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Posted on 2001-07-20
Medium Priority
Last Modified: 2010-04-21
i am new to linux
i have a server program written in c.
its a socket server creats a socket on port 1234 which is listining.then my program goes into an infinite loop for accepting client socket.
i close the program by pressing ctl-c .
i need a method to close the server program properly
so the i can close the Listining socket

Some time after closing the program using ctl-c ,the next time when i run the program ,i get error message ,"Socket already bound"

is there any method or linux command to close the socket

please help me
Question by:v_iyengar
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Accepted Solution

superschlonz earned 100 total points
ID: 6303701
I think you have to use the shutdown method:

shutdown( socket, 2 );
close( socket );

If you want to close a connected socket you first have to shutdown sending, then if you read zero bytes from the socket shutdown both and then close it:

shutdown( clientsock, 1 );
read the buffered data...
shutdown( clientsock, 2 );
close( clientsock );

Have also a look at select and poll functions.


Expert Comment

ID: 6304021
You could also use a signal handler (on SIGUSR1 or something like that) and send some kill signal to the process id.

Author Comment

ID: 6304290
tell me more clearly as i am new to linux
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Author Comment

ID: 6304296
how to trap ctl-c ,what signal should i trap

Author Comment

ID: 6304377
how to trap ctl-c ,what signal should i trap

Assisted Solution

mnashadka earned 100 total points
ID: 6304721
A signal handler will allow a program to clean itself up or perform some other action when it receives a signal.  When you type kill, by default the SIGTERM signal is sent.  Of course, with the -s option, you can specify which signal to send.  Here's an example of a simple signal handler:

#include <signal.h>
#include <unistd.h>
#include <iostream>

void handle_signal(int signal)
  std::cout << "In signal handler" << std::endl;

int main()
  signal(SIGTERM, handle_signal);

  return 0;

When you type kill <process id>, this will spit out the "In signal handler" string and exit.  This is the same kind of thing that you need.  You'll replace handle_signal with whatever cleanup code that you need for your socket or whatever, and when you type kill <process id>, the cleanup that you need will be executed.  I hope this helps.

Author Comment

ID: 6307381
i am using signals now
        sigset(SIGCHLD, child_end);
        signal(SIGTERM, signal_handler);
        signal(SIGQUIT, signal_handler);
        signal(SIGINT, signal_handler);
Now the socket is closing properly
but some time when i press ctl-c and close program the socket is not closed.

do i have to trap some more signal


Expert Comment

ID: 6308414
Ctrl+C should be SIGINT, so there's no reason that your signal handler shouldn't handle it.

Author Comment

ID: 6311496
in my signal handler  i have written the code to close the socket.It works fine. But some time very rarely the socket is not closed
is there any situation when a socket can not be closed ?

Expert Comment

ID: 6979314
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Expert Comment

ID: 9871680
No comment has been added lately, so it's time to clean up this TA.
I will leave the following recommendation for this question in the Cleanup topic area:

Splt: superschlonz {http:#6303701} & mnashadka {http:#6304721}

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