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RedHat 9 (Linux i386) - man page for mlock (redhat section 2)

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 pro-
       cesses do not inherit page locks across a fork.

       Memory locking has two main applications: real-time algorithms and high-security data pro-
       cessing. 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 secu-
       rity software often handles critical bytes like passwords or secret keys  as  data  struc-
       tures.  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.

NOTES
       With  the Linux system call, addr is automatically rounded down to the nearest page bound-
       ary.  However, POSIX 1003.1-2001 allows an implementation 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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