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.