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Man Pages

Manual Reference Pages  -  ASYNC_INIT (3)

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ASYNC_init, ASYNC_cleanup, ASYNC_init_thread, ASYNC_cleanup_thread, ASYNC_start_job, ASYNC_pause_job, ASYNC_in_job, ASYNC_get_wait_fd, ASYNC_get_current_job, ASYNC_wake, ASYNC_clear_wake, ASYNC_block_pause, ASYNC_unblock_pause - asynchronous job management functions



 #include <openssl/async.h>

 int ASYNC_init(int init_thread, size_t max_size, size_t init_size);
 void ASYNC_cleanup(int cleanupthread);

 int ASYNC_init_thread(size_t max_size, size_t init_size);
 void ASYNC_cleanup_thread(void);

 int ASYNC_start_job(ASYNC_JOB **job, int *ret, int (*func)(void *),
                     void *args, size_t size);
 int ASYNC_pause_job(void);

 int ASYNC_get_wait_fd(ASYNC_JOB *job);
 ASYNC_JOB *ASYNC_get_current_job(void);
 void ASYNC_wake(ASYNC_JOB *job);
 void ASYNC_clear_wake(ASYNC_JOB *job);
 void ASYNC_block_pause(void);
 void ASYNC_unblock_pause(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. Before using any of the asynchronous job functions, user code should first call ASYNC_init(). If the user application is multi-threaded, then ASYNC_init_thread() should be called for each thread that will initiate asynchronous jobs. If the init_thread parameter to ASYNC_init() is non-zero then ASYNC_init_thread is automatically called for the current thread. Before user code exits it should free up resources for each thread that was initialised 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. Additionally an application should call ASYNC_cleanup() when all asynchronous work is complete across all threads. If cleanupthread is non-zero then ASYNC_cleanup_thread() is automatically called for the current thread.

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). If a pool is created in this way it must still be cleaned up with an explicit call to ASYNC_cleanup_thread().

An asynchronous job is started by calling the ASYNC_start_job() function. Initially *job should be NULL. 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.

Every ASYNC_JOB has a wait file descriptor associated with it. Calling ASYNC_get_wait_fd() and passing in a pointer to an ASYNC_JOB in the job parameter will return the wait file descriptor associated with that job. This file descriptor can be used to signal that the job should be resumed. 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). Applications can signal that a job is ready to resume using ASYNC_wake() or clear an existing signal using ASYNC_clear_wake().

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_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 using ASYNC_wake(). Once resumed the engine would clear the wake signal by calling ASYNC_clear_wake().

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 reenable 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 aquiring the lock and ASYNC_unblock_pause() immediately before releasing it then this situation cannot occur.


ASYNC_init and ASYNC_init_thread return 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_wait_fd returns the wait file descriptor associated with the ASYNC_JOB provided as an argument.

ASYNC_get_current_job returns a pointer to the currently executing ASYNC_JOB or NULL if not within the context of a job.


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

 #include <stdio.h>
 #include <openssl/async.h>

 int jobfunc(void *arg)
     ASYNC_JOB *currjob;
     unsigned char *msg;

     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);

      * 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().

     /* Return control back to main */

     /* Clear the wake signal */

     printf ("Resumed the job after a pause\n");

     return 1;

 int main(void)
     ASYNC_JOB *job = NULL;
     int ret, waitfd;
     fd_set waitfdset;
     unsigned char msg[13] = "Hello world!";

      * Were only expecting 1 job to be used here so were only creating
      * a pool of 1
     if (!ASYNC_init(1, 1, 1)) {
         printf("Error creating pool\n");
         goto end;


     for (;;) {
         switch(ASYNC_start_job(&job, &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");
         waitfd = ASYNC_get_wait_fd(job);
         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(3), ERR_print_errors(3)


ASYNC_init, ASYNC_init_thread, ASYNC_cleanup, ASYNC_cleanup_thread, ASYNC_start_job, ASYNC_pause_job, ASYNC_get_wait_fd, ASYNC_get_current_job, ASYNC_wake, ASYNC_clear_wake were first added to OpenSSL 1.1.0.
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1.1.0-pre3 ASYNC_START_JOB (3) 2016-02-15

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