minherit man page on FreeBSD

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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     Inheritance is a rather esoteric feature largely superseded by the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.

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.

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			       October 30, 2007				   BSD
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