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MEMGUARD(9)	       FreeBSD Kernel Developer's Manual	   MEMGUARD(9)

     MemGuard -- memory	allocator for debugging	purposes

     options DEBUG_MEMGUARD

     MemGuard is a simple and small replacement	memory allocator designed to
     help detect tamper-after-free scenarios.  These problems are more and
     more common and likely with multithreaded kernels where race conditions
     are more prevalent.

     Currently,	MemGuard can take over malloc(), realloc() and free() for a
     single malloc type.  MemGuard can also guard all allocations larger than
     PAGE_SIZE,	and can	guard a	random fraction	of all allocations.  There is
     also a knob to prevent allocations	smaller	than a specified size from
     being guarded, to limit memory waste.

     To	use MemGuard for a memory type,	either add an entry to


     Or	set the	vm.memguard.desc sysctl(8) variable at run-time:

	   sysctl vm.memguard.desc=<memory_type>

     Where memory_type is a short description of the memory type to monitor.
     Only allocations from that	memory_type made after vm.memguard.desc	is set
     will potentially be guarded.  If vm.memguard.desc is modified at run-time
     then only allocations of the new memory_type will potentially be guarded
     once the sysctl(8)	is set.	 Existing guarded allocations will still be
     properly released by free(9).

     The short description of a	malloc(9) type is the second argument to
     MALLOC_DEFINE(9), so one has to find it in	the kernel source.

     The vm.memguard.divisor boot-time tunable is used to scale	how much of
     the system's physical memory MemGuard is allowed to consume.  The default
     is	10, so up to cnt.v_page_count/10 pages can be used.  MemGuard will
     reserve vm_kmem_max / vm.memguard.divisor bytes of	virtual	address	space,
     limited by	twice the physical memory size.	 The physical limit is
     reported as vm.memguard.phys_limit	and the	virtual	space reserved for
     MemGuard is reported as vm.memguard.mapsize.

     MemGuard will not do page promotions for any allocation smaller than
     vm.memguard.minsize bytes.	 The default is	0, meaning all allocations can
     potentially be guarded.  MemGuard can guard sufficiently large alloca-
     tions randomly, with average frequency of every one in 100000 /
     vm.memguard.frequency allocations.	 The default is	0, meaning no alloca-
     tions are randomly	guarded.

     MemGuard can optionally add unmapped guard	pages around each allocation
     to	detect overflow	and underflow, if vm.memguard.options has the 1	bit
     set.  This	option is enabled by default.  MemGuard	will optionally	guard
     all allocations of	PAGE_SIZE or larger if vm.memguard.options has the 2
     bit set.  This option is off by default.

     sysctl(8),	vmstat(8), contigmalloc(9), malloc(9), redzone(9)

     MemGuard first appeared in	FreeBSD	6.0.

     MemGuard was originally written by	Bosko Milekic <>.
     This manual page was originally written by	Christian Brueffer
     <>.  Additions	have been made by Matthew Fleming
     <> to both the implementation and the documentation.

     Currently,	it is not possible to override UMA zone(9) allocations.

FreeBSD	9.2			August 2, 2010			   FreeBSD 9.2


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