NAME    [Toc]    [Back]

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

SYNOPSIS    [Toc]    [Back]

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

     int
     mlock(void *addr, size_t len);

     int
     munlock(void *addr, size_t len);

DESCRIPTION    [Toc]    [Back]

     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-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  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.   A  single  process
can mlock the
     minimum  of a system-wide ``wired pages'' limit and the perprocess
     RLIMIT_MEMLOCK resource limit.

RETURN VALUES    [Toc]    [Back]

     A return value of 0 indicates that the  call  succeeded  and
all pages in
     the  range  have  either  been locked or unlocked.  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    [Toc]    [Back]

     mlock() will fail if:

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


     [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:

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


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

SEE ALSO    [Toc]    [Back]

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

HISTORY    [Toc]    [Back]

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

BUGS    [Toc]    [Back]

     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.

OpenBSD      3.6                           June      2,      1993