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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  mem-
       ory  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 un-
       mapped 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 determin-
       istic timing, and, like scheduling, paging is one major cause of	 unex-
       pected  program	execution  delays. Real-time applications will usually
       also switch to a	real-time scheduler with sched_setscheduler.   Crypto-
       graphic	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_MEM-
       LOCK_RANGE  is  defined in <unistd.h> and the value PAGESIZE from <lim-
       its.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 implemen-
       tation 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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