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 inheritance characteristic inherit.  Not all implementations will guaran-
     tee that the inheritance characteristic can be set on a page basis; the granularity of changes may be as large as an entire region.  FreeBSD
     is capable of adjusting inheritance characteristics on a page basis.  Inheritance 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     Inheritance is a rather esoteric feature largely superseded by the MAP_SHARED feature of mmap().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 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 indi-
     cate 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