.\" Hey Emacs! This file is -*- nroff -*- source. .\" .\" Copyright (C) Markus Kuhn, 1996 .\" .\" This is free documentation; you can redistribute it and/or .\" modify it under the terms of the GNU General Public License as .\" published by the Free Software Foundation; either version 2 of .\" the License, or (at your option) any later version. .\" .\" The GNU General Public License's references to "object code" .\" and "executables" are to be interpreted as the output of any .\" document formatting or typesetting system, including .\" intermediate and printed output. .\" .\" This manual is distributed in the hope that it will be useful, .\" but WITHOUT ANY WARRANTY; without even the implied warranty of .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the .\" GNU General Public License for more details. .\" .\" You should have received a copy of the GNU General Public .\" License along with this manual; if not, write to the Free .\" Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, .\" USA. .\" .\" 1995-11-26 Markus Kuhn .\" First version written .\" .TH MLOCK 2 "1995-11-26" "Linux 1.3.43" "Linux Programmer's Manual" .SH NAME mlock \- disable paging for some parts of memory .SH SYNOPSIS .nf .B #include .sp \fBint mlock(const void *\fIaddr\fB, size_t \fIlen\fB); .fi .SH DESCRIPTION .B mlock disables paging for the memory in the range starting at .I addr with length .I len bytes. All pages which contain a part of the specified memory range are guaranteed be resident in RAM when the .B mlock system call returns successfully and they are guaranteed stay in RAM until the pages are unlocked again by .B munlock or .B munlockall or until the process terminates or starts another program with .BR exec . Child processes do not inherit page locks across a .BR fork . Memory locking has two main applications: real-time algorithms and high-security data processing. Real-time applications require deterministic timing, and, like scheduling, paging is one major cause of unexpected program execution delays. Real-time applications will usually also switch to a real-time scheduler with .BR sched_setscheduler . Cryptographic security software often handles critical bytes like passwords or secret keys as data structures. As a result of paging, these secrets could be transfered onto a persistent swap store medium, where they might be accessible to the enemy long after the security software has erased the secrets in RAM and terminated. Memory locks do not stack, i.e., pages which have been locked several times by calls to .B mlock or .B mlockall will be unlocked by a single call to .B munlock for the corresponding range or by .BR munlockall . Pages which are mapped to several locations or by several processes stay locked into RAM as long as they are locked at least at one location or by at least one process. On POSIX systems on which .B mlock and .B munlock are available, .B _POSIX_MEMLOCK_RANGE is defined in and the value .B PAGESIZE from indicates the number of bytes per page. .SH RETURN VALUE On success, .B mlock returns zero. On error, \-1 is returned, .I errno is set appropriately, and no changes are made to any locks in the address space of the process. .SH ERRORS .TP 0.8i .B ENOMEM Some of the specified address range does not correspond to mapped pages in the address space of the process or the process tried to exceed the maximum number of allowed locked pages. .TP .B EPERM The calling process does not have appropriate privileges. Only root processes are allowed to lock pages. .TP .B EINVAL .I len was not a positive number. .SH STANDARDS POSIX.1b, SVR4 .SH SEE ALSO .BR munlock(2) , .BR mlockall(2) , and .BR munlockall(2) .