NAME

       ASYNC_WAIT_CTX_new, ASYNC_WAIT_CTX_free, ASYNC_WAIT_CTX_set_wait_fd, ASYNC_WAIT_CTX_get_fd,
       ASYNC_WAIT_CTX_get_all_fds, ASYNC_WAIT_CTX_get_changed_fds, ASYNC_WAIT_CTX_clear_fd,
       ASYNC_WAIT_CTX_set_callback, ASYNC_WAIT_CTX_get_callback, ASYNC_WAIT_CTX_set_status,
       ASYNC_WAIT_CTX_get_status, ASYNC_callback_fn, ASYNC_STATUS_UNSUPPORTED, ASYNC_STATUS_ERR,
       ASYNC_STATUS_OK, ASYNC_STATUS_EAGAIN - functions to manage waiting for asynchronous jobs to complete

SYNOPSIS

        #include <openssl/async.h>

        #define ASYNC_STATUS_UNSUPPORTED    0
        #define ASYNC_STATUS_ERR            1
        #define ASYNC_STATUS_OK             2
        #define ASYNC_STATUS_EAGAIN         3
        typedef int (*ASYNC_callback_fn)(void *arg);
        ASYNC_WAIT_CTX *ASYNC_WAIT_CTX_new(void);
        void ASYNC_WAIT_CTX_free(ASYNC_WAIT_CTX *ctx);
        int ASYNC_WAIT_CTX_set_wait_fd(ASYNC_WAIT_CTX *ctx, const void *key,
                                       OSSL_ASYNC_FD fd,
                                       void *custom_data,
                                       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,
                                       size_t *numfds);
        int ASYNC_WAIT_CTX_get_changed_fds(ASYNC_WAIT_CTX *ctx, OSSL_ASYNC_FD *addfd,
                                           size_t *numaddfds, OSSL_ASYNC_FD *delfd,
                                           size_t *numdelfds);
        int ASYNC_WAIT_CTX_clear_fd(ASYNC_WAIT_CTX *ctx, const void *key);
        int ASYNC_WAIT_CTX_set_callback(ASYNC_WAIT_CTX *ctx,
                                        ASYNC_callback_fn callback,
                                        void *callback_arg);
        int ASYNC_WAIT_CTX_get_callback(ASYNC_WAIT_CTX *ctx,
                                        ASYNC_callback_fn *callback,
                                        void **callback_arg);
        int ASYNC_WAIT_CTX_set_status(ASYNC_WAIT_CTX *ctx, int status);
        int ASYNC_WAIT_CTX_get_status(ASYNC_WAIT_CTX *ctx);

DESCRIPTION

       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 ASYNC_PAUSE (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.

       Implementers 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 ASYNC_WAIT_CTX_clear_fd().

       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.

       As well as a file descriptor, user code may also be notified via a callback. The callback and data
       pointers are stored within the ASYNC_WAIT_CTX along with an additional status field that can be used for
       the notification of retries from an engine. This additional method can be used when the user thinks that
       a file descriptor is too costly in terms of CPU cycles or in some context where a file descriptor is not
       appropriate.

       ASYNC_WAIT_CTX_set_callback() sets the callback and the callback argument. The callback will be called to
       notify user code when an engine completes a cryptography operation. It is a requirement that the callback
       function is small and nonblocking as it will be run in the context of a polling mechanism or an
       interrupt.

       ASYNC_WAIT_CTX_get_callback() returns the callback set in the ASYNC_WAIT_CTX structure.

       ASYNC_WAIT_CTX_set_status() allows an engine to set the current engine status.  The possible status
       values are the following:

       ASYNC_STATUS_UNSUPPORTED
           The engine does not support the callback mechanism. This is the default value.  The engine must call
           ASYNC_WAIT_CTX_set_status() to set the status to some value other than ASYNC_STATUS_UNSUPPORTED if it
           intends to enable the callback mechanism.

       ASYNC_STATUS_ERR
           The engine has a fatal problem with this request. The user code should clean up this session.

       ASYNC_STATUS_OK
           The request has been successfully submitted.

       ASYNC_STATUS_EAGAIN
           The engine has some problem which will be recovered soon, such as a buffer is full, so user code
           should resume the job.

       ASYNC_WAIT_CTX_get_status() allows user code to obtain the current status value.  If the status is any
       value other than ASYNC_STATUS_OK then the user code should not expect to receive a callback from the
       engine even if one has been set.

       An example of the usage of the callback method might be the following. User code would initiate
       cryptographic operations, and the engine code would dispatch this operation to hardware, and if the
       dispatch is successful, then the engine code would call ASYNC_pause_job() to return control to the user
       code. After that, user code can perform other tasks. When the hardware completes the operation, normally
       it is detected by a polling function or an interrupt, as the user code set a callback by calling
       ASYNC_WAIT_CTX_set_callback() previously, then the registered callback will be called.

RETURN VALUES

       ASYNC_WAIT_CTX_new() returns a pointer to the newly allocated ASYNC_WAIT_CTX or NULL on error.

       ASYNC_WAIT_CTX_set_wait_fd, ASYNC_WAIT_CTX_get_fd, ASYNC_WAIT_CTX_get_all_fds,
       ASYNC_WAIT_CTX_get_changed_fds, ASYNC_WAIT_CTX_clear_fd, ASYNC_WAIT_CTX_set_callback,
       ASYNC_WAIT_CTX_get_callback and ASYNC_WAIT_CTX_set_status all return 1 on success or 0 on error.
       ASYNC_WAIT_CTX_get_status() returns the engine status.

NOTES

       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 <openssl/async.h>.

SEE ALSO

       crypto(7), ASYNC_start_job(3)

HISTORY

       ASYNC_WAIT_CTX_new(), ASYNC_WAIT_CTX_free(), ASYNC_WAIT_CTX_set_wait_fd(), ASYNC_WAIT_CTX_get_fd(),
       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.

       ASYNC_WAIT_CTX_set_callback(), ASYNC_WAIT_CTX_get_callback(), ASYNC_WAIT_CTX_set_status(), and
       ASYNC_WAIT_CTX_get_status() were added in OpenSSL 3.0.

COPYRIGHT

       Copyright 2016-2023 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (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
       <https://www.openssl.org/source/license.html>.