minherit(2)

NAME

minherit - control the inheritance of pages

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/mman.h>
int
minherit(void *addr, size_t len, int inherit);

DESCRIPTION

The minherit() system call changes the specified pages to

have the inheritance characteristic inherit. Not all implementations will

guarantee
that the inheritance characteristic can be set on a page ba

sis; the granularity of changes may be as large as an entire region.

FreeBSD is capable of adjusting inheritance characteristics on a page ba

sis. Inheritance only effects children created by fork(). It has no

effect on
exec(). exec'd processes replace their address space en

tirely. This
system call also has no effect on the parent's address space

(other than
to potentially share the address space with its children).

Inheritance is a rather esoteric feature largely superseded

by the
MAP_SHARED feature of mmap(). However, it is possible to

use minherit()
to share a block of memory between parent and child that has

been mapped
MAP_PRIVATE. That is, modifications made by parent or child

are shared
but the original underlying file is left untouched.

INHERIT_SHARE This option causes the address space in

question to be

shared between parent and child. It has

no effect on
how the original underlying backing store

was mapped.

INHERIT_NONE This option prevents the address space in

question

from being inherited at all. The address

space will
be unmapped in the child.

INHERIT_COPY This option causes the child to inherit

the address

space as copy-on-write. This option also

has an
unfortunate side effect of causing the

parent address
space to become copy-on-write when the

parent forks.
If the original mapping was MAP_SHARED,

it will no
longer be shared in the parent after the

parent forks
and there is no way to get the previous

shared-backing-store mapping without unmapping and

remapping the
address space in the parent.

RETURN VALUES

The minherit() function returns the value 0 if successful;

otherwise the
value -1 is returned and the global variable errno is set to

indicate the
error.

ERRORS

The minherit() system call will fail if:

[EINVAL] The virtual address range specified by

the addr and

len arguments is not valid.

[EACCES] The flags specified by the inherit argu

ment were not

valid for the pages specified by the addr

and len
arguments.

SEE ALSO

fork(2), madvise(2), mincore(2), mprotect(2), msync(2), mun

map(2),
rfork(2)

HISTORY

The minherit() system call first appeared in OpenBSD.

BUGS

Once you set inheritance to MAP_PRIVATE or MAP_SHARED, there

is no way to
recover the original copy-on-write semantics short of unmap

ping and
remapping the area.

BSD February 17, 1996

BSD