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Annotated edit history of mlockall(2) version 2, including all changes. View license author blame.
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1 perry 1 !!NAME
2 mlockall - disable paging for calling process
3 !!SYNOPSIS
2 PerryLorier 4 #include <sys/mman.h>
1 perry 5
2 PerryLorier 6 __int mlockall(int __''flags''__);__
1 perry 7
8 !!DESCRIPTION
2 PerryLorier 9 mlockall(2) 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 mlockall(2) system call returns successfully and they are guaranteed to stay in RAM until the pages are unlocked again by munlock(2) or munlockall(2) or until the process terminates or starts
10 another program with __exec__. Child processes do not inherit page locks across a fork(2).
1 perry 11
2 PerryLorier 12 Memory locking has two main applications: real-time algorithms and high-security data processing. Real-time applications require deterministic timing, and, like
13 scheduling, paging is one major cause of unexpected program execution delays. Real-time applications will usually also s witch to a real-time scheduler with
14 __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 mlock(2) is available.
1 perry 15
2 PerryLorier 16 The ''flags'' parameter can be constructed from the bitwise OR of the following constants:
17 ;__MCL_CURRENT__: Lock all pages which are currently mapped into the address space of the process.
18 ;__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.
1 perry 19
2 PerryLorier 20 If __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
21 will fail with [ENOMEM]. If these new pages have been mapped by the the growing stack, then the kernel will deny stack expansion and send a [SIGSEGV].
1 perry 22
2 PerryLorier 23 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.
1 perry 24
2 PerryLorier 25 Memory locks do not stack, i.e., pages which have been locked several times by calls to mlockall(2) or mlock(2) will be unlocked by a single call to munlockall(2). 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.
1 perry 26
2 PerryLorier 27 On POSIX systems on which mlockall and munlockall are available, ___POSIX_MEMLOCK__ is defined in <unistd.h>.
1 perry 28
29 !!RETURN VALUE
2 PerryLorier 30 On success, __mlockall__ returns zero. On error, -1 is returned, ''errno'' is set appropriately.
1 perry 31
32 !!ERRORS
2 PerryLorier 33 ;[ENOMEM]: The process tried to exceed the maximum number of allowed locked pages.
34 ;[EPERM]: The calling process does not have appropriate privileges. Only root processes are allowed to lock pages.
35 ;[EINVAL]: Unknown flags were specified.
1 perry 36
37 !!CONFORMING TO
2 PerryLorier 38 POSIX.1b, SVr4. SVr4 documents an additional [EAGAIN] error code.
1 perry 39
40 !!SEE ALSO
2 PerryLorier 41 munlockall(2), mlock(2), munlock(2)
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