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     Inheritance is a rather esoteric feature largely superseded by the MAP_SHARED feature of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.

     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.