AIO(7)                    Linux Programmer's 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:

       aio_read(3)     Enqueue a read request.  This is the asynchronous
                       analog of read(2).

       aio_write(3)    Enqueue a write request.  This is the asynchronous
                       analog of write(2).

       aio_fsync(3)    Enqueue a sync request for the I/O operations on a
                       file descriptor.  This is the asynchronous analog of
                       fsync(2) and fdatasync(2).

       aio_error(3)    Obtain the error status of an enqueued I/O request.

       aio_return(3)   Obtain the return status of a completed I/O request.

       aio_suspend(3)  Suspend the caller until one or more of a specified
                       set of I/O requests completes.

       aio_cancel(3)   Attempt to cancel outstanding I/O requests on a
                       specified file descriptor.

       lio_listio(3)   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:

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

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

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

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

       aio_reqprio     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 speci‐
                       fied value must be between 0 and the value returned
                       by sysconf(_SC_AIO_PRIO_DELTA_MAX).  This field is
                       ignored for file synchronization operations.

       aio_sigevent    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_SIG‐
                       NAL, and SIGEV_THREAD.  See sigevent(7) for further

       aio_lio_opcode  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:

       aio_init(3)     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

VERSIONS         top

       The POSIX AIO interfaces are provided by glibc since version 2.1.

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008.

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

       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.

EXAMPLE         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 cancellation of each of the outstanding requests using

       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

           $ ./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)

       #define errMsg(msg)  do { perror(msg); } 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 ioRequest *ioList;
           struct aiocb *aiocbList;
           struct sigaction sa;
           int s, j;
           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 */

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

           aiocbList = calloc(numReqs, sizeof(struct aiocb));
           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 (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 cancellation requests */

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

                   for (j = 0; j < numReqs; j++) {
                       if (ioList[j].status == EINPROGRESS) {
                           printf("    Request %d 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 (j = 0; j < numReqs; j++) {
                   if (ioList[j].status == EINPROGRESS) {
                       printf("    for request %d (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 (j = 0; j < numReqs; j++) {
               ssize_t s;

               s = aio_return(ioList[j].aiocbp);
               printf("    for request %d (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, 

COLOPHON         top

       This page is part of release 5.04 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2019-03-06                           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)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)