GET_MEMPOLICY(2)          Linux Programmer's Manual         GET_MEMPOLICY(2)

NAME         top

       get_mempolicy - retrieve NUMA memory policy for a thread

SYNOPSIS         top

       #include <numaif.h>

       int get_mempolicy(int *mode, unsigned long *nodemask,
                         unsigned long maxnode, void *addr,
                         unsigned long flags);

       Link with -lnuma.

DESCRIPTION         top

       get_mempolicy() retrieves the NUMA policy of the calling thread or of
       a memory address, depending on the setting of flags.

       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.

       If flags is specified as 0, then information about the calling
       thread's default policy (as set by set_mempolicy(2)) is returned, in
       the buffers pointed to by mode and 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 get_mempolicy() using
       set_mempolicy(2).  When flags is 0, addr must be specified as NULL.

       If flags specifies MPOL_F_MEMS_ALLOWED (available since Linux
       2.6.24), the mode argument is ignored and the set of nodes (memories)
       that the thread is allowed to specify in subsequent calls to mbind(2)
       or set_mempolicy(2) (in the absence of any mode flags) is returned in
       nodemask.  It is not permitted to combine MPOL_F_MEMS_ALLOWED with
       either MPOL_F_ADDR or MPOL_F_NODE.

       If flags specifies MPOL_F_ADDR, then information is returned about
       the policy governing the memory address given in addr.  This policy
       may be different from the thread's default policy if mbind(2) or one
       of the helper functions described in numa(3) has been used to
       establish a policy for the memory range containing addr.

       If the mode argument is not NULL, then get_mempolicy() will store the
       policy mode and any optional mode flags of the requested NUMA policy
       in the location pointed to by this argument.  If nodemask is not
       NULL, then the nodemask associated with the policy will be stored in
       the location pointed to by this argument.  maxnode specifies the
       number of node IDs that can be stored into nodemask—that is, the
       maximum node ID plus one.  The value specified by maxnode is always
       rounded to a multiple of sizeof(unsigned long)*8.

       If flags specifies both MPOL_F_NODE and MPOL_F_ADDR, get_mempolicy()
       will return the node ID of the node on which the address addr is
       allocated into the location pointed to by mode.  If no page has yet
       been allocated for the specified address, 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

       If flags specifies MPOL_F_NODE, but not MPOL_F_ADDR, and the thread's
       current policy is MPOL_INTERLEAVE, then get_mempolicy() will return
       in the location pointed to by a non-NULL 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.  These allocations
       include pages for memory-mapped files in process memory ranges mapped
       using the mmap(2) call with the MAP_PRIVATE flag for read accesses,
       and in memory ranges mapped with the MAP_SHARED flag for all

       Other flag values are reserved.

       For an overview of the possible policies see set_mempolicy(2).

RETURN VALUE         top

       On success, get_mempolicy() returns 0; on error, -1 is returned and
       errno is set to indicate the error.

ERRORS         top

       EFAULT Part of all of the memory range specified by nodemask and
              maxnode points outside your accessible address space.

       EINVAL The value specified by maxnode is less than the number of node
              IDs supported by the system.  Or flags specified values other
              than MPOL_F_NODE or MPOL_F_ADDR; or flags specified
              MPOL_F_ADDR and addr is NULL, or flags did not specify
              MPOL_F_ADDR and addr is not NULL.  Or, flags specified
              MPOL_F_NODE but not MPOL_F_ADDR and the current thread policy
              is not MPOL_INTERLEAVE.  Or, flags specified
              MPOL_F_MEMS_ALLOWED with either MPOL_F_ADDR or MPOL_F_NODE.
              (And there are other EINVAL cases.)

VERSIONS         top

       The get_mempolicy() system call was added to the Linux kernel in
       version 2.6.7.

CONFORMING TO         top

       This system call is Linux-specific.

NOTES         top

       For information on library support, see numa(7).

SEE ALSO         top

       getcpu(2), mbind(2), mmap(2), set_mempolicy(2), numa(3), numa(7),

COLOPHON         top

       This page is part of release 4.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2016-12-12                 GET_MEMPOLICY(2)