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ASYNC_WAIT_CTX_NEW(3) OpenSSL ASYNC_WAIT_CTX_NEW(3)
ASYNC_WAIT_CTX_new, ASYNC_WAIT_CTX_free, ASYNC_WAIT_CTX_set_wait_fd,
ASYNC_WAIT_CTX_get_changed_fds, ASYNC_WAIT_CTX_clear_fd - functions to
manage waiting for asynchronous jobs to complete
void ASYNC_WAIT_CTX_free(ASYNC_WAIT_CTX *ctx);
int ASYNC_WAIT_CTX_set_wait_fd(ASYNC_WAIT_CTX *ctx, const void *key,
void (*cleanup)(ASYNC_WAIT_CTX *, const void *,
OSSL_ASYNC_FD, void *));
int ASYNC_WAIT_CTX_get_fd(ASYNC_WAIT_CTX *ctx, const void *key,
OSSL_ASYNC_FD *fd, void **custom_data);
int ASYNC_WAIT_CTX_get_all_fds(ASYNC_WAIT_CTX *ctx, OSSL_ASYNC_FD *fd,
int ASYNC_WAIT_CTX_get_changed_fds(ASYNC_WAIT_CTX *ctx, OSSL_ASYNC_FD *addfd,
size_t *numaddfds, OSSL_ASYNC_FD *delfd,
int ASYNC_WAIT_CTX_clear_fd(ASYNC_WAIT_CTX *ctx, const void *key);
For an overview of how asynchronous operations are implemented in
OpenSSL see ASYNC_start_job(3). An ASYNC_WAIT_CTX object represents an
asynchronous "session", i.e. a related set of crypto operations. For
example in SSL terms this would have a one-to-one correspondence with
an SSL connection.
Application code must create an ASYNC_WAIT_CTX using the
ASYNC_WAIT_CTX_new() function prior to calling ASYNC_start_job() (see
ASYNC_start_job(3)). When the job is started it is associated with the
ASYNC_WAIT_CTX for the duration of that job. An ASYNC_WAIT_CTX should
only be used for one ASYNC_JOB at any one time, but can be reused after
an ASYNC_JOB has finished for a subsequent ASYNC_JOB. When the session
is complete (e.g. the SSL connection is closed), application code
cleans up with ASYNC_WAIT_CTX_free().
ASYNC_WAIT_CTXs can have "wait" file descriptors associated with them.
Calling ASYNC_WAIT_CTX_get_all_fds() and passing in a pointer to an
ASYNC_WAIT_CTX in the ctx parameter will return the wait file
descriptors associated with that job in *fd. The number of file
descriptors returned will be stored in *numfds. It is the caller's
responsibility to ensure that sufficient memory has been allocated in
*fd to receive all the file descriptors. Calling
ASYNC_WAIT_CTX_get_all_fds() with a NULL fd value will return no file
descriptors but will still populate *numfds. Therefore, application
code is typically expected to call this function twice: once to get the
number of fds, and then again when sufficient memory has been
allocated. If only one asynchronous engine is being used then normally
this call will only ever return one fd. If multiple asynchronous
engines are being used then more could be returned.
The function ASYNC_WAIT_CTX_get_changed_fds() can be used to detect if
any fds have changed since the last call time ASYNC_start_job()
returned an ASYNC_PAUSE result (or since the ASYNC_WAIT_CTX was created
if no ASYNC_PAUSE result has been received). The numaddfds and
numdelfds parameters will be populated with the number of fds added or
deleted respectively. *addfd and *delfd will be populated with the list
of added and deleted fds respectively. Similarly to
ASYNC_WAIT_CTX_get_all_fds() either of these can be NULL, but if they
are not NULL then the caller is responsible for ensuring sufficient
memory is allocated.
Implementors of async aware code (e.g. engines) are encouraged to
return a stable fd for the lifetime of the ASYNC_WAIT_CTX in order to
reduce the "churn" of regularly changing fds - although no guarantees
of this are provided to applications.
Applications can wait for the file descriptor to be ready for "read"
using a system function call such as select or poll (being ready for
"read" indicates that the job should be resumed). If no file descriptor
is made available then an application will have to periodically "poll"
the job by attempting to restart it to see if it is ready to continue.
Async aware code (e.g. engines) can get the current ASYNC_WAIT_CTX from
the job via ASYNC_get_wait_ctx(3) and provide a file descriptor to use
for waiting on by calling ASYNC_WAIT_CTX_set_wait_fd(). Typically this
would be done by an engine immediately prior to calling
ASYNC_pause_job() and not by end user code. An existing association
with a file descriptor can be obtained using ASYNC_WAIT_CTX_get_fd()
and cleared using ASYNC_WAIT_CTX_clear_fd(). Both of these functions
requires a key value which is unique to the async aware code. This
could be any unique value but a good candidate might be the ENGINE *
for the engine. The custom_data parameter can be any value, and will be
returned in a subsequent call to ASYNC_WAIT_CTX_get_fd(). The
ASYNC_WAIT_CTX_set_wait_fd() function also expects a pointer to a
"cleanup" routine. This can be NULL but if provided will automatically
get called when the ASYNC_WAIT_CTX is freed, and gives the engine the
opportunity to close the fd or any other resources. Note: The "cleanup"
routine does not get called if the fd is cleared directly via a call to
An example of typical usage might be an async capable engine. User code
would initiate cryptographic operations. The engine would initiate
those operations asynchronously and then call
ASYNC_WAIT_CTX_set_wait_fd() followed by ASYNC_pause_job() to return
control to the user code. The user code can then perform other tasks or
wait for the job to be ready by calling "select" or other similar
function on the wait file descriptor. The engine can signal to the user
code that the job should be resumed by making the wait file descriptor
"readable". Once resumed the engine should clear the wake signal on the
wait file descriptor.
ASYNC_WAIT_CTX_new() returns a pointer to the newly allocated
ASYNC_WAIT_CTX or NULL on error.
ASYNC_WAIT_CTX_get_all_fds, ASYNC_WAIT_CTX_get_changed_fds and
ASYNC_WAIT_CTX_clear_fd all return 1 on success or 0 on error.
On Windows platforms the openssl/async.h header is dependent on some of
the types customarily made available by including windows.h. The
application developer is likely to require control over when the latter
is included, commonly as one of the first included headers. Therefore,
it is defined as an application developer's responsibility to include
windows.h prior to async.h.
ASYNC_WAIT_CTX_get_all_fds(), ASYNC_WAIT_CTX_get_changed_fds() and
ASYNC_WAIT_CTX_clear_fd() were added in OpenSSL 1.1.0.
Copyright 2016-2020 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy
in the file LICENSE in the source distribution or at
1.1.1q 2022-07-05 ASYNC_WAIT_CTX_NEW(3)