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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 physical	pages.	Un-
     locking is	performed explicitly 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_user_wired.
     vm.max_user_wired applies to the system as	a whole, so the	amount avail-
     able to a single process at any given time	is the difference between
     vm.max_user_wired and vm.stats.vm.v_user_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 range given	wraps around zero.

     [ENOMEM]		Some portion of	the indicated address range is not al-
			located.  There	was an error faulting/mapping a	page.
			Locking	the indicated range would exceed the per-
			process	or system-wide limits 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 range given	wraps around zero.

     [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 and system-wide resource limits of	locked memory apply to
     the amount	of virtual memory locked, not the amount of locked physical
     pages.  Hence two distinct	locked mappings	of the same physical page
     counts as 2 pages aginst the system limit,	and also against the per-
     process limit if both mappings belong to the same physical	map.

     The per-process resource limit is not currently supported.

FreeBSD	13.0			 May 13, 2019			  FreeBSD 13.0


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