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MLOCK(2) System Calls Manual MLOCK(2)

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

library “libc”

#include <sys/mman.h>

int
mlock(void *addr, size_t len);

int
munlock(void *addr, size_t len);

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. The entire range of memory 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 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.

Portable code should ensure that the addr and len parameters are aligned to a multiple of the page size, even though the NetBSD implementation will round as necessary.

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 indicates 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 indicate the error.

mlock() will fail if:

[]
Locking the indicated range would exceed either the system or per-process limit for locked memory.
[]
The address or length given is not page aligned and the implementation does not round.
[]
Some portion of the indicated address range is not allocated. There was an error faulting/mapping a page.
[]
mlock() was called by non-root on an architecture where locked page accounting is not implemented.

munlock() will fail if:

[]
The address or length given is not page aligned and the implementation does not round.
[]
Some portion of the indicated address range is not allocated. Some portion of the indicated address range is not locked.

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

The mlock() and munlock() functions conform to IEEE Std 1003.1b-1993 (“POSIX.1b”).

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

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.

February 8, 2015 NetBSD-9.2