mlock (2)

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 Copyright (c) 1993
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(The comments found at the beginning of the groff file "man2/mlock.2freebsd".)

NAME

mlock munlock - lock (unlock) physical pages in memory

LIBRARY

Lb libc

SYNOPSIS

In sys/mman.h Ft int Fn mlock const void *addr size_t len Ft int Fn munlock const void *addr size_t len

DESCRIPTION

The Fn mlock system call locks into memory the physical pages associated with the virtual address range starting at Fa addr for Fa len bytes. The Fn munlock system call unlocks pages previously locked by one or more Fn mlock calls. For both, the Fa addr argument should be aligned to a multiple of the page size. If the Fa 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 Fn 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 Fn mlock calls on the same address range. Unlocking is performed explicitly by Fn munlock or implicitly by a call to Fn 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 Fn 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

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.

ERRORS

The Fn mlock system call will fail if:

Bq Er EPERM

security.bsd.unprivileged_mlock is set to 0 and the caller is not the super-user.

Bq Er EINVAL

The address given is not page aligned or the length is negative.

Bq Er EAGAIN

Locking the indicated range would exceed the system limit for locked memory.

Bq Er 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 Fn munlock system call will fail if:

Bq Er EPERM

security.bsd.unprivileged_mlock is set to 0 and the caller is not the super-user.

Bq Er EINVAL

The address given is not page aligned or the length is negative.

Bq Er 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.

Bq Er 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 Fn mlock and Fn munlock system calls first appeared in BSD 4.4

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.