---
title: "hv_vmx_vcpu_get_cap_write_vmcs(_:_:_:_:)"
framework: Hypervisor
role: symbol
role_heading: Function
platforms: [macOS 11.0+]
path: "hypervisor/hv_vmx_vcpu_get_cap_write_vmcs(_:_:_:_:)"
---

# hv_vmx_vcpu_get_cap_write_vmcs(_:_:_:_:)

Returns the allowed_0 and allowed_1 masks for a VMCS field of a vCPU.

## Declaration

```swift
func hv_vmx_vcpu_get_cap_write_vmcs(_ vcpu: hv_vcpuid_t, _ field: UInt32, _ allowed_0: UnsafeMutablePointer<UInt64>, _ allowed_1: UnsafeMutablePointer<UInt64>) -> hv_return_t
```

## Parameters

- `vcpu`: The vCPU ID.
- `field`: The ID of the VMCS field for which to return capabilities.
- `allowed_0`: The pointer to the VMCS allowed_0 mask (written on success).
- `allowed_1`: The pointer to the VMCS allowed_1 mask (written on success).

## Return Value

Return Value HV_SUCCESS if the operation was successful, otherwise an error code specified in hv_return_t.

## Discussion

Discussion Returns the constraints imposed by the Hypervisor framework on the given VMCS field, in the form of allowed_0 and allowed_1 masks, that indicate what bit values may be set when writing the given VMCS field. When writing to a VMCS field, the caller is allowed to set bits that are 0 in its allowed_0 mask to 0, and bits that are 1 in its allowed_1 mask to 1. This means: If allowed_0 = 0, allowed_1 = 0 -> must be NOT SET. If allowed_0 = 0, allowed_1 = 1 -> can be either SET or NOT SET. If allowed_0 = 1, allowed_1 = 0 -> undefined (shouldn’t happen). If allowed_0 = 1, allowed_1 = 1 -> must be SET. important: This function must be called by the owning thread.

## See Also

### Field management

- [Virtual Machine Control Structure (VMCS) Field IDs](hypervisor/virtual-machine-control-structure-vmcs-field-ids.md)
- [hv_vmx_vcpu_read_vmcs(_:_:_:)](hypervisor/hv_vmx_vcpu_read_vmcs(_:_:_:).md)
- [hv_vmx_vcpu_write_vmcs(_:_:_:)](hypervisor/hv_vmx_vcpu_write_vmcs(_:_:_:).md)
- [hv_vmx_vcpu_set_apic_address(_:_:)](hypervisor/hv_vmx_vcpu_set_apic_address(_:_:).md)
- [Virtual Machine control structure (VMCS) Field IDs](hypervisor/1469436-virtual_machine_control_structur-enum.md)