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.\" ========================================================================
.\"
.IX Title "nbd_aio_pread 3"
.TH nbd_aio_pread 3 "2021-02-09" "libnbd-1.6.1" "LIBNBD"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
nbd_aio_pread \- read from the NBD server
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <libnbd.h>
\&
\& typedef struct {
\&   int (*callback) (void *user_data, int *error);
\&   void *user_data;
\&   void (*free) (void *user_data);
\& } nbd_completion_callback;
\&
\& int64_t nbd_aio_pread (struct nbd_handle *h, void *buf,
\&                        size_t count, uint64_t offset,
\&                        nbd_completion_callback completion_callback,
\&                        uint32_t flags);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Issue a read command to the \s-1NBD\s0 server.
.PP
To check if the command completed, call \fBnbd_aio_command_completed\fR\|(3).
Or supply the optional \f(CW\*(C`completion_callback\*(C'\fR which will be invoked
as described in \*(L"Completion callbacks\*(R" in \fBlibnbd\fR\|(3).
.PP
Note that you must ensure \f(CW\*(C`buf\*(C'\fR is valid until the command has
completed.  Other parameters behave as documented in \fBnbd_pread\fR\|(3).
.PP
By default, libnbd will reject attempts to use this function with
parameters that are likely to result in server failure, such as
requesting an unknown command flag.  The \fBnbd_set_strict_mode\fR\|(3)
function can be used to alter which scenarios should await a server
reply rather than failing fast.
.SH "RETURN VALUE"
.IX Header "RETURN VALUE"
This call returns the 64 bit cookie of the command.
The cookie is ≥ \f(CW1\fR.
Cookies are unique (per libnbd handle, not globally).
.SH "ERRORS"
.IX Header "ERRORS"
On error \f(CW\*(C`\-1\*(C'\fR is returned.
.PP
Refer to \*(L"\s-1ERROR HANDLING\*(R"\s0 in \fBlibnbd\fR\|(3)
for how to get further details of the error.
.SH "HANDLE STATE"
.IX Header "HANDLE STATE"
The handle must be
connected with the server,
otherwise this call will return an error.
.SH "VERSION"
.IX Header "VERSION"
This function first appeared in libnbd 1.0.
.PP
If you need to test if this function is available at compile time
check if the following macro is defined:
.PP
.Vb 1
\& #define LIBNBD_HAVE_NBD_AIO_PREAD 1
.Ve
.SH "EXAMPLE"
.IX Header "EXAMPLE"
This example is also available as \fIexamples/aio\-connect\-read.c\fR
in the libnbd source code.
.PP
.Vb 10
\& /* This example shows how to use the AIO (asynchronous) low
\&  * level API to connect to a server and read the disk.
\&  *
\&  * Here are a few ways to try this example:
\&  *
\&  * nbdkit \-U \- linuxdisk . \e
\&  *   \-\-run \*(Aq./aio\-connect\-read $unixsocket\*(Aq
\&  *
\&  * nbdkit \-U \- floppy . \e
\&  *   \-\-run \*(Aq./aio\-connect\-read $unixsocket\*(Aq
\&  *
\&  * nbdkit \-U \- pattern size=1M \e
\&  *   \-\-run \*(Aq./aio\-connect\-read $unixsocket\*(Aq
\&  */
\& 
\& #include <stdio.h>
\& #include <stdlib.h>
\& #include <stdint.h>
\& #include <inttypes.h>
\& #include <assert.h>
\& 
\& #include <libnbd.h>
\& 
\& #define NR_SECTORS 32
\& #define SECTOR_SIZE 512
\& 
\& struct data {
\&   uint64_t offset;
\&   char sector[SECTOR_SIZE];
\& };
\& 
\& static int
\& hexdump (void *user_data, int *error)
\& {
\&   struct data *data = user_data;
\&   FILE *pp;
\& 
\&   printf ("sector at offset 0x%" PRIx64 ":\en",
\&           data\->offset);
\&   pp = popen ("hexdump \-C", "w");
\&   if (pp == NULL) {
\&     perror ("popen: hexdump");
\&     exit (EXIT_FAILURE);
\&   }
\&   fwrite (data\->sector, SECTOR_SIZE, 1, pp);
\&   pclose (pp);
\&   printf ("\en");
\& 
\&   /* Returning 1 from the callback automatically retires
\&    * the command.
\&    */
\&   return 1;
\& }
\& 
\& int
\& main (int argc, char *argv[])
\& {
\&   struct nbd_handle *nbd;
\&   struct data data[NR_SECTORS];
\&   size_t i;
\& 
\&   if (argc != 2) {
\&     fprintf (stderr, "%s socket\en", argv[0]);
\&     exit (EXIT_FAILURE);
\&   }
\& 
\&   /* Create the libnbd handle. */
\&   nbd = nbd_create ();
\&   if (nbd == NULL) {
\&     fprintf (stderr, "%s\en", nbd_get_error ());
\&     exit (EXIT_FAILURE);
\&   }
\& 
\&   /* Connect to the NBD server over a Unix domain socket.
\&    * This only starts the connection.
\&    */
\&   if (nbd_aio_connect_unix (nbd, argv[1]) == \-1) {
\&     fprintf (stderr, "%s\en", nbd_get_error ());
\&     exit (EXIT_FAILURE);
\&   }
\& 
\&   /* Wait for the connection to complete.  The use of
\&    * nbd_poll here is only as an example.  You could also
\&    * integrate this with poll(2), glib or another main
\&    * loop.  Read libnbd(3) and the source file lib/poll.c.
\&    */
\&   while (!nbd_aio_is_ready (nbd)) {
\&     if (nbd_poll (nbd, \-1) == \-1) {
\&       fprintf (stderr, "%s\en", nbd_get_error ());
\&       exit (EXIT_FAILURE);
\&     }
\&   }
\& 
\&   assert (nbd_get_size (nbd) >= NR_SECTORS * SECTOR_SIZE);
\& 
\&   /* Issue read commands for the first NR sectors. */
\&   for (i = 0; i < NR_SECTORS; ++i) {
\&     data[i].offset = i * SECTOR_SIZE;
\& 
\&     /* The callback (hexdump) is called when the command
\&      * completes.  The buffer must continue to exist while
\&      * the command is running.
\&      */
\&     if (nbd_aio_pread (nbd, data[i].sector, SECTOR_SIZE,
\&                        data[i].offset,
\&                        (nbd_completion_callback) {
\&                          .callback = hexdump,
\&                          .user_data = &data[i],
\&                        }, 0) == \-1) {
\&       fprintf (stderr, "%s\en", nbd_get_error ());
\&       exit (EXIT_FAILURE);
\&     }
\&   }
\& 
\&   /* Run the main loop until all the commands have
\&    * completed and retired.  Again the use of nbd_poll
\&    * here is only as an example.
\&    */
\&   while (nbd_aio_in_flight (nbd) > 0) {
\&     if (nbd_poll (nbd, \-1) == \-1) {
\&       fprintf (stderr, "%s\en", nbd_get_error ());
\&       exit (EXIT_FAILURE);
\&     }
\&   }
\& 
\&   /* Close the libnbd handle. */
\&   nbd_close (nbd);
\& 
\&   exit (EXIT_SUCCESS);
\& }
.Ve
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbd_aio_command_completed\fR\|(3),
\&\fBnbd_aio_pread_structured\fR\|(3),
\&\fBnbd_create\fR\|(3),
\&\fBnbd_pread\fR\|(3),
\&\fBnbd_set_strict_mode\fR\|(3),
\&\*(L"Issuing asynchronous commands\*(R" in \fBlibnbd\fR\|(3),
\&\fBlibnbd\fR\|(3).
.SH "AUTHORS"
.IX Header "AUTHORS"
Eric Blake
.PP
Richard W.M. Jones
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright (C) 2019\-2020 Red Hat Inc.
.SH "LICENSE"
.IX Header "LICENSE"
This library is free software; you can redistribute it and/or
modify it under the terms of the \s-1GNU\s0 Lesser General Public
License as published by the Free Software Foundation; either
version 2 of the License, or (at your option) any later version.
.PP
This library is distributed in the hope that it will be useful,
but \s-1WITHOUT ANY WARRANTY\s0; without even the implied warranty of
\&\s-1MERCHANTABILITY\s0 or \s-1FITNESS FOR A PARTICULAR PURPOSE.\s0  See the \s-1GNU\s0
Lesser General Public License for more details.
.PP
You should have received a copy of the \s-1GNU\s0 Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, \s-1MA 02110\-1301 USA\s0