. strlen requires a string to find the length of; so you will need to provide it with such a thing. (Perhaps you meant "sizeof" instead.) Write requires a pointer to the information that is to be written; so you will need to provide it with such a thing.

I am able to correct my errors guys with your suggestions. Thank you.

Code:

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

typedef struct 
{
  char lastname[25];
  char firstname[25];
  char address[25];
  char phonenumber[25];
}addressbook;

addressbook a={0,0};

int main (int argc, char* const argv[])
{
  int ret;
  const char* const socket_name ="/tmp/socket";
  int socket_fd;
  struct sockaddr_un name;
   
  /* Create the socket. */
  socket_fd = socket (PF_LOCAL, SOCK_STREAM, 0);
  
  /* Indicate that this is a server. */
  name.sun_family = AF_LOCAL;
  strcpy (name.sun_path, socket_name);
  bind (socket_fd, (struct sockaddr*)&name, SUN_LEN (&name));
  
  /* Listen for connections. */
  listen (socket_fd, 5);
  
    struct sockaddr_un client_name;
    socklen_t client_name_len;
    int client_socket_fd;
    int client_socket;
    
    /* Accept a connection. */
    client_socket_fd = accept (socket_fd, (struct sockaddr*)&client_name, &client_name_len);
  
    ret = read (client_socket_fd, &a, sizeof(a));
  
    if(ret != 0) {
      printf("error: %s",strerror(errno));
    }
    printf("lastname of person : %s\n", a.lastname);
    printf("firstname of person : %s\n", a.firstname);
    printf("address of person : %s\n", a.address);
    printf("phone number of person : %s\n", a.phonenumber);
    
     /* Close our end of the connection. */
    close (client_socket_fd);
    
  /* Remove the socket file.*/
  close (socket_fd);
  unlink (socket_name);
  return 0;
}

Code:

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


typedef struct 
{
  char lastname[25];
  char firstname[25];
  char address[25];
  char phonenumber[25];
}addressbook;

addressbook a={0,0};

int main (int argc, char* const argv[])
{
  const char* const socket_name = "/tmp/socket";

  const char* const message;
  int socket_fd;
  struct sockaddr_un name;
  
  /* Create the socket. */
  socket_fd = socket (PF_LOCAL, SOCK_STREAM, 0);
  
  /* Store the server’s name in the socket address. */
  name.sun_family = AF_LOCAL;
  strcpy (name.sun_path, socket_name);
  
  /* Connect the socket. */
  connect (socket_fd, (struct sockaddr*)&name, SUN_LEN (&name));
  
  /* Write the text on the command line to the socket.*/
    printf("enter details\n");
    printf("enter lastname of person :\n");
    scanf("%s", a.lastname);
    printf("enter firstname of person :\n");
    scanf("%s", a.firstname);
    printf("enter address of person :\n");
    scanf("%s", a.address);
    printf("enter phone number of person :\n");
    scanf("%s", a.phonenumber);
  write (socket_fd, (struct sockaddr*)&a, sizeof(a));
  
  /*close the socket */
  close (socket_fd);
  return 0;
}

Hi guys, With your help I am able to send data between two processes using sockets. Thank you.
I tried to extend my program. This is to insert and display the details of a person.
there are two processes- server.c and client.c.
for insertion - client process takes the input data and sends to server. Server must read the data and write to a file.
for display - client must retrieve the data from the sever(which is stored in file) and display it.
Can anyone help me with my code. First I am sending the select option for insertion and display then sending the data from client to server. Is this the correct or is there any good approach for this kind of situation.
I also had one error, can anyone please help me.

Code:

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

typedef struct 
{
  char lastname[25];
  char firstname[25];
  char address[25];
  char phonenumber[25];
}addressbook;

addressbook a={0,0};

int main (int argc, char* const argv[])
{
  int ret;
  FILE *fp;
  const char* const socket_name ="/tmp/socket";
  int socket_fd;
  int select;
  struct sockaddr_un name;
   
  /* Create the socket. */
  socket_fd = socket (PF_LOCAL, SOCK_STREAM, 0);
  
  /* Indicate that this is a server. */
  name.sun_family = AF_LOCAL;
  strcpy (name.sun_path, socket_name);
  bind (socket_fd, (struct sockaddr*)&name, SUN_LEN (&name));
  
  /* Listen for connections. */
  listen (socket_fd, 5);
  
  struct sockaddr_un client_name;
  socklen_t client_name_len;
  int client_socket_fd;
        
  /* Accept a connection. */
  client_socket_fd = accept (socket_fd, (struct sockaddr*)&client_name, &client_name_len);
  read (client_socket_fd, &select, sizeof(select));
  if(select == 1){
    ret = read (client_socket_fd, &a, sizeof(a));
    if(ret != 0) {
      printf("error: %s\n",strerror(errno));
    }
    printf("lastname of person : %s\n", a.lastname);
    printf("firstname of person : %s\n", a.firstname);
    printf("address of person : %s\n", a.address);
    printf("phone number of person : %s\n", a.phonenumber);
    fp = fopen("addressbook.dat","a+");
    fwrite(&a, sizeof(addressbook), 1, fp);
    fclose(fp);
  } elseif (select == 2){
    fp = fopen("addressbook.dat", "r");
    fread(&a, sizeof(addressbook), 1, fp); 
    write (client_socket_fd, (struct sockaddr*)&a, sizeof(a));
   }else{
    printf("wrong selection");
   }
      
   /* Close our end of the connection. */
  close (client_socket_fd);
    
  /* Remove the socket file.*/
  close (socket_fd);
  unlink (socket_name);
  return 0;
}

Code:

error: 
socket.c: In function ‘main’:
socket.c:58: error: expected ‘;’ before ‘{’ token

Code:

client.c
#include<stdio.h>
#include<string.h>
#include<sys/socket.h>
#include<sys/un.h>
#include<unistd.h>


typedef struct 
{
  char lastname[25];
  char firstname[25];
  char address[25];
  char phonenumber[25];
}addressbook;

addressbook a={0,0};

int main (int argc, char* const argv[])
{
  const char* const socket_name = "/tmp/socket";

  const char* const message;
  int socket_fd;
  int select;
  struct sockaddr_un name;
  
  /* Create the socket. */
  socket_fd = socket (PF_LOCAL, SOCK_STREAM, 0);
  
  /* Store the server’s name in the socket address. */
  name.sun_family = AF_LOCAL;
  strcpy (name.sun_path, socket_name);
    /* Connect the socket. */
  connect (socket_fd, (struct sockaddr*)&name, SUN_LEN (&name));
  do {
    printf("Address book: \n Press\n 1: Insert \n 2: Display \n Enter:");
    scanf("%d", &select);
    if(select == 1){
      write (socket_fd, select, sizeof(select));
      /* Write the text on the command line to the socket.*/
      printf("enter details\n");
      printf("enter lastname of person :\n");
      scanf("%s", a.lastname);
      printf("enter firstname of person :\n");
      scanf("%s", a.firstname);
      printf("enter address of person :\n");
      scanf("%s", a.address);
      printf("enter phone number of person :\n");
      scanf("%s", a.phonenumber);
      write (socket_fd, (struct sockaddr*)&a, sizeof(a));
    }elseif(select == 2){
      write (socket_fd, select, sizeof(select));
      read (socket_fd, &a, sizeof(a));
      printf("lastname of person : %s\n", a.lastname);
      printf("firstname of person : %s\n", a.firstname);
      printf("address of person : %s\n", a.address);
      printf("phone number of person : %s\n", a.phonenumber);
    }else{
      printf("wrong selection");
    }
    if((select > 4) || (select < 1))
    break;
  }while(1);
  /*close the socket */
   close (socket_fd);
   return 0;
}

Code:

error:
socket1.c: In function ‘main’:
socket1.c:39: warning: passing argument 2 of ‘write’ makes pointer from integer without a cast
/usr/include/unistd.h:363: note: expected ‘const void *’ but argument is of type ‘int’
socket1.c:51: error: expected ‘;’ before ‘{’ token

Can anyone please help me with this. Thank you.

Your overall approach of sending the select option (or "command") to the server, then reading/writing data as appropriate is a fine approach. What you aren't doing well is checking the results of all your IO functions. You need to check the return code of socket, bind, listen, accept, connect, read, write, fopen, fread, fwrite, and maybe some others I missed.

Highlighting the lines that have errors is a big help to us. As for the "expected ';' before '{' token" errors, you used "elseif" insted of "else if" (you need a space). As for the "passing argument 2 of 'write'" error, you're calling it incorrectly. The write function isn't type-aware, it just wants a memory address to the start of the data it's going to send. You need to pass the address of the data it's sending, i.e. &select.

Thank you anduril462. I edited my code, now it is running with no errors.