Client/Server Socket Communication (AF_UNIX)

Question

I'm trying to write a client program and a server program in which when the client connects to the server, the server sends a random string from a file back to it. Here is what I have so far (reading from file omitted):

server.c

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

int listfd;
int connfd;

int main(int argc, char *argv[]){

    /*
    * Create Sockets
    */
    listfd = socket(AF_UNIX, SOCK_STREAM, 0);
    if(listfd == -1)
        exit(-1);

    struct sockaddr saddr = {AF_UNIX, "server"};
    socklen_t saddrlen = sizeof(struct sockaddr) + 6;
    bind(listfd, &saddr, saddrlen);

    listen(listfd, 10);

    fflush(stdout);
    printf("Running...\n");

    /*
    * Listen for connections
    * and send random phrase on accept
    */
    while(1){
        connfd = accept(listfd, NULL, NULL);

        int r = rand() % num_of_lines; //Pick random phrase/hint pair
        write(connfd, phrases[r], strlen(phrases[r]));

        close(connfd);
        sleep(1);
    }

    exit(0);
}

client.c

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

int main(int argc, char *argv[])
{
    int sock;
    int conn;

    struct sockaddr saddr = {AF_UNIX, "server"};
    socklen_t saddrlen = sizeof(struct sockaddr) + 6;

    sock = socket(AF_UNIX, SOCK_STREAM, 0);

    conn = connect(sock, &saddr, saddrlen);

    char BUFF[1024];

    read(sock, BUFF, 1024);

    printf("%s", BUFF);

    return 0;
}

My problem arises when I try to print in the client. I run the server, but when I run the client, it only prints garbage characters, and I'm not entirely sure what's causing this.

Any help would be much appreciated, thank you!

EDIT:

I figured out my problem. Because the server socket was bound to "server", which was also the name of the executable, this caused a lot of issues.

Renaming the sockaddr.sa_data field fixed my problem.

Answer 1

You have a number of problems. This is wrong:

struct sockaddr saddr = {AF_UNIX, "server"};
socklen_t saddrlen = sizeof(struct sockaddr) + 6;

You're telling the kernel "Here's a bag of bytes representing the socket address. It's sizeof(struct sockaddr) + 6 bytes long", but you're only passing in sizeof(struct sockaddr) bytes. Result: the kernel reads your address out of bounds, at best causing the syscall to fail with EFAULT, and at worst causing the kernel to read in garbage data.

The correct way to setup an AF_UNIX socket address is with the socket address type intended for AF_UNIX sockets, namely struct sockaddr_un (see unix(7)):

struct sockaddr_un saddr = {AF_UNIX, "/socket/path"};
bind(listfd, (struct sockaddr *)&saddr, sizeof(saddr));

Likewise the client code which calls connect(2) should also set up the address in the same way.

Next, you need to check for errors. Pretty much any system call here could fail, so you need to check for failures after every call and handle it appropriately (typically close the socket and/or terminate the process with an appropriate error message).

As you mentioned, you also need to choose an appropriate path name for the socket. Named AF_UNIX sockets live in the file system and so cannot share the same name with any other file, including your server and client programs.

Your call to read(2) needs to use the return value to determine the number of bytes read. It does NOT null-terminate its output—if it reads 4 bytes, those 4 bytes will be in your buffer, and everything else afterwards will be indeterminate. Since printf expects its input to be null-terminated, you need to either explicitly null-terminate it yourself, or pass in the length, e.g.:

int n = read(sock, BUFF, sizeof(BUFF));
if (n < 0) { /* Handle error */ }
printf("%.*s", n, BUFF); 

Answer 2

read(sock, BUFF, 1024);

does not NULL terminate the buffer. You need to do it explicitly. It returns the number of bytes read and copies the many bytes to the buffer.

 readlen = read(sock, BUFF, 1024);
 BUFF[readlen] = '\0';

Check the return value of connect and read for any error. Check what server sends.