xref: /openssl/doc/man3/ASYNC_WAIT_CTX_new.pod (revision 4333b89f) 
1=pod
2
3=head1 NAME
4
5ASYNC_WAIT_CTX_new, ASYNC_WAIT_CTX_free, ASYNC_WAIT_CTX_set_wait_fd,
6ASYNC_WAIT_CTX_get_fd, ASYNC_WAIT_CTX_get_all_fds,
7ASYNC_WAIT_CTX_get_changed_fds, ASYNC_WAIT_CTX_clear_fd,
8ASYNC_WAIT_CTX_set_callback, ASYNC_WAIT_CTX_get_callback,
9ASYNC_WAIT_CTX_set_status, ASYNC_WAIT_CTX_get_status, ASYNC_callback_fn,
10ASYNC_STATUS_UNSUPPORTED, ASYNC_STATUS_ERR, ASYNC_STATUS_OK,
11ASYNC_STATUS_EAGAIN
12- functions to manage waiting for asynchronous jobs to complete
13
14=head1 SYNOPSIS
15
16 #include <openssl/async.h>
17
18 #define ASYNC_STATUS_UNSUPPORTED    0
19 #define ASYNC_STATUS_ERR            1
20 #define ASYNC_STATUS_OK             2
21 #define ASYNC_STATUS_EAGAIN         3
22 typedef int (*ASYNC_callback_fn)(void *arg);
23 ASYNC_WAIT_CTX *ASYNC_WAIT_CTX_new(void);
24 void ASYNC_WAIT_CTX_free(ASYNC_WAIT_CTX *ctx);
25 int ASYNC_WAIT_CTX_set_wait_fd(ASYNC_WAIT_CTX *ctx, const void *key,
26                                OSSL_ASYNC_FD fd,
27                                void *custom_data,
28                                void (*cleanup)(ASYNC_WAIT_CTX *, const void *,
29                                                OSSL_ASYNC_FD, void *));
30 int ASYNC_WAIT_CTX_get_fd(ASYNC_WAIT_CTX *ctx, const void *key,
31                           OSSL_ASYNC_FD *fd, void **custom_data);
32 int ASYNC_WAIT_CTX_get_all_fds(ASYNC_WAIT_CTX *ctx, OSSL_ASYNC_FD *fd,
33                                size_t *numfds);
34 int ASYNC_WAIT_CTX_get_changed_fds(ASYNC_WAIT_CTX *ctx, OSSL_ASYNC_FD *addfd,
35                                    size_t *numaddfds, OSSL_ASYNC_FD *delfd,
36                                    size_t *numdelfds);
37 int ASYNC_WAIT_CTX_clear_fd(ASYNC_WAIT_CTX *ctx, const void *key);
38 int ASYNC_WAIT_CTX_set_callback(ASYNC_WAIT_CTX *ctx,
39                                 ASYNC_callback_fn callback,
40                                 void *callback_arg);
41 int ASYNC_WAIT_CTX_get_callback(ASYNC_WAIT_CTX *ctx,
42                                 ASYNC_callback_fn *callback,
43                                 void **callback_arg);
44 int ASYNC_WAIT_CTX_set_status(ASYNC_WAIT_CTX *ctx, int status);
45 int ASYNC_WAIT_CTX_get_status(ASYNC_WAIT_CTX *ctx);
46
47
48=head1 DESCRIPTION
49
50For an overview of how asynchronous operations are implemented in OpenSSL see
51L<ASYNC_start_job(3)>. An B<ASYNC_WAIT_CTX> object represents an asynchronous
52"session", i.e. a related set of crypto operations. For example in SSL terms
53this would have a one-to-one correspondence with an SSL connection.
54
55Application code must create an B<ASYNC_WAIT_CTX> using the ASYNC_WAIT_CTX_new()
56function prior to calling ASYNC_start_job() (see L<ASYNC_start_job(3)>). When
57the job is started it is associated with the B<ASYNC_WAIT_CTX> for the duration
58of that job. An B<ASYNC_WAIT_CTX> should only be used for one B<ASYNC_JOB> at
59any one time, but can be reused after an B<ASYNC_JOB> has finished for a
60subsequent B<ASYNC_JOB>. When the session is complete (e.g. the SSL connection
61is closed), application code cleans up with ASYNC_WAIT_CTX_free().
62
63B<ASYNC_WAIT_CTX>s can have "wait" file descriptors associated with them.
64Calling ASYNC_WAIT_CTX_get_all_fds() and passing in a pointer to an
65B<ASYNC_WAIT_CTX> in the I<ctx> parameter will return the wait file descriptors
66associated with that job in I<*fd>. The number of file descriptors returned will
67be stored in I<*numfds>. It is the caller's responsibility to ensure that
68sufficient memory has been allocated in I<*fd> to receive all the file
69descriptors. Calling ASYNC_WAIT_CTX_get_all_fds() with a NULL I<fd> value will
70return no file descriptors but will still populate I<*numfds>. Therefore,
71application code is typically expected to call this function twice: once to get
72the number of fds, and then again when sufficient memory has been allocated. If
73only one asynchronous engine is being used then normally this call will only
74ever return one fd. If multiple asynchronous engines are being used then more
75could be returned.
76
77The function ASYNC_WAIT_CTX_get_changed_fds() can be used to detect if any fds
78have changed since the last call time ASYNC_start_job() returned B<ASYNC_PAUSE>
79(or since the B<ASYNC_WAIT_CTX> was created if no B<ASYNC_PAUSE> result has
80been received). The I<numaddfds> and I<numdelfds> parameters will be populated
81with the number of fds added or deleted respectively. I<*addfd> and I<*delfd>
82will be populated with the list of added and deleted fds respectively. Similarly
83to ASYNC_WAIT_CTX_get_all_fds() either of these can be NULL, but if they are not
84NULL then the caller is responsible for ensuring sufficient memory is allocated.
85
86Implementors of async aware code (e.g. engines) are encouraged to return a
87stable fd for the lifetime of the B<ASYNC_WAIT_CTX> in order to reduce the
88"churn" of regularly changing fds - although no guarantees of this are provided
89to applications.
90
91Applications can wait for the file descriptor to be ready for "read" using a
92system function call such as select or poll (being ready for "read" indicates
93that the job should be resumed). If no file descriptor is made available then an
94application will have to periodically "poll" the job by attempting to restart it
95to see if it is ready to continue.
96
97Async aware code (e.g. engines) can get the current B<ASYNC_WAIT_CTX> from the
98job via L<ASYNC_get_wait_ctx(3)> and provide a file descriptor to use for
99waiting on by calling ASYNC_WAIT_CTX_set_wait_fd(). Typically this would be done
100by an engine immediately prior to calling ASYNC_pause_job() and not by end user
101code. An existing association with a file descriptor can be obtained using
102ASYNC_WAIT_CTX_get_fd() and cleared using ASYNC_WAIT_CTX_clear_fd(). Both of
103these functions requires a I<key> value which is unique to the async aware
104code.  This could be any unique value but a good candidate might be the
105B<ENGINE *> for the engine. The I<custom_data> parameter can be any value, and
106will be returned in a subsequent call to ASYNC_WAIT_CTX_get_fd(). The
107ASYNC_WAIT_CTX_set_wait_fd() function also expects a pointer to a "cleanup"
108routine. This can be NULL but if provided will automatically get called when
109the B<ASYNC_WAIT_CTX> is freed, and gives the engine the opportunity to close
110the fd or any other resources. Note: The "cleanup" routine does not get called
111if the fd is cleared directly via a call to ASYNC_WAIT_CTX_clear_fd().
112
113An example of typical usage might be an async capable engine. User code would
114initiate cryptographic operations. The engine would initiate those operations
115asynchronously and then call ASYNC_WAIT_CTX_set_wait_fd() followed by
116ASYNC_pause_job() to return control to the user code. The user code can then
117perform other tasks or wait for the job to be ready by calling "select" or other
118similar function on the wait file descriptor. The engine can signal to the user
119code that the job should be resumed by making the wait file descriptor
120"readable". Once resumed the engine should clear the wake signal on the wait
121file descriptor.
122
123As well as a file descriptor, user code may also be notified via a callback. The
124callback and data pointers are stored within the B<ASYNC_WAIT_CTX> along with an
125additional status field that can be used for the notification of retries from an
126engine. This additional method can be used when the user thinks that a file
127descriptor is too costly in terms of CPU cycles or in some context where a file
128descriptor is not appropriate.
129
130ASYNC_WAIT_CTX_set_callback() sets the callback and the callback argument. The
131callback will be called to notify user code when an engine completes a
132cryptography operation. It is a requirement that the callback function is small
133and nonblocking as it will be run in the context of a polling mechanism or an
134interrupt.
135
136ASYNC_WAIT_CTX_get_callback() returns the callback set in the B<ASYNC_WAIT_CTX>
137structure.
138
139ASYNC_WAIT_CTX_set_status() allows an engine to set the current engine status.
140The possible status values are the following:
141
142=over 4
143
144=item B<ASYNC_STATUS_UNSUPPORTED>
145
146The engine does not support the callback mechanism. This is the default value.
147The engine must call ASYNC_WAIT_CTX_set_status() to set the status to some value
148other than B<ASYNC_STATUS_UNSUPPORTED> if it intends to enable the callback
149mechanism.
150
151=item B<ASYNC_STATUS_ERR>
152
153The engine has a fatal problem with this request. The user code should clean up
154this session.
155
156=item B<ASYNC_STATUS_OK>
157
158The request has been successfully submitted.
159
160=item B<ASYNC_STATUS_EAGAIN>
161
162The engine has some problem which will be recovered soon, such as a buffer is
163full, so user code should resume the job.
164
165=back
166
167ASYNC_WAIT_CTX_get_status() allows user code to obtain the current status value.
168If the status is any value other than B<ASYNC_STATUS_OK> then the user code
169should not expect to receive a callback from the engine even if one has been
170set.
171
172An example of the usage of the callback method might be the following. User
173code would initiate cryptographic operations, and the engine code would dispatch
174this operation to hardware, and if the dispatch is successful, then the engine
175code would call ASYNC_pause_job() to return control to the user code. After
176that, user code can perform other tasks. When the hardware completes the
177operation, normally it is detected by a polling function or an interrupt, as the
178user code set a callback by calling ASYNC_WAIT_CTX_set_callback() previously,
179then the registered callback will be called.
180
181=head1 RETURN VALUES
182
183ASYNC_WAIT_CTX_new() returns a pointer to the newly allocated B<ASYNC_WAIT_CTX>
184or NULL on error.
185
186ASYNC_WAIT_CTX_set_wait_fd, ASYNC_WAIT_CTX_get_fd, ASYNC_WAIT_CTX_get_all_fds,
187ASYNC_WAIT_CTX_get_changed_fds, ASYNC_WAIT_CTX_clear_fd,
188ASYNC_WAIT_CTX_set_callback, ASYNC_WAIT_CTX_get_callback and
189ASYNC_WAIT_CTX_set_status all return 1 on success or 0 on error.
190ASYNC_WAIT_CTX_get_status() returns the engine status.
191
192
193=head1 NOTES
194
195On Windows platforms the F<< <openssl/async.h> >> header is dependent on some
196of the types customarily made available by including F<< <windows.h> >>. The
197application developer is likely to require control over when the latter
198is included, commonly as one of the first included headers. Therefore,
199it is defined as an application developer's responsibility to include
200F<< <windows.h> >> prior to F<< <openssl/async.h> >>.
201
202=head1 SEE ALSO
203
204L<crypto(7)>, L<ASYNC_start_job(3)>
205
206=head1 HISTORY
207
208ASYNC_WAIT_CTX_new(), ASYNC_WAIT_CTX_free(), ASYNC_WAIT_CTX_set_wait_fd(),
209ASYNC_WAIT_CTX_get_fd(), ASYNC_WAIT_CTX_get_all_fds(),
210ASYNC_WAIT_CTX_get_changed_fds() and ASYNC_WAIT_CTX_clear_fd()
211were added in OpenSSL 1.1.0.
212
213ASYNC_WAIT_CTX_set_callback(), ASYNC_WAIT_CTX_get_callback(),
214ASYNC_WAIT_CTX_set_status(), and ASYNC_WAIT_CTX_get_status()
215were added in OpenSSL 3.0.
216
217=head1 COPYRIGHT
218
219Copyright 2016-2021 The OpenSSL Project Authors. All Rights Reserved.
220
221Licensed under the Apache License 2.0 (the "License").  You may not use
222this file except in compliance with the License.  You can obtain a copy
223in the file LICENSE in the source distribution or at
224L<https://www.openssl.org/source/license.html>.
225
226=cut
227