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

       mlock - disable paging for some parts of memory

       #include <sys/mman.h>

       int mlock(const void *addr, size_t len);

       mlock disables paging for the memory in the range starting at addr with
       length len bytes. All pages which contain a part of the specified
       memory range are guaranteed be resident in RAM when the mlock system
       call returns successfully and they are guaranteed to stay in RAM until
       the pages are unlocked by munlock or munlockall, until the pages are
       unmapped via munmap, or until the process terminates or starts another
       program with exec.  Child processes do not inherit page locks across a

       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.

       Memory locks do not stack, i.e., pages which have been locked several
       times by calls to mlock or mlockall will be unlocked by a single call
       to munlock for the corresponding range or by 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 mlock and munlock are available,
       _POSIX_MEMLOCK_RANGE is defined in <unistd.h> and the value PAGESIZE
       from <limits.h> indicates the number of bytes per page.

       With the Linux system call, addr is automatically rounded down to the
       nearest page boundary.  However, POSIX 1003.1-2001 allows an
       implementation to require that addr is page aligned, so portable
       applications should ensure this.

       On success, mlock returns zero.  On error, -1 is returned, errno is set
       appropriately, and no changes are made to any locks in the address
       space of the process.

       ENOMEM Some of the specified address range does not correspond to
              mapped pages in the address space of the process or 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 len was not a positive number.

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

       mlockall(2), munlock(2), munlockall(2), munmap(2), setrlimit(2)

Linux 1.3.43                      1995-11-26                          MLOCK(2)


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