MLOCK(2)						      BSD System Calls Manual							  MLOCK(2)

NAME

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

SYNOPSIS

     #include <sys/mman.h>

     int
     mlock(const void *addr, size_t len);

     int
     munlock(const void *addr, size_t len);

DESCRIPTION

     The mlock system call locks a set of physical pages into memory.  The pages are associated with a virtual address range that starts at addr
     and extends for len bytes.  The munlock call unlocks pages that were previously locked by one or more mlock calls.  For both calls, 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-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.  Similarly, a single process may 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, which deallocates the unmapped address range.  Locked mappings are not
     inherited by the child process after a fork(2).

     Because physical memory is a potentially scarce resource, processes are limited in how much memory 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.

RETURN VALUES

     A return value of 0 indicates that the call succeeded and all pages in the range have either been locked or unlocked, as requested.  A return
     value of -1 indicates an error occurred and the locked status of all pages in the range remains unchanged.  In this case, the global location
     errno is set to indicate the error.

ERRORS

     mlock() and munlock() will fail if:

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

     [ENOMEM]		Part or all of the specified address range is not mapped to the process.

     mlock() will fail if:

     [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 allocated.  There was an error faulting/mapping a page.

     munlock() will fail if:

     [ENOMEM]		Some portion of the indicated address range is not allocated.  Some portion of the indicated address range is not locked.

LEGACY SYNOPSIS

     #include <sys/types.h>
     #include <sys/mman.h>

     The include file <sys/types.h> is necessary.

     int
     mlock(caddr_t addr, size_t len);

     int
     munlock(caddr_t addr, size_t len);

     The variable type of addr has changed.

SEE ALSO

     fork(2), mincore(2), minherit(2), mmap(2), munmap(2), setrlimit(2), getpagesize(3), compat(5)

BUGS

     Unlike The Sun implementation, multiple mlock calls on the same address range require the corresponding number of munlock calls to actually
     unlock the pages, i.e.  mlock nests.  This should be considered a consequence 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.

HISTORY

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

BSD
								   June 2, 1993 							       BSD