.\" SPDX-License-Identifier: Linux-man-pages-copyleft-var
.\"
.\" Copyright 2003,2004 Andi Kleen, SuSE Labs.
.\" and Copyright 2007 Lee Schermerhorn, Hewlett Packard
.\"
.\" 2006-02-03, mtk, substantial wording changes and other improvements
.\" 2007-08-27, Lee Schermerhorn <Lee.Schermerhorn@hp.com>
.\"     more precise specification of behavior.
.\"
.TH get_mempolicy 2 2023-07-16 "Linux man-pages 6.05.01"
.SH NAME
get_mempolicy \- retrieve NUMA memory policy for a thread
.SH LIBRARY
NUMA (Non-Uniform Memory Access) policy library
.RI ( libnuma ", " \-lnuma )
.SH SYNOPSIS
.B "#include <numaif.h>"
.nf
.PP
.BI "long get_mempolicy(int *" mode ,
.BI "                   unsigned long " nodemask [(. maxnode " + ULONG_WIDTH - 1)"
.B "                                          / ULONG_WIDTH],"
.BI "                   unsigned long " maxnode ", void *" addr ,
.BI "                   unsigned long " flags );
.fi
.SH DESCRIPTION
.BR get_mempolicy ()
retrieves the NUMA policy of the calling thread or of a memory address,
depending on the setting of
.IR flags .
.PP
A NUMA machine has different
memory controllers with different distances to specific CPUs.
The memory policy defines from which node memory is allocated for
the thread.
.PP
If
.I flags
is specified as 0,
then information about the calling thread's default policy
(as set by
.BR set_mempolicy (2))
is returned, in the buffers pointed to by
.I mode
and
.IR nodemask .
The value returned in these arguments
may be used to restore the thread's policy to its state at
the time of the call to
.BR get_mempolicy ()
using
.BR set_mempolicy (2).
When
.I flags
is 0,
.I addr
must be specified as NULL.
.PP
If
.I flags
specifies
.B MPOL_F_MEMS_ALLOWED
(available since Linux 2.6.24), the
.I mode
argument is ignored and the set of nodes (memories) that the
thread is allowed to specify in subsequent calls to
.BR mbind (2)
or
.BR set_mempolicy (2)
(in the absence of any
.IR "mode flags" )
is returned in
.IR nodemask .
It is not permitted to combine
.B MPOL_F_MEMS_ALLOWED
with either
.B MPOL_F_ADDR
or
.BR MPOL_F_NODE .
.PP
If
.I flags
specifies
.BR MPOL_F_ADDR ,
then information is returned about the policy governing the memory
address given in
.IR addr .
This policy may be different from the thread's default policy if
.BR mbind (2)
or one of the helper functions described in
.BR numa (3)
has been used to establish a policy for the memory range containing
.IR addr .
.PP
If the
.I mode
argument is not NULL, then
.BR get_mempolicy ()
will store the policy mode and any optional
.I "mode flags"
of the requested NUMA policy in the location pointed to by this argument.
If
.I nodemask
is not NULL, then the nodemask associated with the policy will be stored
in the location pointed to by this argument.
.I maxnode
specifies the number of node IDs
that can be stored into
.IR nodemask \[em]that
is, the maximum node ID plus one.
The value specified by
.I maxnode
is always rounded to a multiple of
.IR "sizeof(unsigned\ long)*8" .
.PP
If
.I flags
specifies both
.B MPOL_F_NODE
and
.BR MPOL_F_ADDR ,
.BR get_mempolicy ()
will return the node ID of the node on which the address
.I addr
is allocated into the location pointed to by
.IR mode .
If no page has yet been allocated for the specified address,
.BR get_mempolicy ()
will allocate a page as if the thread had performed a read
(load) access to that address, and return the ID of the node
where that page was allocated.
.PP
If
.I flags
specifies
.BR MPOL_F_NODE ,
but not
.BR MPOL_F_ADDR ,
and the thread's current policy is
.BR MPOL_INTERLEAVE ,
then
.BR get_mempolicy ()
will return in the location pointed to by a non-NULL
.I mode
argument,
the node ID of the next node that will be used for
interleaving of internal kernel pages allocated on behalf of the thread.
.\" Note:  code returns next interleave node via 'mode' argument -Lee Schermerhorn
These allocations include pages for memory-mapped files in
process memory ranges mapped using the
.BR mmap (2)
call with the
.B MAP_PRIVATE
flag for read accesses, and in memory ranges mapped with the
.B MAP_SHARED
flag for all accesses.
.PP
Other flag values are reserved.
.PP
For an overview of the possible policies see
.BR set_mempolicy (2).
.SH RETURN VALUE
On success,
.BR get_mempolicy ()
returns 0;
on error, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EFAULT
Part of all of the memory range specified by
.I nodemask
and
.I maxnode
points outside your accessible address space.
.TP
.B EINVAL
The value specified by
.I maxnode
is less than the number of node IDs supported by the system.
Or
.I flags
specified values other than
.B MPOL_F_NODE
or
.BR MPOL_F_ADDR ;
or
.I flags
specified
.B MPOL_F_ADDR
and
.I addr
is NULL,
or
.I flags
did not specify
.B MPOL_F_ADDR
and
.I addr
is not NULL.
Or,
.I flags
specified
.B MPOL_F_NODE
but not
.B MPOL_F_ADDR
and the current thread policy is not
.BR MPOL_INTERLEAVE .
Or,
.I flags
specified
.B MPOL_F_MEMS_ALLOWED
with either
.B MPOL_F_ADDR
or
.BR MPOL_F_NODE .
(And there are other
.B EINVAL
cases.)
.SH STANDARDS
Linux.
.SH HISTORY
Linux 2.6.7.
.SH NOTES
For information on library support, see
.BR numa (7).
.SH SEE ALSO
.BR getcpu (2),
.BR mbind (2),
.BR mmap (2),
.BR set_mempolicy (2),
.BR numa (3),
.BR numa (7),
.BR numactl (8)