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

CONTENTS

Library
Synopsis
Description
Return Values
Errors
History
Bugs

.Lb libc

.In sys/mman.h int mlock const void *addr size_t len int munlock const 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 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. A single process can mlock the minimum of a system-wide ‘‘wired pages’’ limit and the per-process RLIMIT_MEMLOCK resource limit.

These calls are only available to the super-user.

.Rv -std

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

The mlock system call will fail if:

[EPERM]
	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 either the system or per-process limit for locked memory.
[ENOMEM]
	Some portion of the indicated address range is not allocated. There was an error faulting/mapping a page.

The munlock system call will fail if:

[EPERM]
	The caller is not the super-user.
[EINVAL]
	The address given is not page aligned or the length is negative.
[ENOMEM]
	Some portion of the indicated address range is not allocated.

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

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

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.

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