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OpenDarwin 7.2.1 - man page for mlock (opendarwin section 2)

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

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

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

     int
     mlock(caddr_t addr, size_t len);

     int
     munlock(caddr_t addr, size_t len);

DESCRIPTION
     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 mul-
     tiple 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 map-
     pings 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.

RETURN VALUES
     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
     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 por-
			tion of the indicated address range is not locked.

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

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 dis-
     tinct 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
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