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MINHERIT(2)		       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 inheri-
     tance 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  ad-
     justing  inheritance characteristics on a page basis.  Inheritance only ef-
     fects 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     Inheritance  is  a  rather  esoteric  feature  largely  superseded  by  the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 be-
		    ing 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 ef-
		    fect 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 argu-
			ments.

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 remap-
     ping the area.

FreeBSD ports 15.quarterly	 March 15, 2017 		     MINHERIT(2)

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