mlockall
MLOCKALL(2) Linux Programmer's Manual MLOCKALL(2)
NAME
mlockall - disable paging for calling process
SYNOPSIS
#include <sys/mman.h>
int mlockall(int flags);
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 success-
fully 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 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. For security applications,
only small parts of memory have to be locked, for which mlock is avail-
able.
The flags parameter can be constructed from the bitwise OR of the fol-
lowing 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 sev-
eral 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 available,
_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), munlock(2)
Linux 1.3.43 1995-11-26 MLOCKALL(2)
