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(libc.info)Networks Database

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Networks Database
=================

   Many systems come with a database that records a list of networks
known to the system developer.  This is usually kept either in the file
`/etc/networks' or in an equivalent from a name server.  This data base
is useful for routing programs such as `route', but it is not useful
for programs that simply communicate over the network.  We provide
functions to access this database, which are declared in `netdb.h'.

 - Data Type: struct netent
     This data type is used to represent information about entries in
     the networks database.  It has the following members:

    `char *n_name'
          This is the "official" name of the network.

    `char **n_aliases'
          These are alternative names for the network, represented as a
          vector of strings.  A null pointer terminates the array.

    `int n_addrtype'
          This is the type of the network number; this is always equal
          to `AF_INET' for Internet networks.

    `unsigned long int n_net'
          This is the network number.  Network numbers are returned in
          host byte order; see Note: Byte Order.

   Use the `getnetbyname' or `getnetbyaddr' functions to search the
networks database for information about a specific network.  The
information is returned in a statically-allocated structure; you must
copy the information if you need to save it.

 - Function: struct netent * getnetbyname (const char *NAME)
     The `getnetbyname' function returns information about the network
     named NAME.  It returns a null pointer if there is no such network.

 - Function: struct netent * getnetbyaddr (unsigned long int NET, int
          TYPE)
     The `getnetbyaddr' function returns information about the network
     of type TYPE with number NET.  You should specify a value of
     `AF_INET' for the TYPE argument for Internet networks.

     `getnetbyaddr' returns a null pointer if there is no such network.

   You can also scan the networks database using `setnetent',
`getnetent' and `endnetent'.  Be careful when using these functions
because they are not reentrant.

 - Function: void setnetent (int STAYOPEN)
     This function opens and rewinds the networks database.

     If the STAYOPEN argument is nonzero, this sets a flag so that
     subsequent calls to `getnetbyname' or `getnetbyaddr' will not
     close the database (as they usually would).  This makes for more
     efficiency if you call those functions several times, by avoiding
     reopening the database for each call.

 - Function: struct netent * getnetent (void)
     This function returns the next entry in the networks database.  It
     returns a null pointer if there are no more entries.

 - Function: void endnetent (void)
     This function closes the networks database.