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jamesbon
October 18th, 2010, 04:36 PM
Hi,
I am reading and understanding code from a book.
I am copy pasting the exact code which I have.
Following program is for client.c


#include<sys/types.h>
#include<sys/socket.h>
#include <stdio.h>
#include <sys/un.h>
#include <unistd.h>>
#include <stdlib.h>
int main() {
int sockfd;
int len;
struct sockaddr_un address;
int result;
char ch = 'A';
sockfd = socket(AF_UNIX, SOCK_STREAM, 0);
address.sun_family = AF_UNIX;
strcpy(address.sun_path, "server_socket");

I was not able to understand the use of server_socket in above strcpy function.
Where does this come to picture



len = sizeof(address);
result = connect(sockfd, (struct sockaddr *)&address, len);
if(result == -1) {
perror("oops: client1");
exit(1);
}

write(sockfd, &ch, 1);
read(sockfd, &ch, 1);
printf("char from server = %c\n", ch);
close(sockfd);
exit(0);
}


When I run the code as

gcc client.c -o client.o
then on terminal I see each time letter B why is that happening.


and following program is for server.c


#include<sys/types.h>
#include<sys/socket.h>
#include<stdio.h>
#include <sys/un.h>
#include <unistd.h>
#include<stdlib.h>
int main()
{
int server_sockfd, client_sockfd;
int server_len, client_len;
struct sockaddr_un server_address;
struct sockaddr_un client_address;
unlink("server_socket");
server_sockfd = socket(AF_UNIX, SOCK_STREAM, 0);
server_address.sun_family = AF_UNIX;
strcpy(server_address.sun_path, "server_socket");

I am not clear with use of server_address.sun_path
I understand that server_address is a struct of type sockaddr_un
but what is sun_path I had seen the file linux/un.h
but could not understand.



server_len = sizeof(server_address);
bind(server_sockfd, (struct sockaddr *)&server_address, server_len);
listen(server_sockfd, 5);
while (1) {
char ch;
printf("server waiting\n");
client_len = sizeof(client_address);
client_sockfd = accept(server_sockfd,
(struct sockaddr *)&client_address,
&client_len);
read(client_sockfd, &ch, 1);
ch++;
write(client_sockfd, &ch, 1);
close(client_sockfd);
}
}

dwhitney67
October 18th, 2010, 05:15 PM
In short, it is a socket file. While the server is running, examine the current working directory from which the server was launched... you should see a file with the name "server_socket=".

gmargo
October 18th, 2010, 07:32 PM
When I run the code .... then on terminal I see each time letter B why is that happening.


The client connects to the server and send a single character 'A'. The server receives a connection from the client and reads a single character, increments it ('A' + 1 = 'B'), and sends it back to the client. So the client prints 'char from server = B', indicating that your round trip data transfer was successful.



I am not clear with use of server_address.sun_path
For a unix-family socket (AF_UNIX), the "sun_path" field refers to a path in the filesystem, where a socket device will be created. When the server is running, you should have a socket device file in the current directory named "server_socket" with permissions like "srwxr-xr-x". Note the leading "s" which means socket.


$ ls -lgG | grep socket
srwxr-xr-x 1 0 Oct 18 11:27 server_socket

jamesbon
October 20th, 2010, 05:58 PM
Why is the name sun_path used I mean why is sun word used is it proprietary?

dwhitney67
October 20th, 2010, 06:06 PM
sockaddr_un ---> sun

Conversely, oftentimes in socket related code you will see "sin", which is derived from sockaddr_in... not from Adam & Eve.

jamesbon
October 21st, 2010, 07:31 AM
Ok.