# Object usage

Processes create and use kernel objects to perform work. Just as memory can
be leaked or misused (e.g use-after-free), handles to kernel object can
be leaked or misused (e.g use-after-close).

## Handles tool

To help developers diagnose handle issues use the `handles` tool, below
is a sample of process 29831 which is wlancfg.cmx :

```
$ handles 29831
    handle   koid rkoid     rights type
0xa8d44a0f: 29973       0x0000d0ef vmo
0xa8e44aab: 29847 29846 0x0000f00e channel
0xa8d44a0b: 29972       0x0000d0ef vmo
0xa8e42413:  9931  9930 0x0000f00e channel
0xa8d44a07: 29971       0x0000d0ef vmo
0xa8f44a1f: 29969 29970 0x0000f00e channel
0xa8a44a3b: 29964       0x0000d0ef vmo
0xa8d44a17: 29962 29963 0x0000f00e channel
0xa8844a43: 29961       0x0000d0ef vmo
0xa8f44a4b: 29960       0x0000d0ef vmo
0xa8e44a3f: 29959       0x0000d0ef vmo
0xa8e44a23: 29958       0x0000800f port
0xa8f44a2f: 29957       0x0000d0ef vmo
0xa8644a53: 29911       0x0000d0ef vmo
0xa8a44a7f: 29908       0x0000d0ef vmo
0xa8844a6b: 29907       0x0000d0ef vmo
0xa8f44a63: 29906       0x0000d0ef vmo
0xa8844a6f: 29905       0x0000d0ef vmo
0xa8f44a8b: 29904       0x0000d0ef vmo
0xa8944a9f: 29903       0x0000d0ef vmo
0xa8444a83: 29900       0x0000800f vmar
0xa8e44a77: 29845       0x0000d0ef vmo
0xa8f44a8f:  1034       0x0000d0f7 vmo
0xa8d44aa3:  1129       0x0000d00b log
0xa8d44abf:  1129       0x0000d00b log
0xa8d44abb:  1129       0x0000d00b log
0xa8644aef: 29827 29828 0x0000f00e channel
0xa8844ac3: 29826  8711 0x0007dfcf job
0xa8144afb: 29825 29824 0x0000f00e channel
0xa8e44adb: 29816 29817 0x0000f00e channel
0xa8e44ad3: 29776 29777 0x0000f00e channel
0xa894496b: 29766 29767 0x0000f00e channel
0xa8d44a97: 29833 29831 0x0004d2cf thread
0xa8d44a93: 29832       0x0000801f vmar
0xa8d44aaf: 29831 29826 0x0006d3cf process
0xa8f44a73: 29850       0x0000d00b log
0xa8f44af3: 29768 29769 0x0000f00e channel
0xa8e44aa7: 29834 29835 0x0000f00e channel
38 handles
```

The `handles <pid>` tool dumps the process handle table, which holds all
accessible handles for that particular process at the moment of invocation.

For each handle the tool prints the handle value, the koid of the object it
points to, the related koid (rkoid) if the object has a related object, the
rights of the handle and the type of object.

In the example above, it shows 38 unique handles which map to 36 unique objects;
3 of the handles point to the same "log" object with koid 1129.

It should be noted that not all alive objects might be displayed by the tool.
For example, a thread can be alive even if there are not handles open to it and
VMOs can be held alive by the associated VMAR.

The `handles` tool supports filtering and reverse filtering by object type; use
`handles --help` to see all the options.

## Bad handle policy

Using a handle after it has been [closed](/docs/reference/syscalls/handle_close.md)
or closing a handle that has been already closed are mistakes that can create
hard to diagnose errors.

In order to help developers find these issues, the "bad handle" Job policy can
be activated using [zx_job_set_policy](/docs/reference/syscalls/job_set_policy.md)
with the condition **ZX_POL_BAD_HANDLE** and the action
**ZX_POL_ACTION_ALLOW_EXCEPTION**. When a process is launched under a job with
this policy, any use of an already closed handle will generate an exception
that if not handled will terminate the process and log the offending call stack
or that can be trapped by the [debugger](/docs/development/sdk/documentation/debugger.md)
for interactive troubleshooting.