how can I check the connection is alive or not in socket programming?

I dont really understand the question. What do you mean when you say "connection alive"? 1) Does it mean that the computer is attached to some network? 2) Does it mean it is connected to a specific network, i.e. the internet? 3) Or does it simply mean that you can use [your programming language here, as you didnt specify which one] to create a TCP socket? 4) Does it mean that some socket is open and accessible?

1) You could try and connect to a resource on its network (i.e. its default gateway or other resource that is guaranteed to be accessible).
2) Similar to 1), but use an internet resource (i.e. google.com).
3) Simply type the code to create some TCP socket and see if it fails.
4) Create a socket to the host and port in question, and try to connect to it.

Type netstat

The question was how to do it programmatically, i.e. not using some command-line tool.

Sorry about that. Unless the OP can be more specific, I'd ask him to read about h_errno and perror.

You need to perform some socket operation for that. I think "select" call will work too. Just call the select function on socket for read, write and error with little timeout value (like 1 millisecond) and you can check if the socket is readable, writable or there is some error.
Of course if your socket descriptor is in Error FD_SET then its not alive, and if it is readable and writable it will be alive as well

Hope it works,

Regards,
--Tayyab