Linux Socket Debugging Questions & Answers with Coding Examples

This set of Linux Debugging questions and answers focuses on Unix Domain Sockets.

1. What is the output of this program?

```
   #include<stdio.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       int fd_socket;
```

       fd_socket = socket(AF_UNIX,SOCK_STREAM,0);

```
       printf("%d\n",fd_socket);
```
```
       return 0;
```
```
   }
```

a) -1
b) 0
c) any integer value
d) none of the mentioned
View Answer

Answer: d
Explanation: To use socket(), the header files sys/types.h and sys/socket.h are required.
Output:
[root@localhost sanfoundry]# gcc -o san san.c
san.c: In function ‘main’:
san.c:6:21: error: ‘AF_UNIX’ undeclared (first use in this function)
san.c:6:21: note: each undeclared identifier is reported only once for each function it appears in
san.c:6:29: error: ‘SOCK_STREAM’ undeclared (first use in this function)
[root@localhost sanfoundry]#

2. In this program, the third argument of the socket() is used for _____ potocol.

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```
   #include<stdio.h>
```
```
   #include<sys/types.h>
```
```
   #include<sys/socket.h>
```
```
   int main()
```
```
   {
```
```
       int fd_socket;
```

       if(socket(AF_UNIX,SOCK_STREAM,0) == -1)

```
           perror("socket");
```
```
       return 0;
```
```
   }
```

a) TCP/IP
b) UDP
c) both TCP/IP and UDP
d) none of mentioned
View Answer

Answer: a
Explanation: None.

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3. By this program the soket “san_sock” will create

```
   #include<stdio.h>
```
```
   #include<sys/types.h>
```
```
   #include<sys/un.h>
```
```
   #include<sys/socket.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       struct sockaddr_un add_server;
```
```
       int fd_server;
```

       fd_server = socket(AF_UNIX,SOCK_STREAM,0);

```
       if(fd_server == -1)
```
```
           perror("socket");
```
```
       add_server.sun_family = AF_UNIX;
```

       strcpy(add_server.sun_path,"san_sock");

       if( bind(fd_server,(struct sockaddr*)&add_server,sizeof(add_server)) != 0)

```
           perror("bind");
```
```
       return 0;
```
```
   }
```

a) in the /tmp directory
b) in the /usr directory
c) in the present working directory
d) none of the mentioned
View Answer

Answer: c
Explanation: None.
Output:
[root@localhost sanfoundry]# ls
san.c
[root@localhost sanfoundry]# gcc -o san san.c
[root@localhost sanfoundry]# ./san
[root@localhost sanfoundry]# ls
san san.c san_sock
[root@localhost sanfoundry]#

4. What is the length of of the queue for pending connections in this program?

```
   #include<stdio.h>
```
```
   #include<sys/types.h>
```
```
   #include<sys/un.h>
```
```
   #include<sys/socket.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       struct sockaddr_un add_server;
```
```
       int fd_server;
```

       fd_server = socket(AF_UNIX,SOCK_STREAM,0);

```
       if(fd_server == -1)
```
```
           perror("socket");
```
```
       add_server.sun_family = AF_UNIX;
```

       strcpy(add_server.sun_path,"server_sock2");

       if( bind(fd_server,(struct sockaddr*)&add_server,sizeof(add_server)) != 0)

```
           perror("bind");
```
```
       if( listen(fd_server,3) != 0)
```
```
           perror("listen");
```
```
       return 0;
```
```
   }
```

a) 0
b) 1
c) 2
d) 3
View Answer

Answer: d
Explanation: The second argument of listen() specifies the length for the queue for pending connections.

5. What is the output of the program?

```
   #include<stdio.h>
```
```
   #include<sys/types.h>
```
```
   #include<sys/un.h>
```
```
   #include<sys/socket.h>
```
```
 
```
```
   int main()
```
```
   {
```

       struct sockaddr_un add_server, add_client;

```
       int fd_server, fd_client;
```
```
       int len;
```
```
       char ch;
```

       fd_server = socket(AF_UNIX,SOCK_STREAM,0);

```
       if(fd_server == -1)
```
```
           perror("socket");
```
```
       add_server.sun_family = AF_UNIX;
```

       strcpy(add_server.sun_path,"san_sock");

       if( bind(fd_server,(struct sockaddr*)&add_server,sizeof(add_server)) != 0)

```
           perror("bind");
```
```
       if( listen(fd_server,3) != 0)
```
```
           perror("listen");
```
```
       len = sizeof(add_client);
```

       fd_client = accept(fd_server,(struct sockaddr*)&add_client,&len);

```
       printf("Sanfoundry\n"); 
```
```
       return 0;
```
```
   }
```

a) the program will print the string “Sanfoundry”
b) the process will remain block
c) segmentation fault
d) none of the mentioned
View Answer

Answer: b
Explanation: There is no peding request in the queue for listening socket “san_sock”.
Output:
[root@localhost sanfoundry]# gcc -o san san.c
[root@localhost sanfoundry]# ./san
^Z
[4]+ Stopped ./san
[root@localhost sanfoundry]#

6. What is the output of this program?

```
   #include<stdio.h>
```
```
   #include<sys/types.h>
```
```
   #include<sys/socket.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       int fd;
```

       fd = socket(AF_UNIX,SOCK_STREAM,0);

```
       printf("%d\n",fd);
```
```
       return 0;
```
```
   }
```

a) 0
b) 1
c) 2
d) 3
View Answer

Answer: d
Explanation: The socket() returns the lowest available file descriptor and in this program i.e. 3.
Output:
[root@localhost sanfoundry]# gcc -o san san.c
[root@localhost sanfoundry]# ./san
3
[root@localhost sanfoundry]#

7. What is the output of this program?

```
   #include<stdio.h>
```
```
   #include<sys/types.h>
```
```
   #include<sys/un.h>
```
```
   #include<sys/socket.h>
```
```
   #include<errno.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       struct sockaddr_un addr;
```
```
       int fd;
```

       fd = socket(AF_UNIX,SOCK_STREAM,0);

```
       if (fd == -1)
```
```
           perror("socket");
```
```
       addr.sun_family = AF_UNIX;
```

       strcpy(addr.sun_path,"san_sock");

       if (bind(4,(struct sockaddr*)&addr,sizeof(addr)) == -1)

```
           printf("Sanfoudnry\n");
```
```
       return 0;
```
```
   }
```

a) this program will print the string “Sanfoundry”
b) this program will not print the string “Sanfoundry”
c) segmentation fault
d) none of the mentioned
View Answer

Answer: a
Explanation: The first argument of the bind() is not a valid file descriptor in this program.
Output:
[root@localhost sanfoundry]# gcc -o san san.c
[root@localhost sanfoundry]# ./san
Sanfoudnry
[root@localhost sanfoundry]#

8. What this program is not able to connect with any client program?

```
   #include<stdio.h>
```
```
   #include<sys/types.h>
```
```
   #include<sys/un.h>
```
```
   #include<sys/socket.h>
```
```
 
```
```
   int main()
```
```
   {
```

       struct sockaddr_un add_server, add_client;

```
       int fd_server, fd_client;
```
```
       int len;
```
```
       char ch;
```

       fd_server = socket(AF_UNIX,SOCK_STREAM,0);

```
       if(fd_server == -1)
```
```
           perror("socket");
```
```
       add_server.sun_family = AF_UNIX;
```

       strcpy(add_server.sun_path,"san_sock");

       if( bind(fd_server,(struct sockaddr*)&add_server,sizeof(add_server)) != 0)

```
           perror("bind");
```
```
       len = sizeof(add_client);
```

       fd_client = accept(fd_server,(struct sockaddr*)&add_client,&len);

```
       printf("Sanfoundry\n"); 
```
```
       return 0;
```
```
   }
```

a) the listen() is missing
b) the connect() is missing
c) the read() and write() are missing
d) none of the mentioned
View Answer

Answer: a
Explanation: None.

9. What is the output of this program?

```
   #include<stdio.h>
```
```
   #include<sys/types.h>
```
```
   #include<sys/un.h>
```
```
   #include<sys/socket.h>
```
```
 
```
```
   int main()
```
```
   {
```

       struct sockaddr_un add_server, add_client;

```
       int fd_server, fd_client;
```
```
       int len;
```
```
       char ch;
```

       fd_server = socket(AF_UNIX,SOCK_STREAM,0);

```
       if(fd_server == -1)
```
```
           perror("socket");
```
```
       add_server.sun_family = AF_UNIX;
```

       strcpy(add_server.sun_path,"san_sock");

       if( bind(fd_server,(struct sockaddr*)&add_server,sizeof(add_server)) != 0)

```
           perror("bind");
```
```
       len = sizeof(add_client);
```

       fd_client = connect(fd_server,(struct sockaddr*)&add_client,&len);

```
       printf("Sanfoundry\n"); 
```
```
       return 0;
```
```
   }
```

a) this program will print the string “Sanfoundry”
b) segmentation fault
c) error
d) none of the mentioned
View Answer

Answer: c
Explanation: The syntax of the connect() is wrong. connect() should be used in client program only.
Ouptut:
[root@localhost sanfoundry]# gcc -o san san.c
san.c: In function ‘main’:
san.c:20:46: warning: passing argument 3 of ‘connect’ makes integer from pointer without a cast [enabled by default] /usr/include/sys/socket.h:129:12: note: expected ‘socklen_t’ but argument is of type ‘int *’
[root@localhost sanfoundry]#

10. What is the output of this program?

```
    #include<stdio.h>
```
```
    #include<sys/types.h>
```
```
    #include<netinet/in.h>
```
```
    #include<sys/socket.h>
```
```
    #include<errno.h>
```
```
 
```
```
    int main()
```
```
    {
```
```
        struct sockaddr_in addr;
```
```
        int fd;
```

        fd = socket(AF_UNIX,SOCK_STREAM,0);

```
        if (fd == -1)
```
```
            perror("socket");
```
```
        addr.sun_family = AF_UNIX;
```

        strcpy(addr.sun_path,"san_sock");

        if (bind(4,(struct sockaddr*)&addr,sizeof(addr)) == -1)

```
            printf("Sanfoudnry\n");
```
```
        return 0;
```
```
    }
```

a) error
b) “Sanfoundry”
c) segmentation fault
d) none of the mentioned
View Answer

Answer: a
Explanation: The structure used for AF_UNIX if sockaddr_un.
Output:
[root@localhost sanfoundry]# gcc -o san san.c
san.c: In function ‘main’:
san.c:14:6: error: ‘struct sockaddr_in’ has no member named ‘sun_family’
san.c:15:2: warning: incompatible implicit declaration of built-in function ‘strcpy’ [enabled by default] san.c:15:13: error: ‘struct sockaddr_in’ has no member named ‘sun_path’
[root@localhost sanfoundry]#

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