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NAME    [Toc]    [Back]

       mlockall - disable paging for calling process

SYNOPSIS    [Toc]    [Back]

       #include <sys/mman.h>

       int mlockall(int flags);

DESCRIPTION    [Toc]    [Back]

       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 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 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 is available.

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

       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    [Toc]    [Back]

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

ERRORS    [Toc]    [Back]

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

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

       EINVAL Unknown flags were specified.

CONFORMING TO    [Toc]    [Back]

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

SEE ALSO    [Toc]    [Back]

       munlockall(2), mlock(2), munlock(2)

Linux 1.3.43			  1995-11-26			   MLOCKALL(2)
[ Back ]
 Similar pages
Name OS Title
munlockall Linux reenable paging for calling process
mlock Linux disable paging for some parts of memory
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tt_open HP-UX return the process identifier for the calling process
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