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

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

     #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() call unlocks pages previously locked by one or more mlock()
     calls.  For both, the addr	parameter should be aligned to a multiple of
     the page size.  If	the len	parameter is not a multiple of the page	size,
     it	will be	rounded	up to be so.  The entire range must be allocated.

     After an mlock() call, the	indicated pages	will cause neither a non-resi-
     dent page nor address-translation fault until they	are unlocked.  They
     may still cause protection-violation faults or TLB-miss faults on archi-
     tectures 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	explicitly by
     munlock() or implicitly by	a call to munmap(2) which deallocates the un-
     mapped 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.  A	single process can mlock() the
     minimum of	a system-wide "wired pages" limit and the per-process
     RLIMIT_MEMLOCK resource limit.

     The mlock() and munlock() functions return	the value 0 if successful;
     otherwise the value -1 is returned	and the	global variable	errno is set
     to	indicate the error.

     mlock() will fail if:

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

     [EAGAIN]		Locking	the indicated range would exceed either	the
			system or per-process limit for	locked memory.

     [ENOMEM]		Some portion of	the indicated address range is not al-
			located.  There	was an error faulting/mapping a	page.

     munlock() will fail if:

     [EINVAL]		The address given is not page aligned or addr and size
			specify	a region that would extend beyond the end of
			the address space.

     [ENOMEM]		Some portion of	the indicated address range is not al-
			located.  Some portion of the indicated	address	range
			is not locked.

     fork(2), minherit(2), mlockall(2),	mmap(2), munmap(2), setrlimit(2),

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

     Unlike Sun's implementation, multiple mlock() calls on the	same address
     range require the corresponding number of munlock() calls to actually un-
     lock the pages, i.e., mlock() nests.  This	should be considered a conse-
     quence of the implementation and not a feature.

     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.

FreeBSD	13.0		       January 11, 2019			  FreeBSD 13.0


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