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       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

	#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);

       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

       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 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 max_size argument limits the	number of ASYNC_JOBs that will be held
       in the pool. If 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 max_size.	When the pool
       is first	initialised 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 max_size of 0 (no	upper limit)
       and an init_size	of 0 (no ASYNC_JOBs created up front).

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

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

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

	   The job was successfully started but	was "paused" before it
	   completed (see ASYNC_pause_job() below). A handle to	the job	is
	   placed in *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 *job. The func,
	   args	and size parameters will be ignored when restarting a job.
	   When	restarting a job ASYNC_start_job() must	be called from the
	   same	thread that the	job was	originally started from.

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

       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 *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 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
       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.

       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

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

       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.

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

	#ifdef _WIN32
	# include <windows.h>
	#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;


	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 */

	    /* 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!";


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

	    for	(;;) {
		switch (ASYNC_start_job(&job, ctx, &ret, jobfunc, msg, sizeof(msg))) {
		case ASYNC_ERR:
		    printf("An error occurred\n");
		    goto end;
		    printf("Job	was paused\n");
		    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");
		ASYNC_WAIT_CTX_get_all_fds(ctx,	&waitfd, &numfds);
		FD_SET(waitfd, &waitfdset);
		select(waitfd +	1, &waitfdset, NULL, NULL, NULL);


	    return 0;

       The expected output from	executing the above example program is:

	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

       crypto(7), ERR_print_errors(3)

       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.

       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

1.1.1k				  2021-03-25		    ASYNC_START_JOB(3)


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