mlock(2) - NetBSD Manual Pages

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MLOCK(2)                  NetBSD Programmer's Manual                  MLOCK(2)


NAME

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


SYNOPSIS

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

     int
     mlock(void *addr, size_t len);

     int
     munlock(void *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 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 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 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 in-
     dicates 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 in-
     dicate the error.


ERRORS

     Mlock() will fail if:

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

     [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 allocat-
                   ed.  There was an error faulting/mapping a page.
     Munlock() will fail if:

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

     [ENOMEM]      Some portion of the indicated address range is not allocat-
                   ed.  Some portion of the indicated address range is not
                   locked.


SEE ALSO

     fork(2),  mincore(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 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.


HISTORY

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

4.4BSD                           June 2, 1993                                2