mlockall.2

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 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 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.

RETURN VALUE
       On success, mlockall returns zero.  On error, -1 is returned, and 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.

AVAILABILITY
       On POSIX systems on which mlockall and munlockall are available,
       _POSIX_MEMLOCK is defined in <unistd.h> to a value greater than 0. (See
       also sysconf(3).)

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

SEE ALSO
       munlockall(2), mlock(2), munlock(2), sysconf(3)



Linux 1.3.43                      1995-11-26                       MLOCKALL(2)