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.

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.

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 async.h.

EXAMPLE

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

SEE ALSO