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).