Provided by: libaio-dev_0.3.109-2ubuntu1_amd64 bug

NAME

       io - Asynchronous IO

SYNOPSYS

       #include <errno.h>

       #include <libaio.h>

DESCRIPTION

       The  libaio library defines a new set of I/O operations which can significantly reduce the
       time an application spends waiting at I/O.  The new functions allow a program to  initiate
       one  or  more  I/O  operations  and  then  immediately  resume  normal  work while the I/O
       operations are executed in parallel.

       These functions are part of the library with realtime functions named  libaio.   They  are
       not  actually  part of the libc binary.  The implementation of these functions can be done
       using support in the kernel.

       All IO operations  operate  on  files  which  were  opened  previously.   There  might  be
       arbitrarily  many  operations  running  for one file.  The asynchronous I/O operations are
       controlled using a data structure named struct iocb It is defined in libaio.h as follows.

       typedef struct io_context *io_context_t;

       typedef enum io_iocb_cmd {
               IO_CMD_PREAD = 0,
               IO_CMD_PWRITE = 1,

               IO_CMD_FSYNC = 2,
               IO_CMD_FDSYNC = 3,

               IO_CMD_POLL = 5,
               IO_CMD_NOOP = 6,
       } io_iocb_cmd_t;

       struct io_iocb_common {
               void            *buf;
               unsigned        __pad1;
               long            nbytes;
               unsigned        __pad2;
               long long       offset;
               long long       __pad3, __pad4;
       };      /* result code is the amount read or -'ve errno */

       struct iocb {
               void            *data;
               unsigned        key;
               short           aio_lio_opcode;
               short           aio_reqprio;
               int             aio_fildes;
               union {
                       struct io_iocb_common           c;
                       struct io_iocb_vector           v;
                       struct io_iocb_poll             poll;
                       struct io_iocb_sockaddr saddr;
               } u;
       };

       int aio_fildes
              This element specifies the file descriptor to be used for the operation.   It  must
              be a legal descriptor, otherwise the operation will fail.

              The  device on which the file is opened must allow the seek operation.  I.e., it is
              not possible to use any of the IO operations on devices  like  terminals  where  an
              lseek(2) call would lead to an error.

       long u.c.offset
              This  element  specifies  the  offset  in the file at which the operation (input or
              output) is performed.  Since the operations are carried out in arbitrary order  and
              more than one operation for one file descriptor can be started, one cannot expect a
              current read/write position of the file descriptor.

       void *buf
              This is a pointer to the buffer with the data to be written or the place where  the
              read data is stored.

       long u.c.nbytes
              This element specifies the length of the buffer pointed to by io_buf.

       int aio_reqprio
              Is not currently used.

       IO_CMD_PREAD
              Start  a  read  operation.  Read from the file at position u.c.offset and store the
              next u.c.nbytes bytes in the buffer pointed to by buf.

       IO_CMD_PWRITE
              Start a write operation.  Write u.c.nbytes bytes starting  at  buf  into  the  file
              starting at position u.c.offset.

       IO_CMD_NOP
              Do nothing for this control block.  This value is useful sometimes when an array of
              struct iocb values contains holes, i.e., some of the values  must  not  be  handled
              although the whole array is presented to the io_submit function.

       IO_CMD_FSYNC

       IO_CMD_POLL
              This is experimental.

EXAMPLE

       /*
        * Simplistic version of copy command using async i/o
        *
        * From:  Stephen Hemminger <shemminger@osdl.org>
        * Copy file by using a async I/O state machine.
        * 1. Start read request
        * 2. When read completes turn it into a write request
        * 3. When write completes decrement counter and free resources
        *
        *
        * Usage: aiocp file(s) desination
        */

       #include <unistd.h>
       #include <stdio.h>
       #include <sys/types.h>
       #include <sys/stat.h>
       #include <sys/param.h>
       #include <fcntl.h>
       #include <errno.h>

       #include <libaio.h>

       #define AIO_BLKSIZE (64*1024)
       #define AIO_MAXIO   32

       static int busy = 0;          // # of I/O's in flight
       static int tocopy = 0;        // # of blocks left to copy
       static int dstfd = -1;        // destination file descriptor
       static const char *dstname = NULL;
       static const char *srcname = NULL;

       /* Fatal error handler */
       static void io_error(const char *func, int rc)
       {
           if (rc == -ENOSYS)
            fprintf(stderr, "AIO not in this kernel0);
           else if (rc < 0 && -rc < sys_nerr)
            fprintf(stderr, "%s: %s0, func, sys_errlist[-rc]);
           else
            fprintf(stderr, "%s: error %d0, func, rc);

           if (dstfd > 0)
            close(dstfd);
           if (dstname)
            unlink(dstname);
           exit(1);
       }

       /*
        * Write complete callback.
        * Adjust counts and free resources
        */
       static void wr_done(io_context_t ctx, struct iocb *iocb, long res, long res2)
       {
           if (res2 != 0) {
            io_error("aio write", res2);
           }
           if (res != iocb->u.c.nbytes) {
            fprintf(stderr, "write missed bytes expect %d got %d0, iocb->u.c.nbytes, res2);
            exit(1);
           }
           --tocopy;
           --busy;
           free(iocb->u.c.buf);

           memset(iocb, 0xff, sizeof(iocb));   // paranoia
           free(iocb);
           write(2, "w", 1);
       }

       /*
        * Read complete callback.
        * Change read iocb into a write iocb and start it.
        */
       static void rd_done(io_context_t ctx, struct iocb *iocb, long res, long res2)
       {
           /* library needs accessors to look at iocb? */
           int iosize = iocb->u.c.nbytes;
           char *buf = iocb->u.c.buf;
           off_t offset = iocb->u.c.offset;

           if (res2 != 0)
            io_error("aio read", res2);
           if (res != iosize) {
            fprintf(stderr, "read missing bytes expect %d got %d0, iocb->u.c.nbytes, res);
            exit(1);
           }

           /* turn read into write */
           io_prep_pwrite(iocb, dstfd, buf, iosize, offset);
           io_set_callback(iocb, wr_done);
           if (1 != (res = io_submit(ctx, 1, &iocb)))
            io_error("io_submit write", res);
           write(2, "r", 1);
       }

       int main(int argc, char *const *argv)
       {
           int srcfd;
           struct stat st;
           off_t length = 0, offset = 0;
           io_context_t myctx;

           if (argc != 3 || argv[1][0] == '-') {
            fprintf(stderr, "Usage: aiocp SOURCE DEST");
            exit(1);
           }
           if ((srcfd = open(srcname = argv[1], O_RDONLY)) < 0) {
            perror(srcname);
            exit(1);
           }
           if (fstat(srcfd, &st) < 0) {
            perror("fstat");
            exit(1);
           }
           length = st.st_size;

           if ((dstfd = open(dstname = argv[2], O_WRONLY | O_CREAT, 0666)) < 0) {
            close(srcfd);
            perror(dstname);
            exit(1);
           }

           /* initialize state machine */
           memset(&myctx, 0, sizeof(myctx));
           io_queue_init(AIO_MAXIO, &myctx);
           tocopy = howmany(length, AIO_BLKSIZE);

           while (tocopy > 0) {
            int i, rc;
            /* Submit as many reads as once as possible upto AIO_MAXIO */
            int n = MIN(MIN(AIO_MAXIO - busy, AIO_MAXIO / 2),
                     howmany(length - offset, AIO_BLKSIZE));
            if (n > 0) {
                struct iocb *ioq[n];

                for (i = 0; i < n; i++) {
                 struct iocb *io = (struct iocb *) malloc(sizeof(struct iocb));
                 int iosize = MIN(length - offset, AIO_BLKSIZE);
                 char *buf = (char *) malloc(iosize);

                 if (NULL == buf || NULL == io) {
                     fprintf(stderr, "out of memory0);
                     exit(1);
                 }

                 io_prep_pread(io, srcfd, buf, iosize, offset);
                 io_set_callback(io, rd_done);
                 ioq[i] = io;
                 offset += iosize;
                }

                rc = io_submit(myctx, n, ioq);
                if (rc < 0)
                 io_error("io_submit", rc);

                busy += n;
            }

            // Handle IO's that have completed
            rc = io_queue_run(myctx);
            if (rc < 0)
                io_error("io_queue_run", rc);

            // if we have maximum number of i/o's in flight
            // then wait for one to complete
            if (busy == AIO_MAXIO) {
                rc = io_queue_wait(myctx, NULL);
                if (rc < 0)
                 io_error("io_queue_wait", rc);
            }

           }

           close(srcfd);
           close(dstfd);
           exit(0);
       }

       /*
        * Results look like:
        * [alanm@toolbox ~/MOT3]$ ../taio kernel-source-2.4.8-0.4g.ppc.rpm abc
        * rrrrrrrrrrrrrrrwwwrwrrwwrrwrwwrrwrwrwwrrwrwrrrrwwrwwwrrwrrrwwwwwwwwwwwwwwwww
        * rrrrrrrrrrrrrrwwwrrwrwrwrwrrwwwwwwwwwwwwwwrrrrrrrrrrrrrrrrrrwwwwrwrwwrwrwrwr
        * wrrrrrrrwwwwwwwwwwwwwrrrwrrrwrrwrwwwwwwwwwwrrrrwwrwrrrrrrrrrrrwwwwwwwwwwwrww
        * wwwrrrrrrrrwwrrrwwrwrwrwwwrrrrrrrwwwrrwwwrrwrwwwwwwwwrrrrrrrwwwrrrrrrrwwwwww
        * wwwwwwwrwrrrrrrrrwrrwrrwrrwrwrrrwrrrwrrrwrwwwwwwwwwwwwwwwwwwrrrwwwrrrrrrrrrr
        * rrwrrrrrrwrrwwwwwwwwwwwwwwwwrwwwrrwrwwrrrrrrrrrrrrrrrrrrrwwwwwwwwwwwwwwwwwww
        * rrrrrwrrwrwrwrrwrrrwwwwwwwwrrrrwrrrwrwwrwrrrwrrwrrrrwwwwwwwrwrwwwwrwwrrrwrrr
        * rrrwwwwwwwrrrrwwrrrrrrrrrrrrwrwrrrrwwwwwwwwwwwwwwrwrrrrwwwwrwrrrrwrwwwrrrwww
        * rwwrrrrrrrwrrrrrrrrrrrrwwwwrrrwwwrwrrwwwwwwwwwwwwwwwwwwwwwrrrrrrrwwwwwwwrw
        */

SEE ALSO

       io_cancel(3),    io_fsync(3),    io_getevents(3),    io_prep_fsync(3),   io_prep_pread(3),
       io_prep_pwrite(3),      io_queue_init(3),      io_queue_release(3),       io_queue_run(3),
       io_queue_wait(3), io_set_callback(3), io_submit(3), errno(3).