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)