select call, writefd

[maheshgupta024]

Hi all, when using the select can anyone tell what the writefds used for.

[Roxoff]

Nope, this is too cryptic. You're going to have to give us a little more information if you want help.

Is this an SQL select? What are you trying to achieve?

[Rubberman]

From the select(2) man page:

       Three  independent  sets of file descriptors are watched.  Those listed in readfds
       will be watched to see if characters become available for reading (more precisely,
       to see if a read will not block; in particular, a file descriptor is also ready on
       end-of-file), those in writefds will be watched to see if a write will not  block,
       and those in exceptfds will be watched for exceptions.  On exit, the sets are mod-
       ified in place to indicate which file descriptors actually changed  status.   Each
       of  the three file descriptor sets may be specified as NULL if no file descriptors
       are to be watched for the corresponding class of events.

[maheshgupta024]

thanks for the reply Rubberman, I have gone through this.. but my intention is to make a function for non-blocking connect call in socket i.e., I want to connect to a socket at an IP, if the IP address is not present and if I connect , my aplication is blocked at the connect call for some time(say 30secs). But I dont want it to happen like this. I want to check only say 2secs and give me an error message.
Can I do this stuff like this :

1. Change the socket to non-blocking
2. Connect to the IP Address
3. Then check the socket with select, by adding this fd in the write fds.

Please can you help with this problem, this will be of grt help to me.

[Rubberman]

1. You can set an itimer w/ 2 second timeout before attempting to connect, then check the return from connect call to see if you got an EINTR errno.
2. You can do what you suggested. According to the connect() man page, what you suggest is acceptable and should work. From the connect(2) man page for a connect() call on a non-blocking socked which returns errno == EINPROGRESS:

       EINPROGRESS
              The socket is non-blocking and the connection cannot be  completed  immedi-
              ately.   It is possible to select(2) or poll(2) for completion by selecting
              the socket for writing. After select(2) indicates writability, use getsock-
              opt(2) to read the SO_ERROR option at level SOL_SOCKET to determine whether
              connect() completed  successfully  (SO_ERROR  is  zero)  or  unsuccessfully
              (SO_ERROR  is one of the usual error codes listed here, explaining the rea-
              son for the failure).

As for help doing this, reading the appropriate man pages should get you sorted out. Also, if you are going to do a lot of client/server programming, you should probably invest in Comer and Stevens Internetworking with TCP/IP, Vol. III, the BSD Socket Version.