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          aio - POSIX asynchronous I/O overview

          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 ana-
               log of read(2).

               Enqueue a write request.  This is the asynchronous ana-
               log 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 speci-
               fied 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 argu-
          ment of this type is employed with all of the functions
          listed above.  This structure has the following form:

              #include <aiocb.h>

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              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 aqaio_lio_opcodeaq: */

              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.

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               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

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

               The aio_reqprio field of the aiocb structure was less
               than 0, or was greater than the limit returned by the
               call sysconf(_SC_AIO_PRIO_DELTA_MAX).

          The POSIX AIO interfaces are provided by glibc since version

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

          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 reim-
          plemented using the kernel system calls.

          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

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          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 outstand-
          ing 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;

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              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));

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              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 (int 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 */

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              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 (int 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 (int 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:

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

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

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

              for (int 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);


          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 Sym-
          posium, 2003,

          This page is part of release 5.10 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

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