---
name: "get_mempolicy"
section: "2"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/get_mempolicy"
---

# get\_mempolicy(2)

retrieve NUMA memory policy for a thread

## NAME

get\_mempolicy - retrieve NUMA memory policy for a thread

## LIBRARY

NUMA (Non-Uniform Memory Access) policy library (*libnuma*, *-lnuma*)

## SYNOPSIS

**#include \<numaif.h\>**

```

```

**long get\_mempolicy(**unsigned long maxnode;                    **int \****mode***,**                    **unsigned long** *nodemask***\[(***maxnode* **+ ULONG\_WIDTH - 1)**                                           **/ ULONG\_WIDTH\],**                    **unsigned long** *maxnode***, void \****addr***,**                    **unsigned long** *flags***);**

## DESCRIPTION

**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)](/set_mempolicy/2.md)) 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)](/set_mempolicy/2.md). 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)](/mbind/2.md) or [**set\_mempolicy**(2)](/set_mempolicy/2.md) (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)](/mbind/2.md) or one of the helper functions described in [**numa**(3)](/numa/3.md) 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 allocated.

If *flags* specifies **MPOL\_F\_NODE**, but not **MPOL\_F\_ADDR**, and the thread's current policy is **MPOL\_INTERLEAVE** or **MPOL\_WEIGHTED\_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)](/mmap/2.md) call with the **MAP\_PRIVATE** flag for read accesses, and in memory ranges mapped with the **MAP\_SHARED** flag for all accesses.

Other flag values are reserved.

For an overview of the possible policies see [**set\_mempolicy**(2)](/set_mempolicy/2.md).

## RETURN VALUE

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

## ERRORS

- **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 neither **MPOL\_INTERLEAVE** nor **MPOL\_WEIGHTED\_INTERLEAVE**. Or, *flags* specified **MPOL\_F\_MEMS\_ALLOWED** with either **MPOL\_F\_ADDR** or **MPOL\_F\_NODE**. (And there are other **EINVAL** cases.)

## STANDARDS

Linux.

## HISTORY

Linux 2.6.7.

## NOTES

For information on library support, see [**numa**(7)](/numa/7.md).

## SEE ALSO

[**getcpu**(2)](/getcpu/2.md), [**mbind**(2)](/mbind/2.md), [**mmap**(2)](/mmap/2.md), [**set\_mempolicy**(2)](/set_mempolicy/2.md), [**numa**(3)](/numa/3.md), [**numa**(7)](/numa/7.md), [**numactl**(8)](/numactl/8.md)