1ASYNC_START_JOB(3)                  OpenSSL                 ASYNC_START_JOB(3)
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NAME

6       ASYNC_get_wait_ctx, ASYNC_init_thread, ASYNC_cleanup_thread,
7       ASYNC_start_job, ASYNC_pause_job, ASYNC_get_current_job,
8       ASYNC_block_pause, ASYNC_unblock_pause, ASYNC_is_capable - asynchronous
9       job management functions
10

SYNOPSIS

12        #include <openssl/async.h>
13
14        int ASYNC_init_thread(size_t max_size, size_t init_size);
15        void ASYNC_cleanup_thread(void);
16
17        int ASYNC_start_job(ASYNC_JOB **job, ASYNC_WAIT_CTX *ctx, int *ret,
18                            int (*func)(void *), void *args, size_t size);
19        int ASYNC_pause_job(void);
20
21        ASYNC_JOB *ASYNC_get_current_job(void);
22        ASYNC_WAIT_CTX *ASYNC_get_wait_ctx(ASYNC_JOB *job);
23        void ASYNC_block_pause(void);
24        void ASYNC_unblock_pause(void);
25
26        int ASYNC_is_capable(void);
27

DESCRIPTION

29       OpenSSL implements asynchronous capabilities through an ASYNC_JOB. This
30       represents code that can be started and executes until some event
31       occurs. At that point the code can be paused and control returns to
32       user code until some subsequent event indicates that the job can be
33       resumed.
34
35       The creation of an ASYNC_JOB is a relatively expensive operation.
36       Therefore, for efficiency reasons, jobs can be created up front and
37       reused many times. They are held in a pool until they are needed, at
38       which point they are removed from the pool, used, and then returned to
39       the pool when the job completes. If the user application is multi-
40       threaded, then ASYNC_init_thread() may be called for each thread that
41       will initiate asynchronous jobs. Before user code exits per-thread
42       resources need to be cleaned up. This will normally occur automatically
43       (see OPENSSL_init_crypto(3)) but may be explicitly initiated by using
44       ASYNC_cleanup_thread(). No asynchronous jobs must be outstanding for
45       the thread when ASYNC_cleanup_thread() is called. Failing to ensure
46       this will result in memory leaks.
47
48       The max_size argument limits the number of ASYNC_JOBs that will be held
49       in the pool. If max_size is set to 0 then no upper limit is set. When
50       an ASYNC_JOB is needed but there are none available in the pool already
51       then one will be automatically created, as long as the total of
52       ASYNC_JOBs managed by the pool does not exceed max_size. When the pool
53       is first initialised init_size ASYNC_JOBs will be created immediately.
54       If ASYNC_init_thread() is not called before the pool is first used then
55       it will be called automatically with a max_size of 0 (no upper limit)
56       and an init_size of 0 (no ASYNC_JOBs created up front).
57
58       An asynchronous job is started by calling the ASYNC_start_job()
59       function.  Initially *job should be NULL. ctx should point to an
60       ASYNC_WAIT_CTX object created through the ASYNC_WAIT_CTX_new(3)
61       function. ret should point to a location where the return value of the
62       asynchronous function should be stored on completion of the job. func
63       represents the function that should be started asynchronously. The data
64       pointed to by args and of size size will be copied and then passed as
65       an argument to func when the job starts.  ASYNC_start_job will return
66       one of the following values:
67
68       ASYNC_ERR
69           An error occurred trying to start the job. Check the OpenSSL error
70           queue (e.g.  see ERR_print_errors(3)) for more details.
71
72       ASYNC_NO_JOBS
73           There are no jobs currently available in the pool. This call can be
74           retried again at a later time.
75
76       ASYNC_PAUSE
77           The job was successfully started but was "paused" before it
78           completed (see ASYNC_pause_job() below). A handle to the job is
79           placed in *job. Other work can be performed (if desired) and the
80           job restarted at a later time. To restart a job call
81           ASYNC_start_job() again passing the job handle in *job. The func,
82           args and size parameters will be ignored when restarting a job.
83           When restarting a job ASYNC_start_job() must be called from the
84           same thread that the job was originally started from.
85
86       ASYNC_FINISH
87           The job completed. *job will be NULL and the return value from func
88           will be placed in *ret.
89
90       At any one time there can be a maximum of one job actively running per
91       thread (you can have many that are paused). ASYNC_get_current_job() can
92       be used to get a pointer to the currently executing ASYNC_JOB. If no
93       job is currently executing then this will return NULL.
94
95       If executing within the context of a job (i.e. having been called
96       directly or indirectly by the function "func" passed as an argument to
97       ASYNC_start_job()) then ASYNC_pause_job() will immediately return
98       control to the calling application with ASYNC_PAUSE returned from the
99       ASYNC_start_job() call. A subsequent call to ASYNC_start_job passing in
100       the relevant ASYNC_JOB in the *job parameter will resume execution from
101       the ASYNC_pause_job() call. If ASYNC_pause_job() is called whilst not
102       within the context of a job then no action is taken and
103       ASYNC_pause_job() returns immediately.
104
105       ASYNC_get_wait_ctx() can be used to get a pointer to the ASYNC_WAIT_CTX
106       for the job. ASYNC_WAIT_CTXs can have a "wait" file descriptor
107       associated with them. Applications can wait for the file descriptor to
108       be ready for "read" using a system function call such as select or poll
109       (being ready for "read" indicates that the job should be resumed). If
110       no file descriptor is made available then an application will have to
111       periodically "poll" the job by attempting to restart it to see if it is
112       ready to continue.
113
114       An example of typical usage might be an async capable engine. User code
115       would initiate cryptographic operations. The engine would initiate
116       those operations asynchronously and then call
117       ASYNC_WAIT_CTX_set_wait_fd(3) followed by ASYNC_pause_job() to return
118       control to the user code. The user code can then perform other tasks or
119       wait for the job to be ready by calling "select" or other similar
120       function on the wait file descriptor. The engine can signal to the user
121       code that the job should be resumed by making the wait file descriptor
122       "readable". Once resumed the engine should clear the wake signal on the
123       wait file descriptor.
124
125       The ASYNC_block_pause() function will prevent the currently active job
126       from pausing. The block will remain in place until a subsequent call to
127       ASYNC_unblock_pause(). These functions can be nested, e.g. if you call
128       ASYNC_block_pause() twice then you must call ASYNC_unblock_pause()
129       twice in order to re-enable pausing. If these functions are called
130       while there is no currently active job then they have no effect. This
131       functionality can be useful to avoid deadlock scenarios. For example
132       during the execution of an ASYNC_JOB an application acquires a lock. It
133       then calls some cryptographic function which invokes ASYNC_pause_job().
134       This returns control back to the code that created the ASYNC_JOB. If
135       that code then attempts to acquire the same lock before resuming the
136       original job then a deadlock can occur. By calling ASYNC_block_pause()
137       immediately after acquiring the lock and ASYNC_unblock_pause()
138       immediately before releasing it then this situation cannot occur.
139
140       Some platforms cannot support async operations. The ASYNC_is_capable()
141       function can be used to detect whether the current platform is async
142       capable or not.
143

RETURN VALUES

145       ASYNC_init_thread returns 1 on success or 0 otherwise.
146
147       ASYNC_start_job returns one of ASYNC_ERR, ASYNC_NO_JOBS, ASYNC_PAUSE or
148       ASYNC_FINISH as described above.
149
150       ASYNC_pause_job returns 0 if an error occurred or 1 on success. If
151       called when not within the context of an ASYNC_JOB then this is counted
152       as success so 1 is returned.
153
154       ASYNC_get_current_job returns a pointer to the currently executing
155       ASYNC_JOB or NULL if not within the context of a job.
156
157       ASYNC_get_wait_ctx() returns a pointer to the ASYNC_WAIT_CTX for the
158       job.
159
160       ASYNC_is_capable() returns 1 if the current platform is async capable
161       or 0 otherwise.
162

NOTES

164       On Windows platforms the openssl/async.h header is dependent on some of
165       the types customarily made available by including windows.h. The
166       application developer is likely to require control over when the latter
167       is included, commonly as one of the first included headers. Therefore
168       it is defined as an application developer's responsibility to include
169       windows.h prior to async.h.
170

EXAMPLE

172       The following example demonstrates how to use most of the core async
173       APIs:
174
175        #ifdef _WIN32
176        # include <windows.h>
177        #endif
178        #include <stdio.h>
179        #include <unistd.h>
180        #include <openssl/async.h>
181        #include <openssl/crypto.h>
182
183        int unique = 0;
184
185        void cleanup(ASYNC_WAIT_CTX *ctx, const void *key, OSSL_ASYNC_FD r, void *vw)
186        {
187            OSSL_ASYNC_FD *w = (OSSL_ASYNC_FD *)vw;
188
189            close(r);
190            close(*w);
191            OPENSSL_free(w);
192        }
193
194        int jobfunc(void *arg)
195        {
196            ASYNC_JOB *currjob;
197            unsigned char *msg;
198            int pipefds[2] = {0, 0};
199            OSSL_ASYNC_FD *wptr;
200            char buf = 'X';
201
202            currjob = ASYNC_get_current_job();
203            if (currjob != NULL) {
204                printf("Executing within a job\n");
205            } else {
206                printf("Not executing within a job - should not happen\n");
207                return 0;
208            }
209
210            msg = (unsigned char *)arg;
211            printf("Passed in message is: %s\n", msg);
212
213            if (pipe(pipefds) != 0) {
214                printf("Failed to create pipe\n");
215                return 0;
216            }
217            wptr = OPENSSL_malloc(sizeof(OSSL_ASYNC_FD));
218            if (wptr == NULL) {
219                printf("Failed to malloc\n");
220                return 0;
221            }
222            *wptr = pipefds[1];
223            ASYNC_WAIT_CTX_set_wait_fd(ASYNC_get_wait_ctx(currjob), &unique,
224                                       pipefds[0], wptr, cleanup);
225
226            /*
227             * Normally some external event would cause this to happen at some
228             * later point - but we do it here for demo purposes, i.e.
229             * immediately signalling that the job is ready to be woken up after
230             * we return to main via ASYNC_pause_job().
231             */
232            write(pipefds[1], &buf, 1);
233
234            /* Return control back to main */
235            ASYNC_pause_job();
236
237            /* Clear the wake signal */
238            read(pipefds[0], &buf, 1);
239
240            printf ("Resumed the job after a pause\n");
241
242            return 1;
243        }
244
245        int main(void)
246        {
247            ASYNC_JOB *job = NULL;
248            ASYNC_WAIT_CTX *ctx = NULL;
249            int ret;
250            OSSL_ASYNC_FD waitfd;
251            fd_set waitfdset;
252            size_t numfds;
253            unsigned char msg[13] = "Hello world!";
254
255            printf("Starting...\n");
256
257            ctx = ASYNC_WAIT_CTX_new();
258            if (ctx == NULL) {
259                printf("Failed to create ASYNC_WAIT_CTX\n");
260                abort();
261            }
262
263            for (;;) {
264                switch (ASYNC_start_job(&job, ctx, &ret, jobfunc, msg, sizeof(msg))) {
265                case ASYNC_ERR:
266                case ASYNC_NO_JOBS:
267                    printf("An error occurred\n");
268                    goto end;
269                case ASYNC_PAUSE:
270                    printf("Job was paused\n");
271                    break;
272                case ASYNC_FINISH:
273                    printf("Job finished with return value %d\n", ret);
274                    goto end;
275                }
276
277                /* Wait for the job to be woken */
278                printf("Waiting for the job to be woken up\n");
279
280                if (!ASYNC_WAIT_CTX_get_all_fds(ctx, NULL, &numfds)
281                        || numfds > 1) {
282                    printf("Unexpected number of fds\n");
283                    abort();
284                }
285                ASYNC_WAIT_CTX_get_all_fds(ctx, &waitfd, &numfds);
286                FD_ZERO(&waitfdset);
287                FD_SET(waitfd, &waitfdset);
288                select(waitfd + 1, &waitfdset, NULL, NULL, NULL);
289            }
290
291        end:
292            ASYNC_WAIT_CTX_free(ctx);
293            printf("Finishing\n");
294
295            return 0;
296        }
297
298       The expected output from executing the above example program is:
299
300        Starting...
301        Executing within a job
302        Passed in message is: Hello world!
303        Job was paused
304        Waiting for the job to be woken up
305        Resumed the job after a pause
306        Job finished with return value 1
307        Finishing
308

SEE ALSO

310       crypto(7), ERR_print_errors(3)
311

HISTORY

313       ASYNC_init_thread, ASYNC_cleanup_thread, ASYNC_start_job,
314       ASYNC_pause_job, ASYNC_get_current_job, ASYNC_get_wait_ctx(),
315       ASYNC_block_pause(), ASYNC_unblock_pause() and ASYNC_is_capable() were
316       first added to OpenSSL 1.1.0.
317
319       Copyright 2015-2016 The OpenSSL Project Authors. All Rights Reserved.
320
321       Licensed under the OpenSSL license (the "License").  You may not use
322       this file except in compliance with the License.  You can obtain a copy
323       in the file LICENSE in the source distribution or at
324       <https://www.openssl.org/source/license.html>.
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3281.1.1                             2018-09-11                ASYNC_START_JOB(3)
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