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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
     guarantee 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     Inheritance is a rather esoteric feature largely superseded by the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 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.