=head1 NAME

nbd_zero - send write zeroes command to the NBD server

=head1 SYNOPSIS

 #include <libnbd.h>

 int nbd_zero (
       struct nbd_handle *h, uint64_t count,
       uint64_t offset, uint32_t flags
     );

=head1 DESCRIPTION

Issue a write zeroes command to the NBD server,
which if supported
by the server causes a zeroes to be written efficiently
starting at C<offset> and ending at C<offset> + C<count> - 1.
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
L<nbd_can_zero(3)> is false or L<nbd_is_read_only(3)> is true.

Note that not all servers can support a C<count> of 4GiB or larger;
L<nbd_get_extended_headers_negotiated(3)> indicates which servers
will parse a request larger than 32 bits.
The NBD protocol does not yet have a way for a client to learn if
the server will enforce an even smaller maximum zero size, although
a future extension may add a constraint visible in
L<nbd_get_block_size(3)>.  Also, some servers may permit a larger
zero request only when the C<LIBNBD_CMD_FLAG_FAST_ZERO> is in use.

The C<flags> parameter may be C<0> for no flags, or may contain
C<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
L<nbd_can_fua(3)>), C<LIBNBD_CMD_FLAG_NO_HOLE> meaning that
the server should favor writing actual allocated zeroes over
punching a hole, and/or C<LIBNBD_CMD_FLAG_FAST_ZERO> meaning
that the server must fail quickly if writing zeroes is no
faster than a normal write (if that is supported - some servers
cannot do this, see L<nbd_can_fast_zero(3)>).

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 L<nbd_set_strict_mode(3)>
function can be used to alter which scenarios should await a server
reply rather than failing fast.

=head1 RETURN VALUE

If the call is successful the function returns C<0>.

=head1 ERRORS

On error C<-1> is returned.

Refer to L<libnbd(3)/ERROR HANDLING>
for how to get further details of the error.

The following parameters must not be NULL: C<h>.
For more information see L<libnbd(3)/Non-NULL parameters>.

=head1 HANDLE STATE

nbd_zero
can be called when the handle is in the following state:

 ┌─────────────────────────────────────┬─────────────────────────┐
 │ Handle created, before connecting   │ ❌ error                │
 │ Connecting                          │ ❌ error                │
 │ Connecting & handshaking (opt_mode) │ ❌ error                │
 │ Connected to the server             │ ✅ allowed              │
 │ Connection shut down                │ ❌ error                │
 │ Handle dead                         │ ❌ error                │
 └─────────────────────────────────────┴─────────────────────────┘

=head1 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_ZERO 1

=head1 SEE ALSO

L<nbd_aio_zero(3)>,
L<nbd_can_fast_zero(3)>,
L<nbd_can_fua(3)>,
L<nbd_can_zero(3)>,
L<nbd_create(3)>,
L<nbd_get_block_size(3)>,
L<nbd_get_extended_headers_negotiated(3)>,
L<nbd_is_read_only(3)>,
L<nbd_set_strict_mode(3)>,
L<libnbd(3)>.

=head1 AUTHORS

Eric Blake

Richard W.M. Jones

=head1 COPYRIGHT

Copyright Red Hat
