bsd/man/man2/mlock.2

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  34 .\"     @(#)mlock.2     8.2 (Berkeley) 12/11/93
  35 .\"
  36 .Dd June 2, 1993
  37 .Dt MLOCK 2
  38 .Os
  39 .Sh NAME
  40 .Nm mlock ,
  41 .Nm munlock
  42 .Nd lock (unlock) physical pages in memory
  43 .Sh SYNOPSIS
  44 .Fd #include <sys/types.h>
  45 .Fd #include <sys/mman.h>
  46 .Ft int
  47 .Fn mlock "caddr_t addr" "size_t len"
  48 .Ft int
  49 .Fn munlock "caddr_t addr" "size_t len"
  50 .Sh DESCRIPTION
  51 The
  52 .Nm mlock
  53 system call
  54 locks into memory the physical pages associated with the virtual address
  55 range starting at
  56 .Fa addr
  57 for
  58 .Fa len
  59 bytes.
  60 The
  61 .Nm munlock
  62 call unlocks pages previously locked by one or more
  63 .Nm mlock
  64 calls.
  65 For both, the
  66 .Fa addr
  67 parameter should be aligned to a multiple of the page size.
  68 If the
  69 .Fa len
  70 parameter is not a multiple of the page size, it will be rounded up
  71 to be so.
  72 The entire range must be allocated.
  73 .Pp
  74 After an
  75 .Nm mlock
  76 call, the indicated pages will cause neither a non-resident page
  77 nor address-translation fault until they are unlocked.
  78 They may still cause protection-violation faults or TLB-miss faults on
  79 architectures with software-managed TLBs.
  80 The physical pages remain in memory until all locked mappings for the pages
  81 are removed.
  82 Multiple processes may have the same physical pages locked via their own
  83 virtual address mappings.
  84 A single process may likewise have pages multiply-locked via different virtual
  85 mappings of the same pages or via nested
  86 .Nm mlock
  87 calls on the same address range.
  88 Unlocking is performed explicitly by
  89 .Nm munlock
  90 or implicitly by a call to
  91 .Nm munmap
  92 which deallocates the unmapped address range.
  93 Locked mappings are not inherited by the child process after a
  94 .Xr fork 2 .
  95 .Pp
  96 Since physical memory is a potentially scarce resource, processes are
  97 limited in how much they can lock down.
  98 A single process can
  99 .Nm mlock
 100 the minimum of
 101 a system-wide ``wired pages'' limit and
 102 the per-process
 103 .Li RLIMIT_MEMLOCK
 104 resource limit.
 105 .Sh RETURN VALUES
 106 A return value of 0 indicates that the call
 107 succeeded and all pages in the range have either been locked or unlocked.
 108 A return value of -1 indicates an error occurred and the locked
 109 status of all pages in the range remains unchanged.
 110 In this case, the global location
 111 .Va errno
 112 is set to indicate the error.
 113 .Sh ERRORS
 114 .Fn Mlock
 115 will fail if:
 116 .Bl -tag -width Er
 117 .It Bq Er EINVAL
 118 The address given is not page aligned or the length is negative.
 119 .It Bq Er EAGAIN
 120 Locking the indicated range would exceed either the system or per-process
 121 limit for locked memory.
 122 .It Bq Er ENOMEM
 123 Some portion of the indicated address range is not allocated.
 124 There was an error faulting/mapping a page.
 125 .El
 126 .Fn Munlock
 127 will fail if:
 128 .Bl -tag -width Er
 129 .It Bq Er EINVAL
 130 The address given is not page aligned or the length is negative.
 131 .It Bq Er ENOMEM
 132 Some portion of the indicated address range is not allocated.
 133 Some portion of the indicated address range is not locked.
 134 .El
 135 .Sh "SEE ALSO"
 136 .Xr fork 2 ,
 137 .Xr mincore 2 ,
 138 .Xr minherit 2 ,
 139 .Xr mmap 2 ,
 140 .Xr munmap 2 ,
 141 .Xr setrlimit 2 ,
 142 .Xr getpagesize 3
 143 .Sh BUGS
 144 Unlike The Sun implementation, multiple
 145 .Nm mlock
 146 calls on the same address range require the corresponding number of
 147 .Nm munlock
 148 calls to actually unlock the pages, i.e.
 149 .Nm mlock
 150 nests.
 151 This should be considered a consequence of the implementation
 152 and not a feature.
 153 .Pp
 154 The per-process resource limit is a limit on the amount of virtual
 155 memory locked, while the system-wide limit is for the number of locked
 156 physical pages.
 157 Hence a process with two distinct locked mappings of the same physical page
 158 counts as 2 pages against the per-process limit and as only a single page
 159 in the system limit.
 160 .Sh HISTORY
 161 The
 162 .Fn mlock
 163 and
 164 .Fn munlock
 165 functions first appeared in 4.4BSD.