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

User · Answer

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

User · Answer

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

User · Answer

the error i received was   cockpit socket  Failed to listen on sockets  Address already in use   the fix I discovered is    I had to disable selinux in  usr lib systemd system cockpit service i changed the line     ExecStartPre  usr sbin remotectl certificate --ensure --user root --group cockpit-ws --selinux-type etc t   to    ExecStartPre  usr sbin remotectl certificate --ensure --user root --group cockpit-ws     so as you can see i took out the argument about selinux then i ran   systemctl daemon-reload systemctl start cockpit service   then I browsed to    I accepted the self-signed certificate and was able to login successfully to cockpit and use it normally   this is all on a fedora25 machine  the 9090 port had already been added using firewall-cmd

User · Answer

Just type  unlink  SOCKET NAME    in the terminal  then the error should no longer exist

User · Answer

As already said  your socket probably enter in TIME WAIT state  This issue is well described by Thomas A  Fine here   To summary  socket closing process follow diagram below     Thomas says      Looking at the diagram above  it is clear that TIME WAIT can be   avoided if the remote end initiates the closure  So the server can   avoid problems by letting the client close first  The application   protocol must be designed so that the client knows when to close  The   server can safely close in response to an EOF from the client  however   it will also need to set a timeout when it is expecting an EOF in case   the client has left the network ungracefully  In many cases simply   waiting a few seconds before the server closes will be adequate    Using SO REUSEADDR is commonly suggested on internet  but Thomas add      Oddly  using SO REUSEADDR can actually lead to more difficult  address   already in use  errors  SO REUSADDR permits you to use a port that is   stuck in TIME WAIT  but you still can not use that port to establish a   connection to the last place it connected to  What  Suppose I pick   local port 1010  and connect to foobar com port 300  and then close   locally  leaving that port in TIME WAIT  I can reuse local port 1010   right away to connect to anywhere except for foobar com port 300

User · Answer

For AF UNIX you can use call unlink  path   after close   socket in  server  app

User · Answer

Even icfantv s answer to this question is already perfect  I still have more findings in my test   As a server socket in listening status  if it only in listening status  and even it accepts request and getting data from the client side  but without any data sending action  We still could restart the server at once after it s stopped  But if any data sending action happens in the server side to the client  the same service same port  restart will have this error   Address already in use    I think this is caused by the TCP IP design principles  When the server send the data back to client  it must ensure the data sending succeed  in order to do this  the OS Linux  need monitor the connection even the server application closed this socket  But I still believe kernel socket designer could improve this issue

User · Answer

Try netstat like this  netstat -ntp  without the -l  It will show tcp connection in TIME WAIT state

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

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