xref: /freebsd/lib/libsys/minherit.2 (revision 8269e7673cf033aba67dab8264fe719920c70f87)

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.Dd March 15, 2017
.Dt MINHERIT 2
.Os
.Sh NAME
.Nm minherit
.Nd control the inheritance of pages
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/mman.h
.Ft int
.Fn minherit "void *addr" "size_t len" "int inherit"
.Sh 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).
.Pp
Inheritance is a rather esoteric feature largely superseded by the
.Dv 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
.Dv MAP_PRIVATE .
That is, modifications made by parent or child are shared but
the original underlying file is left untouched.
.Bl -tag -width ".Dv INHERIT_SHARE"
.It Dv 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.
.It Dv INHERIT_NONE
This option prevents the address space in question from being inherited
at all.
The address space will be unmapped in the child.
.It Dv 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
.Dv 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.
.It Dv INHERIT_ZERO
This option causes the address space in question to be mapped as new
anonymous pages,
which would be initialized to all zero bytes,
in the child process.
.El
.Sh RETURN VALUES
.Rv -std minherit
.Sh ERRORS
The
.Fn minherit
system call will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The virtual address range specified by the
.Fa addr
and
.Fa len
arguments is not valid.
.It 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.
.El
.Sh SEE ALSO
.Xr fork 2 ,
.Xr madvise 2 ,
.Xr mincore 2 ,
.Xr mprotect 2 ,
.Xr msync 2 ,
.Xr munmap 2 ,
.Xr rfork 2
.Sh HISTORY
The
.Fn minherit
system call first appeared in
.Ox
and then in
.Fx 2.2 .
.Pp
The
.Dv INHERIT_ZERO
support first appeared in
.Ox 5.6
and then in
.Fx 12.0 .
.Sh BUGS
Once you set inheritance to
.Dv MAP_PRIVATE
or
.Dv MAP_SHARED ,
there is no way to recover the original copy-on-write semantics
short of unmapping and remapping the area.