NAME    [Toc]    [Back]

     mlock, munlock - lock (unlock) physical pages in memory

LIBRARY    [Toc]    [Back]

     Standard C Library (libc, -lc)

SYNOPSIS    [Toc]    [Back]

     #include <sys/mman.h>

     int
     mlock(void *addr, size_t len);

     int
     munlock(void *addr, size_t len);

DESCRIPTION    [Toc]    [Back]

     The mlock system call locks into memory the physical pages associated
     with the virtual address range starting at addr for len bytes.  The
     munlock call unlocks pages previously locked by one or more mlock calls.
     For both, the addr parameter should be aligned to a multiple of the page
     size.  If the len parameter is not a multiple of the page size, it will
     be rounded up to be so.  The entire range must be allocated.

     After an mlock call, the indicated pages will cause neither a non-resident
 page nor address-translation fault until they are unlocked.  They
     may still cause protection-violation faults or TLB-miss faults on architectures
 with software-managed TLBs.  The physical pages remain in memory
     until all locked mappings for the pages are removed.  Multiple processes
     may have the same physical pages locked via their own virtual address
     mappings.  A single process may likewise have pages multiply-locked via
     different virtual mappings of the same pages or via nested mlock calls on
     the same address range.  Unlocking is performed explicitly by munlock or
     implicitly by a call to munmap which deallocates the unmapped address
     range.  Locked mappings are not inherited by the child process after a
     fork(2).

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

RETURN VALUES    [Toc]    [Back]

     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    [Toc]    [Back]

     mlock() will fail if:

     [EINVAL]           The address given is not page aligned or the length is
                        negative.

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

     [ENOMEM]           Some portion of the indicated address range is not
                        allocated.  There was an error faulting/mapping a
                        page.

     [EPERM]            mlock() was called by non-root on an architecture were
                        locked page accounting is not implemented.

     munlock() will fail if:

     [EINVAL]           The address given is not page aligned or the length is
                        negative.

     [ENOMEM]           Some portion of the indicated address range is not
                        allocated.  Some portion of the indicated address
                        range is not locked.

SEE ALSO    [Toc]    [Back]

     fork(2), mincore(2), mmap(2), munmap(2), setrlimit(2), getpagesize(3)

STANDARDS    [Toc]    [Back]

     The mlock() and munlock() functions conform to IEEE Std 1003.1b-1993
     (``POSIX.1'').

HISTORY    [Toc]    [Back]

     The mlock() and munlock() functions first appeared in 4.4BSD.

BUGS    [Toc]    [Back]

     The per-process resource limit is a limit on the amount of virtual memory
     locked, while the system-wide limit is for the number of locked physical
     pages.  Hence a process with two distinct locked mappings of the same
     physical page counts as 2 pages against the per-process limit and as only
     a single page in the system limit.

BSD                              June 2, 1993                              BSD