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 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.
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.
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>.
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
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 2015-2021 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>.