Provided by: libssl-doc_3.0.8-1ubuntu1_all 
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
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);
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 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:
ASYNC_ERR
An error occurred trying to start the job. Check the OpenSSL error queue (e.g. see
ERR_print_errors(3)) for more details.
ASYNC_NO_JOBS
There are no jobs currently available in the pool. This call can be retried again at a
later time.
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 *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.
ASYNC_FINISH
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 contain two different ways to notify applications that a job is ready to
be resumed. One is a "wait" file descriptor, and the other is a "callback" mechanism.
The "wait" file descriptor associated with ASYNC_WAIT_CTX is used for applications to 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.
ASYNC_WAIT_CTXs also have a "callback" mechanism to notify applications. The callback is
set by an application, and it will be automatically called when an engine completes a
cryptography operation, so that the application can resume the paused work flow without
polling. An engine could be written to look whether the callback has been set. If it has
then it would use the callback mechanism in preference to the file descriptor
notifications. If a callback is not set then the engine may use file descriptor based
notifications. Please note that not all engines may support the callback mechanism, so the
callback may not be used even if it has been set. See ASYNC_WAIT_CTX_new() for more
details.
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.
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.
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 <openssl/async.h>.
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
SEE ALSO
crypto(7), ERR_print_errors(3)
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.
COPYRIGHT
Copyright 2015-2021 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (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 <https://www.openssl.org/source/license.html>.