[#ASYNC_WAIT_CTX_set_callback]
= ASYNC_WAIT_CTX_set_callback
:mrdocs:

Register a callback notified when an engine completes an async operation.

== Synopsis

Declared in `<openssl/async.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
int
ASYNC_WAIT_CTX_set_callback(
    xref:ASYNC_WAIT_CTX.adoc[ASYNC_WAIT_CTX]* ctx,
    xref:ASYNC_callback_fn.adoc[ASYNC_callback_fn] callback,
    void* callback_arg);
----

== Description

Prefer this when a wait file descriptor is too costly or unavailable. Pair with ASYNC_WAIT_CTX_set_status() so the engine can report ASYNC_STATUS_* progress.

== Return Value

1 on success, or 0 on error.

== Parameters

[cols="1,4"]
|===
| Name| Description
| *ctx*
| Wait context that stores the callback.
| *callback*
| Function invoked from a poll or interrupt context; must be small and nonblocking.
| *callback_arg*
| User argument passed to `callback.`
|===


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