=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 2015-2019 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