io(3)

NAME
io - Asynchronous IO

SYNOPSIS
#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(3) 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) destination
 */

#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 kernel\n");
    else if (rc < 0 && -rc < sys_nerr)
	fprintf(stderr, "%s: %s\n", func, sys_errlist[-rc]);
    else
	fprintf(stderr, "%s: error %d\n", 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 %d\n", 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 %d\n", 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 up to 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 memory\n");
		    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).