mlock



MLOCK(2)                      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 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(2) 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

     The mlock() and munlock() functions return the value 0 if successful;
     otherwise the value -1 is returned and the global variable 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 addr and size
                        specify a region that would extend beyond the end of
                        the address space.

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


SEE ALSO

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


HISTORY

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


BUGS

     Unlike Sun's 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 6.4                    November 15, 2014                   OpenBSD 6.4

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