minherit (2)

Leading comments

 $FreeBSD: releng/11.0/lib/libc/sys/minherit.2 173184 2007-10-30 15:28:43Z keramida $

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(The comments found at the beginning of the groff file "man2/minherit.2freebsd".)

NAME

minherit - control the inheritance of pages

LIBRARY

Lb libc

SYNOPSIS

In sys/mman.h Ft int Fn minherit void *addr size_t len int inherit

DESCRIPTION

The Fn minherit system call changes the specified pages to have the inheritance characteristic Fa inherit . Not all implementations will guarantee that the inheritance characteristic can be set on a page basis; the granularity of changes may be as large as an entire region. Fx is capable of adjusting inheritance characteristics on a page basis. Inheritance only effects children created by Fn fork . It has no effect on Fn exec . exec'd processes replace their address space entirely. 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Inheritance is a rather esoteric feature largely superseded by the MAP_SHARED feature of Fn mmap . However, it is possible to use Fn 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

Rv -std minherit

ERRORS

The Fn minherit system call will fail if:

Bq Er EINVAL

The virtual address range specified by the Fa addr and Fa len arguments is not valid.

Bq Er EACCES

The flags specified by the Fa inherit argument were not valid for the pages specified by the Fa addr and Fa len arguments.

SEE ALSO

fork(2), madvise(2), mincore(2), mprotect(2), msync(2), munmap(2), rfork(2)

HISTORY

The Fn minherit system call first appeared in Ox and then in Fx 2.2 .

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 unmapping and remapping the area.