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MLOCK(2)		  FreeBSD System Calls Manual		      MLOCK(2)

     mlock, munlock -- lock (unlock) physical pages in memory

     Standard C	Library	(libc, -lc)

     #include <sys/mman.h>

     mlock(const void *addr, size_t len);

     munlock(const void	*addr, size_t len);

     The mlock() system	call locks into	memory the physical pages associated
     with the virtual address range starting at	addr for len bytes.  The
     munlock() system call unlocks pages previously locked by one or more
     mlock() calls.  For both, the addr	argument should	be aligned to a	multi-
     ple of the	page size.  If the len argument	is not a multiple of the page
     size, it will be rounded up to be so.  The	entire range must be allo-

     After an mlock() system call, the indicated pages will cause neither a
     non-resident page nor address-translation fault until they	are unlocked.
     They may still cause protection-violation faults or TLB-miss faults on
     architectures with	software-managed TLBs.	The physical pages remain in
     memory until all locked mappings for the pages are	removed.  Multiple
     processes may have	the same physical pages	locked via their own virtual
     address mappings.	A single process may likewise have pages multiply-
     locked via	different virtual mappings of the same pages or	via nested
     mlock() calls on the same address range.  Unlocking is performed explic-
     itly by munlock() or implicitly by	a call to munmap() which deallocates
     the unmapped address range.  Locked mappings are not inherited by the
     child process after a fork(2).

     Since physical memory is a	potentially scarce resource, processes are
     limited in	how much they can lock down.  The amount of memory that	a sin-
     gle process can mlock() is	limited	by both	the per-process	RLIMIT_MEMLOCK
     resource limit and	the system-wide	``wired	pages''	limit vm.max_wired.
     vm.max_wired applies to the system	as a whole, so the amount available to
     a single process at any given time	is the difference between vm.max_wired
     and vm.stats.vm.v_wire_count.

     If	security.bsd.unprivileged_mlock	is set to 0 these calls	are only
     available to the super-user.

     Upon successful completion, the value 0 is	returned; otherwise the
     value -1 is returned and the global variable errno	is set to indicate the

     If	the call succeeds, all pages in	the range become locked	(unlocked);
     otherwise the locked status of all	pages in the range remains unchanged.

     The mlock() system	call will fail if:

     [EPERM]		security.bsd.unprivileged_mlock	is set to 0 and	the
			caller is not the super-user.

     [EINVAL]		The address given is not page aligned or the length is

     [EAGAIN]		Locking	the indicated range would exceed the system
			limit for locked memory.

     [ENOMEM]		Some portion of	the indicated address range is not
			allocated.  There was an error faulting/mapping	a
			page.  Locking the indicated range would exceed	the
			per-process limit for locked memory.
     The munlock() system call will fail if:

     [EPERM]		security.bsd.unprivileged_mlock	is set to 0 and	the
			caller is not the super-user.

     [EINVAL]		The address given is not page aligned or the length is

     [ENOMEM]		Some or	all of the address range specified by the addr
			and len	arguments does not correspond to valid mapped
			pages in the address space of the process.

     [ENOMEM]		Locking	the pages mapped by the	specified range	would
			exceed a limit on the amount of	memory that the
			process	may lock.

     fork(2), mincore(2), minherit(2), mlockall(2), mmap(2), munlockall(2),
     munmap(2),	setrlimit(2), getpagesize(3)

     The mlock() and munlock() system calls first appeared in 4.4BSD.

     Allocating	too much wired memory can lead to a memory-allocation deadlock
     which requires a reboot to	recover	from.

     The per-process resource limit is a limit on the amount of	virtual	memory
     locked, while the system-wide limit is for	the number of locked physical
     pages.  Hence a process with two distinct locked mappings of the same
     physical page counts as 2 pages against the per-process limit and as only
     a single page in the system limit.

     The per-process resource limit is not currently supported.

FreeBSD	11.1			 May 17, 2014			  FreeBSD 11.1


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