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 previ-
     ously locked by one or more mlock calls.  For both calls, the addr param-
     eter should be aligned to a multiple of the page size.  If the len param-
     eter 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.  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 deallo-
     cates 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 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.

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

BSD                              June 2, 1993                              BSD