source: vbox/trunk/src/libs/openssl-3.0.1/doc/man3/ASYNC_start_job.pod@ 94081

=pod

=head1 NAME

ASYNC_get_wait_ctx,
ASYNC_init_thread, ASYNC_cleanup_thread, ASYNC_start_job, ASYNC_pause_job,
ASYNC_get_current_job, ASYNC_block_pause, ASYNC_unblock_pause, ASYNC_is_capable
- asynchronous job management functions

=head1 SYNOPSIS

 #include <openssl/async.h>

 int ASYNC_init_thread(size_t max_size, size_t init_size);
 void ASYNC_cleanup_thread(void);

 int ASYNC_start_job(ASYNC_JOB **job, ASYNC_WAIT_CTX *ctx, int *ret,
                     int (*func)(void *), void *args, size_t size);
 int ASYNC_pause_job(void);

 ASYNC_JOB *ASYNC_get_current_job(void);
 ASYNC_WAIT_CTX *ASYNC_get_wait_ctx(ASYNC_JOB *job);
 void ASYNC_block_pause(void);
 void ASYNC_unblock_pause(void);

 int ASYNC_is_capable(void);

=head1 DESCRIPTION

OpenSSL implements asynchronous capabilities through an ASYNC_JOB. This
represents code that can be started and executes until some event occurs. At
that point the code can be paused and control returns to user code until some
subsequent event indicates that the job can be resumed.

The creation of an ASYNC_JOB is a relatively expensive operation. Therefore, for
efficiency reasons, jobs can be created up front and reused many times. They are
held in a pool until they are needed, at which point they are removed from the
pool, used, and then returned to the pool when the job completes. If the user
application is multi-threaded, then ASYNC_init_thread() may be called for each
thread that will initiate asynchronous jobs. Before
user code exits per-thread resources need to be cleaned up. This will normally
occur automatically (see L<OPENSSL_init_crypto(3)>) but may be explicitly
initiated by using ASYNC_cleanup_thread(). No asynchronous jobs must be
outstanding for the thread when ASYNC_cleanup_thread() is called. Failing to
ensure this will result in memory leaks.

The B<max_size> argument limits the number of ASYNC_JOBs that will be held in
the pool. If B<max_size> is set to 0 then no upper limit is set. When an
ASYNC_JOB is needed but there are none available in the pool already then one
will be automatically created, as long as the total of ASYNC_JOBs managed by the
pool does not exceed B<max_size>. When the pool is first initialised
B<init_size> ASYNC_JOBs will be created immediately. If ASYNC_init_thread() is
not called before the pool is first used then it will be called automatically
with a B<max_size> of 0 (no upper limit) and an B<init_size> of 0 (no ASYNC_JOBs
created up front).

An asynchronous job is started by calling the ASYNC_start_job() function.
Initially B<*job> should be NULL. B<ctx> should point to an ASYNC_WAIT_CTX
object created through the L<ASYNC_WAIT_CTX_new(3)> function. B<ret> should
point to a location where the return value of the asynchronous function should
be stored on completion of the job. B<func> represents the function that should
be started asynchronously. The data pointed to by B<args> and of size B<size>
will be copied and then passed as an argument to B<func> when the job starts.
ASYNC_start_job will return one of the following values:

=over 4

=item B<ASYNC_ERR>

An error occurred trying to start the job. Check the OpenSSL error queue (e.g.
see L<ERR_print_errors(3)>) for more details.

=item B<ASYNC_NO_JOBS>

There are no jobs currently available in the pool. This call can be retried
again at a later time.

=item B<ASYNC_PAUSE>

The job was successfully started but was "paused" before it completed (see
ASYNC_pause_job() below). A handle to the job is placed in B<*job>. Other work
can be performed (if desired) and the job restarted at a later time. To restart
a job call ASYNC_start_job() again passing the job handle in B<*job>. The
B<func>, B<args> and B<size> parameters will be ignored when restarting a job.
When restarting a job ASYNC_start_job() B<must> be called from the same thread
that the job was originally started from.

=item B<ASYNC_FINISH>

The job completed. B<*job> will be NULL and the return value from B<func> will
be placed in B<*ret>.

=back

At any one time there can be a maximum of one job actively running per thread
(you can have many that are paused). ASYNC_get_current_job() can be used to get
a pointer to the currently executing ASYNC_JOB. If no job is currently executing
then this will return NULL.

If executing within the context of a job (i.e. having been called directly or
indirectly by the function "func" passed as an argument to ASYNC_start_job())
then ASYNC_pause_job() will immediately return control to the calling
application with ASYNC_PAUSE returned from the ASYNC_start_job() call. A
subsequent call to ASYNC_start_job passing in the relevant ASYNC_JOB in the
B<*job> parameter will resume execution from the ASYNC_pause_job() call. If
ASYNC_pause_job() is called whilst not within the context of a job then no
action is taken and ASYNC_pause_job() returns immediately.

ASYNC_get_wait_ctx() can be used to get a pointer to the ASYNC_WAIT_CTX
for the B<job>. ASYNC_WAIT_CTXs can have a "wait" file descriptor associated
with them. 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.

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 L<ASYNC_WAIT_CTX_set_wait_fd(3)> 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.

The ASYNC_block_pause() function will prevent the currently active job from
pausing. The block will remain in place until a subsequent call to
ASYNC_unblock_pause(). These functions can be nested, e.g. if you call
ASYNC_block_pause() twice then you must call ASYNC_unblock_pause() twice in
order to re-enable pausing. If these functions are called while there is no
currently active job then they have no effect. This functionality can be useful
to avoid deadlock scenarios. For example during the execution of an ASYNC_JOB an
application acquires a lock. It then calls some cryptographic function which
invokes ASYNC_pause_job(). This returns control back to the code that created
the ASYNC_JOB. If that code then attempts to acquire the same lock before
resuming the original job then a deadlock can occur. By calling
ASYNC_block_pause() immediately after acquiring the lock and
ASYNC_unblock_pause() immediately before releasing it then this situation cannot
occur.

Some platforms cannot support async operations. The ASYNC_is_capable() function
can be used to detect whether the current platform is async capable or not.

=head1 RETURN VALUES

ASYNC_init_thread returns 1 on success or 0 otherwise.

ASYNC_start_job returns one of ASYNC_ERR, ASYNC_NO_JOBS, ASYNC_PAUSE or
ASYNC_FINISH as described above.

ASYNC_pause_job returns 0 if an error occurred or 1 on success. If called when
not within the context of an ASYNC_JOB then this is counted as success so 1 is
returned.

ASYNC_get_current_job returns a pointer to the currently executing ASYNC_JOB or
NULL if not within the context of a job.

ASYNC_get_wait_ctx() returns a pointer to the ASYNC_WAIT_CTX for the job.

ASYNC_is_capable() returns 1 if the current platform is async capable or 0
otherwise.

=head1 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 async.h.

=head1 EXAMPLES

The following example demonstrates how to use most of the core async APIs:

 #ifdef _WIN32
 # include <windows.h>
 #endif
 #include <stdio.h>
 #include <unistd.h>
 #include <openssl/async.h>
 #include <openssl/crypto.h>

 int unique = 0;

 void cleanup(ASYNC_WAIT_CTX *ctx, const void *key, OSSL_ASYNC_FD r, void *vw)
 {
     OSSL_ASYNC_FD *w = (OSSL_ASYNC_FD *)vw;

     close(r);
     close(*w);
     OPENSSL_free(w);
 }

 int jobfunc(void *arg)
 {
     ASYNC_JOB *currjob;
     unsigned char *msg;
     int pipefds[2] = {0, 0};
     OSSL_ASYNC_FD *wptr;
     char buf = 'X';

     currjob = ASYNC_get_current_job();
     if (currjob != NULL) {
         printf("Executing within a job\n");
     } else {
         printf("Not executing within a job - should not happen\n");
         return 0;
     }

     msg = (unsigned char *)arg;
     printf("Passed in message is: %s\n", msg);

     if (pipe(pipefds) != 0) {
         printf("Failed to create pipe\n");
         return 0;
     }
     wptr = OPENSSL_malloc(sizeof(OSSL_ASYNC_FD));
     if (wptr == NULL) {
         printf("Failed to malloc\n");
         return 0;
     }
     *wptr = pipefds[1];
     ASYNC_WAIT_CTX_set_wait_fd(ASYNC_get_wait_ctx(currjob), &unique,
                                pipefds[0], wptr, cleanup);

     /*
      * Normally some external event would cause this to happen at some
      * later point - but we do it here for demo purposes, i.e.
      * immediately signalling that the job is ready to be woken up after
      * we return to main via ASYNC_pause_job().
      */
     write(pipefds[1], &buf, 1);

     /* Return control back to main */
     ASYNC_pause_job();

     /* Clear the wake signal */
     read(pipefds[0], &buf, 1);

     printf ("Resumed the job after a pause\n");

     return 1;
 }

 int main(void)
 {
     ASYNC_JOB *job = NULL;
     ASYNC_WAIT_CTX *ctx = NULL;
     int ret;
     OSSL_ASYNC_FD waitfd;
     fd_set waitfdset;
     size_t numfds;
     unsigned char msg[13] = "Hello world!";

     printf("Starting...\n");

     ctx = ASYNC_WAIT_CTX_new();
     if (ctx == NULL) {
         printf("Failed to create ASYNC_WAIT_CTX\n");
         abort();
     }

     for (;;) {
         switch (ASYNC_start_job(&job, ctx, &ret, jobfunc, msg, sizeof(msg))) {
         case ASYNC_ERR:
         case ASYNC_NO_JOBS:
             printf("An error occurred\n");
             goto end;
         case ASYNC_PAUSE:
             printf("Job was paused\n");
             break;
         case ASYNC_FINISH:
             printf("Job finished with return value %d\n", ret);
             goto end;
         }

         /* Wait for the job to be woken */
         printf("Waiting for the job to be woken up\n");

         if (!ASYNC_WAIT_CTX_get_all_fds(ctx, NULL, &numfds)
                 || numfds > 1) {
             printf("Unexpected number of fds\n");
             abort();
         }
         ASYNC_WAIT_CTX_get_all_fds(ctx, &waitfd, &numfds);
         FD_ZERO(&waitfdset);
         FD_SET(waitfd, &waitfdset);
         select(waitfd + 1, &waitfdset, NULL, NULL, NULL);
     }

 end:
     ASYNC_WAIT_CTX_free(ctx);
     printf("Finishing\n");

     return 0;
 }

The expected output from executing the above example program is:

 Starting...
 Executing within a job
 Passed in message is: Hello world!
 Job was paused
 Waiting for the job to be woken up
 Resumed the job after a pause
 Job finished with return value 1
 Finishing

=head1 SEE ALSO

L<crypto(7)>, L<ERR_print_errors(3)>

=head1 HISTORY

ASYNC_init_thread, ASYNC_cleanup_thread,
ASYNC_start_job, ASYNC_pause_job, ASYNC_get_current_job, ASYNC_get_wait_ctx(),
ASYNC_block_pause(), ASYNC_unblock_pause() and ASYNC_is_capable() were first
added in OpenSSL 1.1.0.

=head1 COPYRIGHT

Copyright 2015-2020 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
L<https://www.openssl.org/source/license.html>.

=cut