NAME

nbd_pwrite - write to the NBD server

SYNOPSIS

#include <libnbd.h>

int nbd_pwrite (struct nbd_handle *h, const void *buf, size_t count, uint64_t offset, uint32_t flags);

DESCRIPTION

Issue a write command to the NBD server, writing the data in buf to the range starting at offset and ending at offset + count - 1. NBD can only write all or nothing using this call. The call returns when the command has been acknowledged by the server, or there is an error.

The flags parameter may be 0 for no flags, or may contain LIBNBD_CMD_FLAG_FUA meaning that the server should not return until the data has been committed to permanent storage (if that is supported - some servers cannot do this, see nbd_can_fua(3)).

RETURN VALUE

If the call is successful the function returns 0.

ERRORS

On error -1 is returned.

Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error.

HANDLE STATE

The handle must be connected and finished handshaking with the server, otherwise this call will return an error.

VERSION

This function first appeared in libnbd 1.0.

If you need to test if this function is available at compile time check if the following macro is defined:

 #define LIBNBD_HAVE_NBD_PWRITE 1

EXAMPLE

This example is also available as examples/reads-and-writes.c in the libnbd source code.

 /* This example shows how to do synchronous reads
  * and writes randomly over the first megabyte of an
  * NBD server.  Note this will destroy any existing
  * content on the NBD server.
  *
  * To test it with nbdkit and a RAM disk:
  *
  * nbdkit -U - memory 1M \
  *     --run './simple-reads-and-writes $unixsocket'
  */
 
 #include <stdio.h>
 #include <stdlib.h>
 #include <inttypes.h>
 #include <assert.h>
 #include <time.h>
 
 #include <libnbd.h>
 
 int
 main (int argc, char *argv[])
 {
   struct nbd_handle *nbd;
   char buf[512];
   size_t i;
   int64_t exportsize;
   uint64_t offset;
 
   srand (time (NULL));
 
   if (argc != 2) {
     fprintf (stderr, "%s socket\n", argv[0]);
     exit (EXIT_FAILURE);
   }
 
   /* Create the libnbd handle. */
   nbd = nbd_create ();
   if (nbd == NULL) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }
 
   /* Connect to the NBD server over a
    * Unix domain socket.
    */
   if (nbd_connect_unix (nbd, argv[1]) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }
 
   /* Get the size of the disk and check
    * it's large enough.
    */
   exportsize = nbd_get_size (nbd);
   if (exportsize == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }
   assert (exportsize >= sizeof buf);
 
   /* Check that the server is writable. */
   if (nbd_is_read_only (nbd) == 1) {
     fprintf (stderr, "%s: "
              "error: this NBD export is read-only\n",
              argv[0]);
     exit (EXIT_FAILURE);
   }
 
   for (i = 0; i < sizeof buf; ++i)
     buf[i] = rand ();
 
   /* 1000 writes. */
   for (i = 0; i < 1000; ++i) {
     offset = rand () % (exportsize - sizeof buf);
 
     if (nbd_pwrite (nbd, buf, sizeof buf,
                     offset, 0) == -1) {
       fprintf (stderr, "%s\n", nbd_get_error ());
       exit (EXIT_FAILURE);
     }
   }
 
   /* 1000 reads and writes. */
   for (i = 0; i < 1000; ++i) {
     offset = rand () % (exportsize - sizeof buf);
     if (nbd_pread (nbd, buf, sizeof buf,
                    offset, 0) == -1) {
       fprintf (stderr, "%s\n", nbd_get_error ());
       exit (EXIT_FAILURE);
     }
 
     offset = rand () % (exportsize - sizeof buf);
     if (nbd_pwrite (nbd, buf, sizeof buf,
                     offset, 0) == -1) {
       fprintf (stderr, "%s\n", nbd_get_error ());
       exit (EXIT_FAILURE);
     }
   }
 
   /* Sends a graceful shutdown to the server. */
   if (nbd_shutdown (nbd, 0) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }
 
   nbd_close (nbd);
 
   exit (EXIT_SUCCESS);
 }

SEE ALSO

nbd_can_fua(3), nbd_can_write(3), nbd_aio_pwrite(3), nbd_create(3), libnbd(3).

AUTHORS

Eric Blake

Richard W.M. Jones

COPYRIGHT

Copyright (C) 2019 Red Hat Inc.

LICENSE

This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with this library; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA