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.IX Title "nbd_pwrite 3"
.TH nbd_pwrite 3 2026-07-16 libnbd-1.24.3 LIBNBD
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH NAME
nbd_pwrite \- write to the NBD server
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 1
\& #include <libnbd.h>
\&
\& int nbd_pwrite (
\&       struct nbd_handle *h, const void *buf, size_t count,
\&       uint64_t offset, uint32_t flags
\&     );
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
Issue a write command to the NBD server, writing the data in
\&\f(CW\*(C`buf\*(C'\fR to the range starting at \f(CW\*(C`offset\*(C'\fR and ending at
\&\f(CW\*(C`offset\*(C'\fR + \f(CW\*(C`count\*(C'\fR \- 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.  Note this will
generally return an error if \fBnbd_is_read_only\fR\|(3) is true.
.PP
Note that libnbd defaults to enforcing a maximum write buffer
of the lesser of 64MiB or any maximum payload size advertised
by the server; attempts to exceed this will generally result in
a client\-side \f(CW\*(C`ERANGE\*(C'\fR error, rather than a server\-side
disconnection.  The actual limit can be learned with
\&\fBnbd_get_block_size\fR\|(3).
.PP
The \f(CW\*(C`flags\*(C'\fR parameter may be \f(CW0\fR for no flags, or may contain
\&\f(CW\*(C`LIBNBD_CMD_FLAG_FUA\*(C'\fR 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
\&\fBnbd_can_fua\fR\|(3)).  For convenience, unless \f(CWnbd_set_strict_flags(3)\fR
was used to disable \f(CW\*(C`LIBNBD_STRICT_AUTO_FLAG\*(C'\fR, libnbd ignores the
presence or absence of the flag \f(CW\*(C`LIBNBD_CMD_FLAG_PAYLOAD_LEN\*(C'\fR
in \f(CW\*(C`flags\*(C'\fR, while correctly using the flag over the wire
according to whether extended headers were negotiated.
.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"
If the call is successful the function returns \f(CW0\fR.
.SH ERRORS
.IX Header "ERRORS"
On error \f(CW\-1\fR is returned.
.PP
Refer to "ERROR HANDLING" in \fBlibnbd\fR\|(3)
for how to get further details of the error.
.PP
The following parameters must not be NULL: \f(CW\*(C`h\*(C'\fR, \f(CW\*(C`buf\*(C'\fR.
For more information see "Non\-NULL parameters" in \fBlibnbd\fR\|(3).
.SH "HANDLE STATE"
.IX Header "HANDLE STATE"
nbd_pwrite
can be called when the handle is in the following state:
.PP
.Vb 8
\& ┌─────────────────────────────────────┬─────────────────────────┐
\& │ Handle created, before connecting   │ ❌ error                │
\& │ Connecting                          │ ❌ error                │
\& │ Connecting & handshaking (opt_mode) │ ❌ error                │
\& │ Connected to the server             │ ✅ allowed              │
\& │ Connection shut down                │ ❌ error                │
\& │ Handle dead                         │ ❌ error                │
\& └─────────────────────────────────────┴─────────────────────────┘
.Ve
.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_PWRITE 1
.Ve
.SH EXAMPLE
.IX Header "EXAMPLE"
This example is also available as \fIexamples/reads\-and\-writes.c\fR
in the libnbd source code.
.PP
.Vb 10
\& /* 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 \e
\&  *     \-\-run \*(Aq./simple\-reads\-and\-writes $unixsocket\*(Aq
\&  */
\&
\& #include <stdio.h>
\& #include <stdlib.h>
\& #include <stdint.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\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.
\&    */
\&   if (nbd_connect_unix (nbd, argv[1]) == \-1) {
\&     fprintf (stderr, "%s\en", nbd_get_error ());
\&     exit (EXIT_FAILURE);
\&   }
\&
\&   /* Get the size of the disk and check
\&    * it\*(Aqs large enough.
\&    */
\&   exportsize = nbd_get_size (nbd);
\&   if (exportsize == \-1) {
\&     fprintf (stderr, "%s\en", 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\en",
\&              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\en", 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\en", nbd_get_error ());
\&       exit (EXIT_FAILURE);
\&     }
\&
\&     offset = rand () % (exportsize \- sizeof buf);
\&     if (nbd_pwrite (nbd, buf, sizeof buf,
\&                     offset, 0) == \-1) {
\&       fprintf (stderr, "%s\en", nbd_get_error ());
\&       exit (EXIT_FAILURE);
\&     }
\&   }
\&
\&   /* Sends a graceful shutdown to the server. */
\&   if (nbd_shutdown (nbd, 0) == \-1) {
\&     fprintf (stderr, "%s\en", nbd_get_error ());
\&     exit (EXIT_FAILURE);
\&   }
\&
\&   nbd_close (nbd);
\&
\&   exit (EXIT_SUCCESS);
\& }
.Ve
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbd_aio_pwrite\fR\|(3),
\&\fBnbd_can_fua\fR\|(3),
\&\fBnbd_create\fR\|(3),
\&\fBnbd_get_block_size\fR\|(3),
\&\fBnbd_is_read_only\fR\|(3),
\&\fBnbd_set_strict_mode\fR\|(3),
\&\fBlibnbd\fR\|(3).
.SH AUTHORS
.IX Header "AUTHORS"
Eric Blake
.PP
Richard W.M. Jones
.SH COPYRIGHT
.IX Header "COPYRIGHT"
Copyright Red Hat
.SH LICENSE
.IX Header "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.
.PP
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.
.PP
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