# The `.m` namespace

## Modules

From kdb+ 5.0 onwards, the `.m` namespace was repurposed to hold the local namespaces of modules. 
Objects in the new `.m` namespace should not be accessed directly but only by using the interface provided by the respective module.

Existing code that uses `.m` for [`memory domains`](#memory-backed-files) will not work in kdb+ 5.0+ and should be updated to remove this usage to avoid unexpected issues.

## Memory backed files

V4.0 only (since 2020.03.17). For V5.0 and above see [`modules`](#modules).

Memory can be backed by a filesystem, allowing use of DAX-enabled filesystems as a non-persistent memory extension for kdb+.

[Command-line option `-m path`](../basics/cmdline.md#-m-memory-domain) directs kdb+ to use the filesystem path specified as a separate memory domain. This splits every thread’s heap into two:

domain | description
-------|------------
0      | regular anonymous memory, active and used for all allocs by default
1      | filesystem-backed memory

The `.m` namespace is reserved for objects in memory domain 1, however names from other namespaces can reference them too, e.g. `a:.m.a:1 2 3`

`\d .m` changes current memory domain to 1, causing it to be used by all further allocs. `\d .anyotherns` sets it back to 0.

`.m.x:x` ensures the entirety of `.m.x` is in memory domain 1, performing a deep copy of `x` as needed. (Objects of types `100h`-`103h`, `112h` are not copied and remain in memory domain 0.)

Lambdas defined in `.m` set current memory domain to 1 during execution. This will nest, since other lambdas don’t change memory domains:
```q
q)\d .myns
q)g:{til x}
q)\d .m
q)w:{system"w"};f:{.myns.g x}
q)\d .
q)x:.m.f 1000000;.m.w` / x allocated in domain 1
```

[Internal function `-120!x`](../basics/internal.md#-120x-memory-domain) returns `x`’s memory domain, currently 0 or 1.
```q
q)-120!'(1 2 3;.m.x:1 2 3)
0 1
```

[System command `\w`](../basics/syscmds.md#w-workspace) returns memory info for the current memory domain only.
```q
q)value each ("\\d .m";"\\w";"\\d .";"\\w")
::
353968 67108864 67108864 0 0 8589934592
::
354032 67108864 67108864 0 0 8589934592
```

[Command-line option `-w limit`](../basics/cmdline.md#-w-workspace) (M1/m2) is no longer thread-local, but memory domain-local. Command-line option `-w`, and [system command `\w`](../basics/syscmds.md#w-workspace) set limit for memory domain 0.