How to find the socket connection state in C

Question

I have a TCP connection  Server just reads data from the client  Now  if the connection is lost  the client will get an error while writing the data to the pipe  broken pipe   but the server still listens on that pipe  Is there any way I can find if the connection is UP or NOT

User · Answer

The only way to reliably detect if a socket is still connected is to periodically try to send data  Its usually more convenient to define an application level  ping  packet that the clients ignore  but if the protocol is already specced out without such a capability you should be able to configure tcp sockets to do this by setting the SO KEEPALIVE socket option  I ve linked to the winsock documentation  but the same functionality should be available on all BSD-like socket stacks

User · Answer

get sock opt may be somewhat useful  however  another way would to have a signal handler installed for SIGPIPE  Basically whenever you the socket connection breaks  the kernel will send a SIGPIPE signal to the process and then you can do the needful  But this still does not provide the solution for knowing the status of the connection  hope this helps

User · Answer

On Windows you can query the precise state of any port on any network-adapter using  GetExtendedTcpTable   You can filter it to only those related to your process  etc and do as you wish periodically monitoring as needed  This is  an alternative  approach   You could also duplicate the socket handle and set up an IOCP Overlapped i o wait on the socket and monitor it that way as well

User · Answer

For BSD sockets I d check out Beej s guide   When recv returns 0 you know the other side disconnected     Now you might actually be asking  what is the easiest way to detect the other side disconnecting   One way of doing it is to have a thread always doing a recv   That thread will be able to instantly tell when the client disconnects

User · Answer

you can use SS ISCONNECTED macro in getsockopt   function  SS ISCONNECTED is define in socketvar h

User · Answer

I had a similar problem  I wanted to know whether the server is connected to client or the client is connected to server  In such circumstances the return value of the recv function can come in handy  If the socket is not connected it will return 0 bytes  Thus using this I broke the loop and did not have to use any extra threads of functions  You might also use this same if experts feel this is the correct method

User · Answer

You could call getsockopt just like the following   int error   0  socklen t len   sizeof  error   int retval   getsockopt  socket fd  SOL SOCKET  SO ERROR   amp error   amp len     To test if the socket is up   if  retval    0           there was a problem getting the error code        fprintf stderr   error getting socket error code   s n   strerror retval        return     if  error    0           socket has a non zero error status        fprintf stderr   socket error   s n   strerror error

User · Answer

Very simple  as pictured in the recv   To check that you will want to read 1 byte from the socket with MSG PEEK and MSG DONT WAIT  This will not dequeue data  PEEK  and the operation is nonblocking  DONT WAIT    while  recv client- gt socket NULL 1  MSG PEEK   MSG DONTWAIT     0        sleep rand     2      Sleep for a bit to avoid spam     fflush stdin       printf  I am alive   d n   socket         When the client has disconnected  this line will execute printf  Client  d went away    n   client- gt socket     Found the example here

User · Answer

You should try to use  getpeername function   now when the connection is down you will get in errno  ENOTCONN - The socket is not connected  which means for you DOWN   else  if no other failures  there the return code will 0 --  which means UP   resources   man page  http   man7 org linux man-pages man2 getpeername 2 html

User · Answer

There is an easy way to check socket connection state via poll call  First  you need to poll socket  whether it has POLLIN event    If socket is not closed and there is data to read then read will return more than zero  If there is no new data on socket  then POLLIN will be set to 0 in revents If socket is closed then POLLIN flag will be set to one and read will return 0    Here is small code snippet   int client socket 1  client socket 2  if   client socket 1   accept listen socket  NULL  NULL    lt  0        perror  Unable to accept s1        abort      if   client socket 2   accept listen socket  NULL  NULL    lt  0        perror  Unable to accept s2        abort      pollfd pfd     client socket 1 POLLIN 0   client socket 2 POLLIN 0    char sock buf 1024    while  true        poll pfd 2 5       if  pfd 0  revents  amp  POLLIN                int sock readden   read client socket 1  sock buf  sizeof sock buf            if  sock readden    0              break          if  sock readden  gt  0              write client socket 2  sock buf  sock readden             if  pfd 1  revents  amp  POLLIN                int sock readden   read client socket 2  sock buf  sizeof sock buf            if  sock readden    0              break          if  sock readden  gt  0              write client socket 1  sock buf  sock readden

User · Answer

TCP keepalive socket option  SO KEEPALIVE  would help in this scenario and close server socket in case of connection loss

[c] How to find the socket connection state in C?

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