oracular (2) mlock.2freebsd.gz
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 physical pages. 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_user_wired. vm.max_user_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_user_wired and vm.stats.vm.v_user_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 range given wraps around zero.
[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 or
system-wide limits 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 range given wraps around zero.
[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 and system-wide resource limits of locked memory apply to the amount of virtual memory
locked, not the amount of locked physical pages. Hence two distinct locked mappings of the same physical
page counts as 2 pages aginst the system limit, and also against the per-process limit if both mappings
belong to the same physical map.
The per-process resource limit is not currently supported.