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MLOCK(2) System Calls Manual MLOCK(2)

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

#include <sys/mman.h>

int
mlock(const void *addr, size_t len);

int
munlock(const void *addr, size_t len);

The () system call locks into memory the physical pages associated with the virtual address range starting at addr for len bytes. The () 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 () 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 () the minimum of a system-wide “wired pages” limit and the per-process RLIMIT_MEMLOCK resource limit.

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.

mlock() will fail if:

[]
The address given is not page aligned or the length is negative.
[]
Locking the indicated range would exceed either the system or per-process limit for locked memory.
[]
Some portion of the indicated address range is not allocated. There was an error faulting/mapping a page.

munlock() will fail if:

[]
The address given is not page aligned or addr and size specify a region that would extend beyond the end of the address space.
[]
Some portion of the indicated address range is not allocated. Some portion of the indicated address range is not locked.

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

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

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.

January 11, 2019 OpenBSD-7.6