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MLOCK(2)                  FreeBSD System Calls Manual                 MLOCK(2)

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

LIBRARY
     Standard C Library (libc, -lc)

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() system call unlocks pages previously locked by one or more
     mlock() calls.  For both, the addr argument should be aligned to a
     multiple of the page size.  If the len argument 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() system 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.  The amount of memory that a
     single process can mlock() is limited by both the per-process
     RLIMIT_MEMLOCK resource limit and the system-wide ``wired pages'' limit
     vm.max_wired.  vm.max_wired applies to the system as a whole, so the
     amount available to a single process at any given time is the difference
     between vm.max_wired and vm.stats.vm.v_wire_count.

     If security.bsd.unprivileged_mlock is set to 0 these calls are only
     available to the super-user.

RETURN VALUES
     Upon successful completion, the value 0 is returned; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

     If the call succeeds, all pages in the range become locked (unlocked);
     otherwise the locked status of all pages in the range remains unchanged.

ERRORS
     The mlock() system call will fail if:

     [EPERM]            security.bsd.unprivileged_mlock is set to 0 and the
                        caller is not the super-user.

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

     [EAGAIN]           Locking the indicated range would exceed the system
                        limit for locked memory.

     [ENOMEM]           Some portion of the indicated address range is not
                        allocated.  There was an error faulting/mapping a
                        page.  Locking the indicated range would exceed the
                        per-process limit for locked memory.
     The munlock() system call will fail if:

     [EPERM]            security.bsd.unprivileged_mlock is set to 0 and the
                        caller is not the super-user.

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

     [ENOMEM]           Some or all of the address range specified by the addr
                        and len arguments does not correspond to valid mapped
                        pages in the address space of the process.

     [ENOMEM]           Locking the pages mapped by the specified range would
                        exceed a limit on the amount of memory that the
                        process may lock.

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

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

BUGS
     Allocating too much wired memory can lead to a memory-allocation deadlock
     which requires a reboot to recover from.

     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.

     The per-process resource limit is not currently supported.

FreeBSD 11.0-PRERELEASE          May 17, 2014          FreeBSD 11.0-PRERELEASE

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | HISTORY | BUGS

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