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MLOCK(2) | System Calls Manual | MLOCK(2) |
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¶
Themlock
() 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. 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_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¶
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¶
Themlock
() 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 given is not page aligned or the length is negative.
- [
EAGAIN
] - Locking the indicated range would exceed the system limit for locked memory.
- [
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.
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 given is not page aligned or the length is negative.
- [
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¶
Themlock
() 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 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.May 17, 2014 | Debian |