Error: Address already in use while binding socket with address but the port number is shown free by `netstat`

Question

I tried to bind my socket(server socket) at port number 8000. It worked and did the job for me. At the end of the code I close the socket as well. The very next instant I run my code again and it shows me that the address is already in use. I have printed the meaning of error values strerror(errno); to see if my code working properly at each point. To check if the port is free I checked it using netstat but it shows that port number 8000 is free. It has happened with me a lot of times. Every time I then wait for a few more secs and then it starts working again. I am using c language. So what is he reason for this behavior by my OS.

After a few more secs I run the code and then it works.

anirudh@anirudh-Aspire-5920:~/Desktop/testing$ sudo ./a.out  Socket Creation: Success File open: Success Socket Bind: Address already in use Socket Listen: Address already in use ^C anirudh@anirudh-Aspire-5920:~/Desktop/testing$ sudo netstat -lntp Active Internet connections (only servers) Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN      1348/lighttpd    tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      984/sshd         tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN      1131/cupsd       tcp        0      0 0.0.0.0:3306            0.0.0.0:*               LISTEN      1211/mysqld      tcp6       0      0 :::22                   :::*                    LISTEN      984/sshd         tcp6       0      0 ::1:631                 :::*                    LISTEN      1131/cupsd       anirudh@anirudh-Aspire-5920:~/Desktop/testing$ sudo ./a.out  Socket Creation: Success File open: Success Socket Bind: Address already in use Socket Listen: Address already in use ^C anirudh@anirudh-Aspire-5920:~/Desktop/testing$  

Answer 1

I've run into that same issue as well. It's because you're closing your connection to the socket, but not the socket itself. The socket can enter a TIME_WAIT state (to ensure all data has been transmitted, TCP guarantees delivery if possible) and take up to 4 minutes to release.

or, for a REALLY detailed/technical explanation, check this link

It's certainly annoying, but it's not a bug. See the comment from @Vereb on this answer below on the use of SO_REUSEADDR.

Answer 2

I know its been a while since the question was asked but I was able to find a solution:

int sockfd; int option = 1; sockfd = socket(AF_INET, SOCK_STREAM, 0); setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &option, sizeof(option)); 

This set the socket able to be reused immediately.

I apologize if this is "wrong". I'm not very experienced with sockets