RedHat 9 (Linux i386) - man page for mlockall (redhat section 2)

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MLOCKALL(2)			    Linux Programmer's Manual			      MLOCKALL(2)

NAME
       mlockall - disable paging for calling process

SYNOPSIS
       #include <sys/mman.h>

       int mlockall(int flags);

DESCRIPTION
       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 mlockall system  call  returns  successfully
       and  they  are  guaranteed to stay in RAM until the pages are unlocked again by munlock or
       munlockall or until the process terminates or starts another  program  with  exec.   Child
       processes do not inherit page locks across a fork.

       Memory locking has two main applications: real-time algorithms and high-security data pro-
       cessing. 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 sched_setscheduler.  Cryptographic secu-
       rity  software  often  handles critical bytes like passwords or secret keys as data struc-
       tures. 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 is available.

       The flags parameter can be constructed from the bitwise OR of the following constants:

       MCL_CURRENT Lock  all  pages  which  are  currently  mapped  into the address space of the
		   process.

       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.

       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 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.

       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
       mlockall or mlock will be unlocked by a single call to 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 mlockall and munlockall are available, _POSIX_MEMLOCK is defined
       in <unistd.h>.

RETURN VALUE
       On success, mlockall returns zero.  On error, -1 is returned, errno is set appropriately.

ERRORS
       ENOMEM The process tried to exceed the maximum number of allowed locked pages.

       EPERM  The calling process does not have appropriate privileges. Only root  processes  are
	      allowed to lock pages.

       EINVAL Unknown flags were specified.

CONFORMING TO
       POSIX.1b, SVr4.	 SVr4 documents an additional EAGAIN error code.

SEE ALSO
       munlockall(2), mlock(2), munlock(2)

Linux 1.3.43				    1995-11-26				      MLOCKALL(2)
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