CODEWITHSUNDEEP
c++linuxsocketsselect
Ali Nadalizadeh
Ali Nadalizadeh

Reputation: 2826

Socket select() works in Windows and times out in Linux

I'm porting a windows network application to linux and faced a timeout problem with select call on linux. The following function blocks for the entire timeout value and returns while I checked with a packet sniffer that client has already sent the data.

int recvTimeOutTCP( SOCKET socket, long sec, long usec )
{
  struct timeval timeout;
  fd_set fds;.

  timeout.tv_sec = sec;
  timeout.tv_usec = usec;
  FD_ZERO( &fds );
  FD_SET( socket, &fds );

  // Possible return values:
  // -1: error occurred
  // 0: timed out
  // > 0: data ready to be read
  cerr << "Waiting on fd " << socket << endl;
  return select(1, &fds, 0, 0, &timeout);
}

Upvotes: 7

Views: 8759

Answers (5)

Loki Astari
Loki Astari

Reputation: 264331

From the man page of select:

int select(int nfds,
           fd_set* restrict readfds,
           fd_set* restrict writefds,
           fd_set* restrict errorfds, 
           struct timeval* restrict timeout);

The first nfds descriptors are checked in each set; i.e., the descriptors from 0 through nfds-1 in the descriptor sets are examined.

Thus the first parameter to select should be socket + 1.

return select(socket + 1, &fds, 0, 0, &timeout);

Upvotes: 3

woolstar
woolstar

Reputation: 5083

The issue is that the fd_set in linux is a bit array ( originally it was just a int, but then you could only watch the first 16 io's of your process ). In windows fd_set is an array of sockets with a length at the front (which is why windows doesn't need to know how many bits to watch).

The poll() function takes an array of records to watch on linux and has other benefits which make it a better choice than select().

int recvTimeOutTCP( SOCKET socket, long msec )
{
    int iret ;
    struct polldf   sockpoll ;

    sockpoll.fd= socket ;
    sockpoll.events= POLLIN ;

    return poll(& sockpoll, 1, msec) ;
}

Upvotes: 3

Nikolai Fetissov
Nikolai Fetissov

Reputation: 84151

select on Windows ignores the first parameter. From MSDN:

C++
int select(
  __in     int nfds,
  __inout  fd_set *readfds,
  __inout  fd_set *writefds,
  __inout  fd_set *exceptfds,
  __in     const struct timeval *timeout
);

Parameters

nfds [in]

    Ignored. The nfds parameter is included only for
    compatibility with Berkeley sockets.
...

Upvotes: 3

tatsu
tatsu

Reputation: 115

The first parameter to select(...) is the number of file descriptor to check in the set. Your call is telling it to only look at file descriptor 0, which is almost certainly not what socket is set to.

Upvotes: 1

epatel
epatel

Reputation: 46051

I think the first parameter to select() should be socket+1.

You really should use another name as socket also is used for other things. Usually sock is used.

Upvotes: 13

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