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NAME

       aio - POSIX asynchronous I/O overview

DESCRIPTION

       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 call.

       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_filedes     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 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.

       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_SIGNAL, and  SIGEV_THREAD.
                       See sigevent(7) for further details.

       aio_lio_opcode  The type of operation to be performed; used only for lio_listio(3).

       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.

NOTES

       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 userspace 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.

ERRORS

       EINVAL 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).

VERSIONS

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

CONFORMING TO

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

EXAMPLE

       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 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
           aio_error():
               for request 0 (descriptor 3): In progress
               for request 1 (descriptor 4): In progress
           abc
           I/O completion signal received
           aio_error():
               for request 0 (descriptor 3): I/O succeeded
               for request 1 (descriptor 4): In progress
           aio_error():
               for request 1 (descriptor 4): In progress
           x
           I/O completion signal received
           aio_error():
               for request 1 (descriptor 4): I/O succeeded
           All I/O requests completed
           aio_return():
               for request 0 (descriptor 3): 4
               for request 1 (descriptor 4): 2

   Program source

       #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)
       {
           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 */
       }

       int
       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",
                       argv[0]);
               exit(EXIT_FAILURE);
           }

           numReqs = argc - 1;

           /* Allocate our arrays */

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

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

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

           sa.sa_flags = SA_RESTART;
           sigemptyset(&sa.sa_mask);

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

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

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

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

               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 =
                                       &ioList[j];

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

           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,
                                   ioList[j].aiocbp->aio_fildes);
                           s = aio_cancel(ioList[j].aiocbp->aio_fildes,
                                   ioList[j].aiocbp);
                           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");
                           else
                               errMsg("aio_cancel");
                       }
                   }

                   gotSIGQUIT = 0;
               }

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

               printf("aio_error():\n");
               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");
                           break;
                       case EINPROGRESS:
                           printf("In progress\n");
                           break;
                       case ECANCELED:
                           printf("Canceled\n");
                           break;
                       default:
                           errMsg("aio_error");
                           break;
                       }

                       if (ioList[j].status != EINPROGRESS)
                           openReqs--;
                   }
               }
           }

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

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

           printf("aio_return():\n");
           for (j = 0; j < numReqs; j++) {
               ssize_t s;

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

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       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),
       http://www.squid-cache.org/~adrian/Reprint-Pulavarty-OLS2003.pdf

COLOPHON

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