aio(7) — Linux manual page


AIO(7)              Miscellaneous Information Manual              AIO(7)

NAME         top

       aio - POSIX asynchronous I/O overview

DESCRIPTION         top

       The POSIX asynchronous I/O (AIO) interface allows applications to
       initiate one or more I/O operations that are performed
       asynchronously (i.e., in the background).  The application can
       elect to be notified of completion of the I/O operation in a
       variety of ways: by delivery of a signal, by instantiation of a
       thread, or no notification at all.

       The POSIX AIO interface consists of the following functions:

              Enqueue a read request.  This is the asynchronous analog
              of read(2).

              Enqueue a write request.  This is the asynchronous analog
              of write(2).

              Enqueue a sync request for the I/O operations on a file
              descriptor.  This is the asynchronous analog of fsync(2)
              and fdatasync(2).

              Obtain the error status of an enqueued I/O request.

              Obtain the return status of a completed I/O request.

              Suspend the caller until one or more of a specified set of
              I/O requests completes.

              Attempt to cancel outstanding I/O requests on a specified
              file descriptor.

              Enqueue multiple I/O requests using a single function

       The aiocb ("asynchronous I/O control block") structure defines
       parameters that control an I/O operation.  An argument of this
       type is employed with all of the functions listed above.  This
       structure has the following form:

           #include <aiocb.h>

           struct aiocb {
               /* The order of these fields is implementation-dependent */

               int             aio_fildes;     /* File descriptor */
               off_t           aio_offset;     /* File offset */
               volatile void  *aio_buf;        /* Location of buffer */
               size_t          aio_nbytes;     /* Length of transfer */
               int             aio_reqprio;    /* Request priority */
               struct sigevent aio_sigevent;   /* Notification method */
               int             aio_lio_opcode; /* Operation to be performed;
                                                  lio_listio() only */

               /* Various implementation-internal fields not shown */

           /* Operation codes for 'aio_lio_opcode': */

           enum { LIO_READ, LIO_WRITE, LIO_NOP };

       The fields of this structure are as follows:

              The file descriptor on which the I/O operation is to be

              This is the file offset at which the I/O operation is to
              be performed.

              This is the buffer used to transfer data for a read or
              write operation.

              This is the size of the buffer pointed to by aio_buf.

              This field specifies a value that is subtracted from the
              calling thread's real-time priority in order to determine
              the priority for execution of this I/O request (see
              pthread_setschedparam(3)).  The specified value must be
              between 0 and the value returned by
              sysconf(_SC_AIO_PRIO_DELTA_MAX).  This field is ignored
              for file synchronization operations.

              This field is a structure that specifies how the caller is
              to be notified when the asynchronous I/O operation
              completes.  Possible values for aio_sigevent.sigev_notify
              are SIGEV_NONE, SIGEV_SIGNAL, and SIGEV_THREAD.  See
              sigevent(7) for further details.

              The type of operation to be performed; used only for

       In addition to the standard functions listed above, the GNU C
       library provides the following extension to the POSIX AIO API:

              Set parameters for tuning the behavior of the glibc POSIX
              AIO implementation.

ERRORS         top

       EINVAL The aio_reqprio field of the aiocb structure was less than
              0, or was greater than the limit returned by the call

STANDARDS         top


HISTORY         top

       POSIX.1-2001.  glibc 2.1.

NOTES         top

       It is a good idea to zero out the control block buffer before use
       (see memset(3)).  The control block buffer and the buffer pointed
       to by aio_buf must not be changed while the I/O operation is in
       progress.  These buffers must remain valid until the I/O
       operation completes.

       Simultaneous asynchronous read or write operations using the same
       aiocb structure yield undefined results.

       The current Linux POSIX AIO implementation is provided in user
       space by glibc.  This has a number of limitations, most notably
       that maintaining multiple threads to perform I/O operations is
       expensive and scales poorly.  Work has been in progress for some
       time on a kernel state-machine-based implementation of
       asynchronous I/O (see io_submit(2), io_setup(2), io_cancel(2),
       io_destroy(2), io_getevents(2)), but this implementation hasn't
       yet matured to the point where the POSIX AIO implementation can
       be completely reimplemented using the kernel system calls.

EXAMPLES         top

       The program below opens each of the files named in its command-
       line arguments and queues a request on the resulting file
       descriptor using aio_read(3).  The program then loops,
       periodically monitoring each of the I/O operations that is still
       in progress using aio_error(3).  Each of the I/O requests is set
       up to provide notification by delivery of a signal.  After all
       I/O requests have completed, the program retrieves their status
       using aio_return(3).

       The SIGQUIT signal (generated by typing control-\) causes the
       program to request cancelation of each of the outstanding
       requests using aio_cancel(3).

       Here is an example of what we might see when running this
       program.  In this example, the program queues two requests to
       standard input, and these are satisfied by two lines of input
       containing "abc" and "x".

           $ ./a.out /dev/stdin /dev/stdin
           opened /dev/stdin on descriptor 3
           opened /dev/stdin on descriptor 4
               for request 0 (descriptor 3): In progress
               for request 1 (descriptor 4): In progress
           I/O completion signal received
               for request 0 (descriptor 3): I/O succeeded
               for request 1 (descriptor 4): In progress
               for request 1 (descriptor 4): In progress
           I/O completion signal received
               for request 1 (descriptor 4): I/O succeeded
           All I/O requests completed
               for request 0 (descriptor 3): 4
               for request 1 (descriptor 4): 2

   Program source

       #include <fcntl.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <stdio.h>
       #include <errno.h>
       #include <aio.h>
       #include <signal.h>

       #define BUF_SIZE 20     /* Size of buffers for read operations */

       #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)

       struct ioRequest {      /* Application-defined structure for tracking
                                  I/O requests */
           int           reqNum;
           int           status;
           struct aiocb *aiocbp;

       static volatile sig_atomic_t gotSIGQUIT = 0;
                               /* On delivery of SIGQUIT, we attempt to
                                  cancel all outstanding I/O requests */

       static void             /* Handler for SIGQUIT */
       quitHandler(int sig)
           gotSIGQUIT = 1;

       #define IO_SIGNAL SIGUSR1   /* Signal used to notify I/O completion */

       static void                 /* Handler for I/O completion signal */
       aioSigHandler(int sig, siginfo_t *si, void *ucontext)
           if (si->si_code == SI_ASYNCIO) {
               write(STDOUT_FILENO, "I/O completion signal received\n", 31);

               /* The corresponding ioRequest structure would be available as
                      struct ioRequest *ioReq = si->si_value.sival_ptr;
                  and the file descriptor would then be available via
                      ioReq->aiocbp->aio_fildes */

       main(int argc, char *argv[])
           struct sigaction sa;
           int s;
           int numReqs;        /* Total number of queued I/O requests */
           int openReqs;       /* Number of I/O requests still in progress */

           if (argc < 2) {
               fprintf(stderr, "Usage: %s <pathname> <pathname>...\n",

           numReqs = argc - 1;

           /* Allocate our arrays. */

           struct ioRequest *ioList = calloc(numReqs, sizeof(*ioList));
           if (ioList == NULL)

           struct aiocb *aiocbList = calloc(numReqs, sizeof(*aiocbList));
           if (aiocbList == NULL)

           /* Establish handlers for SIGQUIT and the I/O completion signal. */

           sa.sa_flags = SA_RESTART;

           sa.sa_handler = quitHandler;
           if (sigaction(SIGQUIT, &sa, NULL) == -1)

           sa.sa_flags = SA_RESTART | SA_SIGINFO;
           sa.sa_sigaction = aioSigHandler;
           if (sigaction(IO_SIGNAL, &sa, NULL) == -1)

           /* Open each file specified on the command line, and queue
              a read request on the resulting file descriptor. */

           for (size_t j = 0; j < numReqs; j++) {
               ioList[j].reqNum = j;
               ioList[j].status = EINPROGRESS;
               ioList[j].aiocbp = &aiocbList[j];

               ioList[j].aiocbp->aio_fildes = open(argv[j + 1], O_RDONLY);
               if (ioList[j].aiocbp->aio_fildes == -1)
               printf("opened %s on descriptor %d\n", argv[j + 1],

               ioList[j].aiocbp->aio_buf = malloc(BUF_SIZE);
               if (ioList[j].aiocbp->aio_buf == NULL)

               ioList[j].aiocbp->aio_nbytes = BUF_SIZE;
               ioList[j].aiocbp->aio_reqprio = 0;
               ioList[j].aiocbp->aio_offset = 0;
               ioList[j].aiocbp->aio_sigevent.sigev_notify = SIGEV_SIGNAL;
               ioList[j].aiocbp->aio_sigevent.sigev_signo = IO_SIGNAL;
               ioList[j].aiocbp->aio_sigevent.sigev_value.sival_ptr =

               s = aio_read(ioList[j].aiocbp);
               if (s == -1)

           openReqs = numReqs;

           /* Loop, monitoring status of I/O requests. */

           while (openReqs > 0) {
               sleep(3);       /* Delay between each monitoring step */

               if (gotSIGQUIT) {

                   /* On receipt of SIGQUIT, attempt to cancel each of the
                      outstanding I/O requests, and display status returned
                      from the cancelation requests. */

                   printf("got SIGQUIT; canceling I/O requests: \n");

                   for (size_t j = 0; j < numReqs; j++) {
                       if (ioList[j].status == EINPROGRESS) {
                           printf("    Request %zu on descriptor %d:", j,
                           s = aio_cancel(ioList[j].aiocbp->aio_fildes,
                           if (s == AIO_CANCELED)
                               printf("I/O canceled\n");
                           else if (s == AIO_NOTCANCELED)
                               printf("I/O not canceled\n");
                           else if (s == AIO_ALLDONE)
                               printf("I/O all done\n");

                   gotSIGQUIT = 0;

               /* Check the status of each I/O request that is still
                  in progress. */

               for (size_t j = 0; j < numReqs; j++) {
                   if (ioList[j].status == EINPROGRESS) {
                       printf("    for request %zu (descriptor %d): ",
                               j, ioList[j].aiocbp->aio_fildes);
                       ioList[j].status = aio_error(ioList[j].aiocbp);

                       switch (ioList[j].status) {
                       case 0:
                           printf("I/O succeeded\n");
                       case EINPROGRESS:
                           printf("In progress\n");
                       case ECANCELED:

                       if (ioList[j].status != EINPROGRESS)

           printf("All I/O requests completed\n");

           /* Check status return of all I/O requests. */

           for (size_t j = 0; j < numReqs; j++) {
               ssize_t s;

               s = aio_return(ioList[j].aiocbp);
               printf("    for request %zu (descriptor %d): %zd\n",
                       j, ioList[j].aiocbp->aio_fildes, s);


SEE ALSO         top

       io_cancel(2), io_destroy(2), io_getevents(2), io_setup(2),
       io_submit(2), aio_cancel(3), aio_error(3), aio_init(3),
       aio_read(3), aio_return(3), aio_write(3), lio_listio(3)

       "Asynchronous I/O Support in Linux 2.5", Bhattacharya, Pratt,
       Pulavarty, and Morgan, Proceedings of the Linux Symposium, 2003,
       ⟨ man-pages (unreleased)     (date)                           AIO(7)

Pages that refer to this page: io_cancel(2)io_destroy(2)io_getevents(2)io_setup(2)io_submit(2)aio_cancel(3)aiocb(3type)aio_error(3)aio_fsync(3)aio_init(3)aio_read(3)aio_return(3)aio_suspend(3)aio_write(3)lio_listio(3)sigevent(7)