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minherit(2)
MINHERIT(2) BSD System Calls Manual 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 inher‐
itance characteristic inherit. Not all implementations will guarantee
that the inheritance characteristic can be set on a page basis; the gran‐
ularity of changes may be as large as an entire region. FreeBSD is capa‐
ble of adjusting inheritance characteristics on a page basis. Inheri‐
tance only effects children created by fork(). It has no effect on
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Inheritance is a rather esoteric feature largely superseded by the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 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 map‐
ping 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 unmap‐
ping and remapping the address space in the parent.
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.
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 argument 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), munmap(2),
rfork(2)
HISTORY
The minherit() system call first appeared in OpenBSD and then in
FreeBSD 2.2.
The INHERIT_ZERO support first appeared in OpenBSD 5.6 and then in
FreeBSD 12.0.
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.
BSD March 15, 2017 BSD