.\" Copyright 2003,2004 Andi Kleen, SuSE Labs. .\" and Copyright 2007 Lee Schermerhorn, Hewlett Packard .\" .\" %%%LICENSE_START(VERBATIM_PROF) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .\" 2006-02-03, mtk, substantial wording changes and other improvements .\" 2007-08-27, Lee Schermerhorn .\" more precise specification of behavior. .\" .TH GET_MEMPOLICY 2 2017-09-15 Linux "Linux Programmer's Manual" .SH NAME get_mempolicy \- retrieve NUMA memory policy for a thread .SH SYNOPSIS .B "#include " .nf .PP .BI "long get_mempolicy(int *" mode ", unsigned long *" nodemask , .BI " unsigned long " maxnode ", void *" addr , .BI " unsigned long " flags ); .PP Link with \fI\-lnuma\fP. .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 .BR 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 \(emthat 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 VERSIONS The .BR get_mempolicy () system call was added to the Linux kernel in version 2.6.7. .SH CONFORMING TO This system call is Linux-specific. .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) .SH COLOPHON This page is part of release 5.10 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.