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  algo-
     rithms and high-security data processing. Real-time applica-
     tions 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  pass-
     words or secret keys as data structures. As a result of pag-
     ing, 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  logical  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.

     On POSIX systems on which mlockall and munlockall are avail-
     able, _POSIX_MEMLOCK is defined in <unistd.h>.

RETURN VALUE

     On  success,  mlockall  returns  zero.   On  error,  -1   is
     returned, 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.

CONFORMING TO

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

SEE ALSO

     munlockall(2), mlock(2), and munlock(2).