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ASYNC_start_job(3)                  OpenSSL                 ASYNC_start_job(3)

       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

       libcrypto, -lcrypto

        #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:
                case ASYNC_NO_JOBS:
                    printf("An error occurred\n");
                    goto end;
                case ASYNC_PAUSE:
                    printf("Job was paused\n");
                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");
                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

1.1.1i                            2020-12-10                ASYNC_start_job(3)