DragonFly On-Line Manual Pages

MLOCKALL(2)              DragonFly System Calls Manual             MLOCKALL(2)

NAME
     mlockall, munlockall - lock (unlock) the address space of a process

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/types.h>
     #include <sys/mman.h>

     int
     mlockall(int flags);

     int
     munlockall(void);

DESCRIPTION
     The mlockall() system call locks into memory the physical pages
     associated with the address space of a process until the address space is
     unlocked, the process exits, fork()s, or execs another program image.
     Any pages which are copy-on-write at the time of the function call will
     be force faulted.  Locked pages will not be paged to swap backing store.

     The following flags affect the behavior of mlockall():

     MCL_CURRENT      Lock all pages currently mapped into the process's
                      address space.

     MCL_FUTURE       Lock all pages mapped into the process's address space
                      in the future, at the time the mapping is established.
                      Note that this may cause future mappings to fail if
                      those mappings cause resource limits to be exceeded.

     Since physical memory is a potentially scarce resource, processes are
     limited in how much they can lock down.  A single process can lock the
     minimum of a system-wide "wired pages" limit and the per-process
     RLIMIT_MEMLOCK resource limit.

     mlockall() has two limitations, necessary to keep it functional on modern
     systems.  The first is that writable file-backed MAP_PRIVATE pages (such
     as shared library mappings) which have not yet been write-faulted will
     retain their read-only mapping to the file backing store and not be
     force-copied.  If we were to force copy these pages, it would cause
     immense unnecessary overheads for the program.  So any unmodified but
     writable pages which are currently in the pmap read-only will still take
     a COW fault if written to.

     The second limitation is that when a fork() is issued, all writable pages
     will be made copy-on-write (COW) in both the parent and the child.  The
     child of course does not inherit the locked memory state, but this action
     will cause any locked pages in the parent to become copy-on-write and
     they will be faulted if written to.  So they will not be quite as locked
     as might have been intended in this situation.

     The munlockall() call unlocks any locked memory regions in the process
     address space.  Any regions mapped after an munlockall() call will not be
     locked.

RETURN VALUES
     A return value of 0 indicates that the call succeeded and all pages in
     the range have either been locked or unlocked.  A return value of -1
     indicates an error occurred and the locked status of all pages in the
     range remains unchanged.  In this case, the global location errno is set
     to indicate the error.

ERRORS
     mlockall() will fail if:

     [EINVAL]           The flags argument is zero, or includes unimplemented
                        flags.

     [ENOMEM]           Locking the indicated range would exceed either the
                        system or per-process limit for locked memory.

     [EPERM]            The calling process does not have the appropriate
                        privilege to perform the requested operation.

SEE ALSO
     mincore(2), mlock(2), setrlimit(2)

STANDARDS
     The mlockall() and munlockall() functions are believed to conform to IEEE
     Std 1003.1-2001 ("POSIX.1").

HISTORY
     The mlockall() and munlockall() functions first appeared in
     DragonFly 2.9.

BUGS
     How could there be any bugs?  This is soooo simple...

     These system calls are not recommended for general use.  They are
     obviously not thread-safe, and the larger application context from which
     they are called might be hostile to such actions due to non-deterministic
     resource limits in the system.  In a modern system, even semi-realtime
     and interactive processes are already detected and handled by the system
     schedule.  Please use mlock() and munlock() to lock specific address
     ranges instead of locking the entire address space.

DragonFly 6.3-DEVELOPMENT        May 16, 2022        DragonFly 6.3-DEVELOPMENT