.\" 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., 59 Temple Place, Suite 330, Boston, MA 02111, .\" USA. .\" .\" 1995-11-26 Markus Kuhn .\" First version written .\" Modified, 27 May 2004, Michael Kerrisk .\" Added notes on capability requirements .\" Modified, 2004-11-23, mtk, Noted buggy MCL_FUTURE behaviour in 2.4.x .\" Modified, 2004-11-25, mtk, 2.4 limits locks to half of physical mem. .\" Modified, 2004-11-25, mtk, Noted MCL_FUTURE bug in 2.4.x, x < 18 .\" .TH MLOCKALL 2 2004-11-25 "Linux 2.6.9" "Linux Programmer's Manual" .SH NAME mlockall \- disable paging for calling process .SH SYNOPSIS .nf .B #include .sp \fBint mlockall(int \fIflags\fB); .fi .SH DESCRIPTION .B mlockall disables paging for all pages mapped into the address space of the calling process. This includes the pages of the code, data and stack segment, as well as shared libraries, user space kernel data, shared memory and memory mapped files. All mapped pages are guaranteed to be resident in RAM when the .B mlockall system call returns successfully and they are guaranteed to 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. For security applications, only small parts of memory have to be locked, for which .B mlock is available. The .I flags parameter can be constructed from the bitwise OR of the following constants: .TP 1.2i .B MCL_CURRENT Lock all pages which are currently mapped into the address space of the process. .TP .B MCL_FUTURE Lock all pages which will become mapped into the address space of the process in the future. These could be for instance new pages required by a growing heap and stack as well as new memory mapped files or shared memory regions. .PP If .B MCL_FUTURE has been specified and the number of locked pages exceeds the upper limit of allowed locked pages, then the system call which caused the new mapping will fail with .BR ENOMEM . If these new pages have been mapped by the the growing stack, then the kernel will deny stack expansion and send a .BR SIGSEGV . Real-time processes should reserve enough locked stack pages before entering the time-critical section, so that no page fault can be caused by function calls. This can be achieved by calling a function which has a sufficiently large automatic variable and which writes to the memory occupied by this large array in order to touch these stack pages. This way, enough pages will be mapped for the stack and can be locked into RAM. The dummy writes ensure that not even copy-on-write page faults can occur in the critical section. Memory locks do not stack, i.e., pages which have been locked several times by calls to .B mlockall or .B mlock will be unlocked by a single call to .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. .SH "NOTES" In Linux 2.4 and earlier, the kernel prevents a single process from locking more than half of RAM. .SH "RETURN VALUE" On success, .B mlockall returns zero. On error, \-1 is returned, and .I errno is set appropriately. .SH ERRORS .TP .B EINVAL Unknown flags were specified. .TP .B ENOMEM The process tried to exceed the maximum number of allowed locked pages. .TP .B EPERM The calling process has insufficient privilege to call .BR mlockall . Under Linux the .B CAP_IPC_LOCK capability is required. .SH "BUGS" In the 2.4 series Linux kernels up to and including 2.4.17, a bug caused the .B MCL_FUTURE flag to be inherited across a .BR fork (2). This was rectified in kernel 2.4.18. .SH AVAILABILITY On POSIX systems on which .B mlockall and .B munlockall are available, .B _POSIX_MEMLOCK is defined in to a value greater than 0. (See also .BR sysconf (3).) .\" POSIX 1003.1-2001: It shall be defined to -1 or 0 or 200112L. .\" -1: unavailable, 0: ask using sysconf(). .\" glibc defines it to 1. .SH "CONFORMING TO" POSIX.1b, SVr4. SVr4 documents an additional EAGAIN error code. .SH "SEE ALSO" .BR mlock (2), .BR munlock (2), .BR munlockall (2), .BR setrlimit (2), .BR sysconf (3), .BR capabilities (7)