Query on RAM Memory

Operating Systems Linux Red Hat Query on RAM Memory

09-30-2011

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LEARN ABOUT REDHAT

mlock

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

NAME

       mlock - disable paging for some parts of memory

SYNOPSIS

       #include <sys/mman.h>

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

DESCRIPTION

       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 fork.

       Memory locking has two main applications: real-time algorithms and high-security data processing. Real-time applications require  determin-
       istic  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  <lim-
       its.h> indicates the number of bytes per page.

NOTES

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

RETURN VALUE

       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.

ERRORS

       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.

CONFORMING TO

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

SEE ALSO

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

Linux 1.3.43							    1995-11-26								  MLOCK(2)

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