ASYNC_START_JOB(3SSL) OpenSSL ASYNC_START_JOB(3SSL)
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,
ASYNC_stack_alloc_fn, ASYNC_stack_free_fn, ASYNC_set_mem_functions,
ASYNC_get_mem_functions - 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);
typedef void *(*ASYNC_stack_alloc_fn)(size_t *num);
typedef void (*ASYNC_stack_free_fn)(void *addr);
int ASYNC_set_mem_functions(ASYNC_stack_alloc_fn alloc_fn,
ASYNC_stack_free_fn free_fn);
void ASYNC_get_mem_functions(ASYNC_stack_alloc_fn *alloc_fn,
ASYNC_stack_free_fn *free_fn);
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.
It's OpenSSL specific implementation of cooperative multitasking.
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_WAIT_CTX is used to know when
a job is ready to be restarted.
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 (see ASYNC_WAIT_CTX_new(3)). 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(2) or poll(2) (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.
Custom memory allocation functions are supported for the POSIX platform.
Custom memory allocation functions allow alternative methods of
allocating stack memory such as mmap, or using stack memory from the
current thread. Using an ASYNC_stack_alloc_fn callback also allows
manipulation of the stack size, which defaults to 32k. The stack size
can be altered by allocating a stack of a size different to the
requested size, and passing back the new stack size in the callback's
*num parameter.
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.
ASYNC_set_mem_functions returns 1 if custom stack allocators are
supported by the current platform and no allocations have already
occurred 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);
/*
* Create a way to inform the calling thread when this job is ready
* to resume, in this example we're using file descriptors.
* For offloading the task to an asynchronous ENGINE it's not necessary,
* the ENGINE should handle that internally.
*/
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 (like a network read being ready,
* disk access being finished, or some hardware offload operation
* completing) 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 just before calling a blocking
* method. The main thread will wait until pipefds[0] is ready
* for reading before returning control to this thread.
*/
ASYNC_pause_job();
/* Perform the blocking call (it won't block with this example code) */
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;
}
/* Get the file descriptor we can use to wait for the job
* to be ready to be woken up
*/
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);
/* Wait for the job to be ready for wakeup */
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. ASYNC_set_mem_functions(),
ASYNC_get_mem_functions() were added in OpenSSL 3.2.
COPYRIGHT
Copyright 2015-2024 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>.
3.5.7 2026-06-09 ASYNC_START_JOB(3SSL)
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