libval_async man page

val_async_submit() — submits a request for asynchronous processing of DNS queries.

val_async_select_info() — set the appropriate file descriptors for outstanding asynchronous requests.

val_async_check_wait() — handle timeouts or processes DNS responses to outstanding queries.

val_async_cancel() — cancel an asynchronous query request.

val_async_cancel_all() — cancel all asynchronous queries for a given context.


#include <validator/validator.h>

typedef int (*val_async_event_cb)(val_async_status *async_status,
                   int event, val_context_t *context,
                   void *cb_data, val_cb_params_t *cbp);

int val_async_submit(val_context_t *context,
                   const char * name, int class,
                   int type, unsigned int flags,
                   val_async_event_cb callback, void *cb_data,
                   val_async_status **async_status);

int val_async_select_info(val_context_t *context,
                   fd_set *fds,
                   int *num_fds,
                   struct timeval *timeout);

int val_async_check_wait(val_context_t *context,
                   fd_set *pending_desc, int *nfds,
                   struct timeval *tv, unsigned int flags);

int val_async_cancel(val_context_t *context,
                   val_async_status *as,
                   unsigned int flags);

int val_async_cancel_all(val_context_t *context,
                   unsigned int flags);


The asynchronous DNSSEC validator API allows an application to submit multiple requests, which can be processed in parallel. In most cases, this will result in validation completing much sooner than a series of synchronous requests.

The ctx parameter in the various functions below specifies the validation context, which can be set to NULL for default values (see libval(3) and dnsval.conf for more details on validation contexts and alidation policy).

The val_async_submit() function submits a request for asynchronous processing of DNS queries for the data associated with the given domain name, class and type.  The async_status object uniquely identifies a particular  request and provides a handle for future operations on that  asynchronous request, including cancelling it prior to lookup completion.

The flags parameter affects when and how often the  callback is called. The following flags are defined.


Don't use any internal cache for answers to this query.


Don't send any new queries. Answers will be returned  from the internal cache.


Caller doesn't care about the answer results.  This can be used for priming the cache.


Don't call any callbacks.


Call callbacks with results, but don't call any callbacks when the request is canceled.


Call the callback function with interim results. If this flag is not specified, the callback function will only be called when all validation results are ready.

When results from the asynchronous call become available, the  callback function (if non-NULL) will be called with  the cb_data value, originally supplied to the val_async_submit()  call, as one of its arguments. The results from the lookup are returned in cb_data, which is a pointer to the val_cb_params_t structure shown below.

    typedef struct val_cb_params_s {
        val_status_t             val_status;
        char                    *name;
        int                      class_h;
        int                      type_h;
        int                      retval;
        struct val_result_chain *results;
        struct val_answer_chain *answers;
    } val_cb_params_t;

The val_cb_params_t structure contains the orginal query parameters in name, class_h and type_h respectively, the  return value for the lookup operation in retval, pointers to the results and answers chains (see libval(3) for more details),  and the final validation status of the lookup operation in val_status. The application must release the memory associated with results and answers using the val_free_result_chain() and val_free_answer_chain() respectively (see libval(3) for more details).

On completion of the asynchronous lookup operation, an event code is returned in event.  The following event types are defined:


The request was completed.


The request is still being processed, but some interim results are available.


The request was canceled. The val_status, results and answers members of the callback parameter structure are undefined.

The val_async_select_info() function examines all outstanding asynchronous requests for the given context and sets the appropriate file descriptors, timeout value and maximum file descriptor value in preparation for a call to select(3).

The file descriptor for each socket awating a response is set in the fds parameter and max_fd is set to the highest file descriptor number of any pending asynchronous request unless that value is less than the current vaule of max_fd, in which case it is left unchanged. The timeout field is set to the lowest timeout value of any pending asynchronous query timeout which is less than the current value in this field by the application.

After the application calls select(3), it must also call  val_async_check_wait() with the fd_set and the number of ready file  descriptors, ndfs, returned by select(). The val_async_check_wait() function handles timeouts or processes DNS responses to outstanding queries.  It also call callbacks for completed requests.

val_async_check_wait() provides two modes of operation.  The first is for use with an application that has its own select() loop.  The applications sets its own file descriptors, calls val_async_select_info() to set file descriptors for pending queries and calls select(). The fds and nfds parameters from select are passed in to val_async_check_wait and the timeout value is ignored.  If responses for a file descriptor are processed, the  the appropriate file descriptor in fds is cleared and nfds is decremented.

In the second mode of operation, the application can set fds and nfds to NULL specify a value for timeout. Here,   val_async_select_info() and select() are called internally and any responses received before the timeout value expires are processed.

The val_async_cancel() function can be used to cancel the asynchronous request identified by its handle as, while val_async_cancel_all() can be used to cancel all asynchronous  requests associated with a given context. The following flag may be set for the cancellation request.


Do not call completed or cancelled callbacks.

Return Values

The val_async_submit() function returns VAL_NO_ERROR on success  and one of VAL_RESOURCE_UNAVAILABLE, VAL_BAD_ARGUMENT or VAL_INTERNAL_ERROR on failure.

val_async_select_info() returns VAL_NO_ERROR on success and VAL_BAD_ARGUMENT if an illegal argument was passed to the function.

val_async_check_wait() returns 0 when no pending requests are found and a positive integer when requests are still pending. A value less than zero on error.

val_async_cancel() and val_async_cancel_all() return VAL_NO_ERROR on success.


Robert Story

See Also



Referenced By

val_async_cancel(3), val_async_cancel_all(3), val_async_check_wait(3), val_async_select_info(3) and val_async_submit(3) are aliases of libval_async(3).

2013-03-11 perl v5.12.4 Programmer's Manual