I can contribute with how it is done in Mac OS X / Darwin.

First, the following framework needs to be linked against:
#include <SystemConfiguration/SystemConfiguration.h>

Then the following function can be used:

/*!
         @function SCNetworkCheckReachabilityByName
         @discussion Determines if the given network host/node name is
                 reachable using the current network configuration.
         @param nodename The node name of the desired host. This name
would
                 be the same as that passed to gethostbyname() or
getaddrinfo().
         @param flags A pointer to memory that will be filled with a
                 set of SCNetworkConnectionFlags detailing the
reachability
                 of the specified node name.
         @result TRUE if the network connection flags are valid; FALSE
if the
                 status could not be determined.
  */
Boolean
SCNetworkCheckReachabilityByName (
                                         const char
*nodename,
                                         SCNetworkConnectionFlags
*flags
                                         );

The flags structure is described like this:

/*!
         @enum SCNetworkConnectionFlags
         @discussion Flags that indicate whether the specified network
                 nodename/address is reachable, requires a connection,
                 requires some user intervention in establishing the
                 connection, and whether the calling application must
                 initiate the connection using the (TBD???) API.

         @constant kSCNetworkFlagsTransientConnection
                 This flag indicates that the specified nodename/address
can
                 be reached via a transient (e.g. PPP) connection.

         @constant kSCNetworkFlagsReachable
                 This flag indicates that the specified nodename/address
can
                 be reached using the current network configuration.

         @constant kSCNetworkFlagsConnectionRequired
                 This flag indicates that the specified nodename/address
can
                 be reached using the current network configuration but a
                 connection must first be established.

                 As an example, this status would be returned for a
dialup
                 connection that was not currently active but could
handle
                 network traffic for the target system.

         @constant kSCNetworkFlagsConnectionAutomatic
                 This flag indicates that the specified nodename/address
can
                 be reached using the current network configuration but a
                 connection must first be established. Any traffic
directed
                 to the specified name/address will initiate the
connection.

         @constant kSCNetworkFlagsInterventionRequired
                 This flag indicates that the specified nodename/address
can
                 be reached using the current network configuration but a
                 connection must first be established. In addition, some
                 form of user intervention will be required to establish
                 this connection (e.g. providing a password,
authentication
                 token, etc.).

         @constant kSCNetworkFlagsIsLocalAddress
                 This flag indicates that the specified nodename/address
                 is one associated with a network interface on the
current
                 system.

         @constant kSCNetworkFlagsIsDirect
                 This flag indicates that network traffic to the
specified
                 nodename/address will not go through a gateway but is
routed
                 directly to one of the interfaces in the system.
  */
enum {
         kSCNetworkFlagsTransientConnection = 1<<0,
         kSCNetworkFlagsReachable = 1<<1,
         kSCNetworkFlagsConnectionRequired = 1<<2,
         kSCNetworkFlagsConnectionAutomatic = 1<<3,
         kSCNetworkFlagsInterventionRequired = 1<<4,
         kSCNetworkFlagsIsLocalAddress = 1<<16,
         kSCNetworkFlagsIsDirect = 1<<17
};
typedef uint32_t SCNetworkConnectionFlags;

The function returns the result of the query in the structure, so it is
possible to see wether it is possible to establish a connection to the
named host. Typical usage:

SCNetworkConnectionFlags flags;
if( !SCNetworkCheckReachabilityByName( "www.apple.com", &flags );
        ; //handle error

if( flags & kSCNetworkFlagsReachable )
        ; // network is reachable

Some additional documentation exists in the SCNetwork.h header, and
other SystemConfiguration headers.

/ Regards, David Remahl

On måndag 14 juli 2003, at 20.23PM, Michael Roberts wrote:

> Is there a list of "ways to figure out connectedness" for various
> OS's?  That'd be a pretty interesting new feature for libcurl: a
> system-independent way of determining connectivity.  People ask about
> it often enough.
>
> Michael
>
> Andrew Francis wrote:
>
> On Mon, Jul 14, 2003 at 10:37:41AM +0100, Paul Johnson wrote:
>
> Fair point. I'm on win32 trying to talk to a unix server across an
> internet
> connection. curl has initalised successfully and returned with no
> errors,
> but I need to make my own program stop right there if an auto-dial flag
> isn't set and theres no internet connection.
>
> Take a look at InternetGetConnectedState and InternetDial, in
> <wininet.h>. You want something along these lines:
>
> DWORD inetInfo;
> if(InternetGetConnectedState(&inetInfo, 0) == FALSE) {
> if(!weAreAllowedToConnect)
> bombout(); // not connected and not allowed to fix that
>
> if(InternetDial(0, 0, 0, &inetInfo, 0) != ERROR_SUCCESS)
> bombout(); // we tried to dial up but couldn't
> }
>
> I've pretty much paraphrased this from some source I have lying around
> but I wrote it eons ago and don't remember what all of those zeroes
> mean,
> so I wouldn't recommend using this blindly without going to the docs
> first.
>
> You want to do this _before_ initialising WinSock. (I believe
> curl_easy_init() takes care of that these days, so do this before
> hitting curl).
>
> Cheers

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Received on 2003-07-14