ares_gethostbyname - Initiate a host query by name


#include <ares.h>
typedef void (*ares_host_callback)(void *arg, int status,
                                   int timeouts,
                                   struct hostent *hostent)
void ares_gethostbyname(ares_channel_t *channel, const char *name,
                        int family, ares_host_callback callback,
                        void *arg)


The ares_gethostbyname function initiates a host query by name on the name service channel identified by channel. The parameter name gives the hostname as a NUL-terminated C string, and family gives the desired type of address for the resulting host entry. When the query is complete or has failed, the ares library will invoke callback. Completion or failure of the query may happen immediately, or may happen during a later call to ares_process, ares_destroy or ares_cancel.

The callback argument arg is copied from the ares_gethostbyname argument arg. The callback argument status indicates whether the query succeeded and, if not, how it failed. It may have any of the following values:

ARES_SUCCESS The host lookup completed successfully.

ARES_ENOTIMP The ares library does not know how to find addresses of type family.

ARES_EBADNAME The hostname name is composed entirely of numbers and periods, but is not a valid representation of an Internet address.

ARES_ENODATA There was no data returned to extract a result from.

ARES_ENOTFOUND The name name was not found.

ARES_ENOMEM Memory was exhausted.

ARES_ECANCELLED The query was cancelled.

ARES_EDESTRUCTION The name service channel channel is being destroyed; the query will not be completed.

The callback argument timeouts reports how many times a query timed out during the execution of the given request.

On successful completion of the query, the callback argument hostent points to a struct hostent containing the name of the host returned by the query. The callback need not and should not attempt to free the memory pointed to by hostent; the ares library will free it when the callback returns. If the query did not complete successfully, hostent will be NULL.

If this is called from a thread other than which the main program event loop is running, care needs to be taken to ensure any file descriptor lists are updated immediately within the eventloop. When the associated callback is called, it is called with a channel lock so care must be taken to ensure any processing is minimal to prevent DNS channel stalls.

See also

ares_process(3), ares_gethostbyaddr(3)


Greg Hudson, MIT Information Systems
Copyright 1998 by the Massachusetts Institute of Technology.

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