c++ - What is exceptional condition pending fd in select function? -

the prototype select function is

select(int no_of_fd,fd_set read_fds,fd_set write_fds,fd_set expection_condition_fd,const struct timeval *timeout) 

what exception condition fd's represent?

when fd set ?

it used to:

1. the presence of out-of-band data when so_oobinline not enabled.

2. detect non-blocking connect() failure on windows. if connection successful, socket put write_fds, otherwise put except_fds instead. documented on msdn select() function:

   in summary, socket identified in particular set when select returns if:

   readfds:
   

   • if listen has been called , connection pending, accept succeed.
     
   • data available reading (includes oob data if so_oobinline enabled).
     
   • connection has been closed/reset/terminated.

   writefds:
   

   • if processing connect call (nonblocking), connection has succeeded.
     
   • data can sent.

   exceptfds:
   

   • if processing connect call (nonblocking), connection attempt failed.
     
   • oob data available reading (only if so_oobinline disabled).

   in case of failure, can query socket particular error code using getsockopt(sol_socket, so_error), if needed.

   on other platforms, such linux, select() puts socket write_fds regardless of whether connection succeeded or failed, have query error code differentiate. documented on linux man page connect(2) function:

   return value
   if connection or binding succeeds, 0 returned. on error, -1 returned, , errno set appropriately.
   ...
   einprogress
   socket nonblocking , connection cannot completed immediately. it possible select(2) or poll(2) completion selecting socket writing. after select(2) indicates writability, use getsockopt(2) read so_error option @ level sol_socket determine whether connect() completed (so_error zero) or unsuccessfully (so_error 1 of usual error codes listed here, explaining reason failure).