ASYNC_start_job.3ssl man page

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:

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.

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.


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 APIs:

#ifdef _WIN32
# include <windows.h&gt;
#include <stdio.h&gt;
#include <unistd.h&gt;
#include &lt;openssl/async.h&gt;
#include &lt;openssl/crypto.h&gt;

int unique = 0;

void cleanup(ASYNC_WAIT_CTX *ctx, const void *key, OSSL_ASYNC_FD r, void *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;
    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

See Also

crypto(3), 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 to OpenSSL 1.1.0.


Explore man page connections for ASYNC_start_job.3ssl(3).