zx_vmo_create_child

NAME

Create a child of a VM Object.

SYNOPSIS

#include <zircon/syscalls.h>

zx_status_t zx_vmo_create_child(zx_handle_t handle,
                                uint32_t options,
                                uint64_t offset,
                                uint64_t size,
                                zx_handle_t* out);

DESCRIPTION

zx_vmo_create_child() creates a new virtual memory object (VMO) a child of an existing vmo. The behavior of the semantics depends on the type of the child.

One handle is returned on success, representing an object with the requested size.

options must contain exactly one of the following flags to specify the child type:

  • ZX_VMO_CHILD_SNAPSHOT - Create a child that behaves as if an eager copy is performed. When a write occurs, both the parent and child perform a lazy copy. Lazy copying allows both the child and the parent to diverge from each other. Any reads from ranges outside of the parent VMO's size contain zeros, and writes allocate new zero filled pages. This flag is not supported on:

    • VMOs with pinned regions.
    • VMOs created with or descended from zx_vmo_create_physical()
    • VMOs backed by a user pager. For information on VMO syscall interactions with children, see NOTES.
  • ZX_VMO_CHILD_SNAPSHOT_AT_LEAST_ON_WRITE - Create a child that behaves with at least copy on write semantics. Any write operation on the child brings in a copy of the page from the parent, after which its contents may diverge from the parent. Until a page is written to, and copied, reads are permitted, although not guaranteed, to return changing values if the parent performs writes. This flag may not be used for VMOs created with zx_vmo_create_physical() or descendants of such a VMO. For information on VMO syscall interactions with children, see NOTES.

  • ZX_VMO_CHILD_SLICE - Create a slice that has direct read/write access into a section of the parent. All operations on the slice vmo behave as if they were done on the parent. A slice differs from a duplicate handle to the parent by allowing access to only a subrange of the parent vmo, and allowing for the ZX_VMO_ZERO_CHILDREN signal to be used. This flag may be used with vmos created with zx_vmo_create_physical() and their descendants.

An alias child type of ZX_VMO_CHILD_COPY_ON_WRITE is also defined and is equivalent to ZX_VMO_CHILD_SNAPSHOT_AT_LEAST_ON_WRITE. This alias is intended to be used when ZX_VMO_CHILD_SNAPSHOT is desired, but the VMO may be backed by a user pager. In all other cases, you should use one of the precise child types as this alias may change or become deprecated.

In addition, options can contain zero or more of the following flags to further specify the child's behavior:

  • ZX_VMO_CHILD_RESIZEABLE - Create a resizeable child VMO.

  • ZX_VMO_CHILD_NO_WRITE - Create a child that cannot be written to.

offset must be page aligned.

offset + size may not exceed the range of a 64bit unsigned value.

Both offset and size may start or extend beyond the original VMO's size.

The size of the VMO will be rounded up to the next page size boundary.

By default the rights of the child handled will be the same as the original with a few exceptions. See zx_vmo_create() for a discussion of the details of each right.

In all cases if ZX_VMO_NO_WRITE is set then ZX_RIGHT_WRITE will be removed.

If options is ZX_VMO_CHILD_COPY_ON_WRITE or ZX_VMO_CHILD_PRIVATE_PAGER_COPY and ZX_VMO_CHILD_NO_WRITE is not set then ZX_RIGHT_WRITE will be added and ZX_RIGHT_EXECUTE will be removed.

NOTES

Creating a child VMO causes the existing (source) VMO ZX_VMO_ZERO_CHILDREN signal to become inactive. Only when the last child is destroyed and no mappings of those child into address spaces exist, will ZX_VMO_ZERO_CHILDREN become active again.

Non-slice child vmos will interact with the VMO syscalls in the following ways:

  • The COMMIT mode of zx_vmo_op_range() on a child will commit pages into the child that have the same content as its parent's corresponding pages. If those pages are supplied by a pager, this operation will also commit those pages in the parent. Otherwise, if those pages are not comitted in the parent, zero-filled pages will be comitted directly into child, without affecting the parent.
  • The DECOMMIT mode of zx_vmo_op_range() is not supported.

RIGHTS

handle must be of type ZX_OBJ_TYPE_VMO and have ZX_RIGHT_DUPLICATE and have ZX_RIGHT_READ.

RETURN VALUE

zx_vmo_create_child() returns ZX_OK on success. In the event of failure, a negative error value is returned.

ERRORS

ERR_BAD_TYPE Input handle is not a VMO.

ZX_ERR_ACCESS_DENIED Input handle does not have sufficient rights.

ZX_ERR_INVALID_ARGS out is an invalid pointer or NULL or the offset is not page aligned.

ZX_ERR_OUT_OF_RANGE offset + size is too large.

ZX_ERR_NO_MEMORY Failure due to lack of memory. There is no good way for userspace to handle this (unlikely) error. In a future build this error will no longer occur.

ERR_BAD_STATE A COW child could not be created because the vmo has some pinned pages.

SEE ALSO