madvise(2)

1MADVISE(2)                 Linux Programmer's Manual                MADVISE(2)
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NAME

6       madvise - give advice about use of memory
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SYNOPSIS

9       #include <sys/mman.h>
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11       int madvise(void *addr, size_t length, int advice);
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13   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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15       madvise(): _BSD_SOURCE
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DESCRIPTION

18       The madvise() system call advises the kernel about how to handle paging
19       input/output in the address range beginning at address  addr  and  with
20       size  length bytes.  It allows an application to tell the kernel how it
21       expects to use some mapped or shared memory areas, so that  the  kernel
22       can  choose  appropriate  read-ahead and caching techniques.  This call
23       does not influence the semantics of the application (except in the case
24       of  MADV_DONTNEED),  but  may influence its performance.  The kernel is
25       free to ignore the advice.
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27       The advice is indicated in the advice argument which can be
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29       MADV_NORMAL
30              No special treatment.  This is the default.
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32       MADV_RANDOM
33              Expect page references in random order.  (Hence, read ahead  may
34              be less useful than normally.)
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36       MADV_SEQUENTIAL
37              Expect  page  references  in sequential order.  (Hence, pages in
38              the given range can be aggressively read ahead, and may be freed
39              soon after they are accessed.)
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41       MADV_WILLNEED
42              Expect  access  in  the near future.  (Hence, it might be a good
43              idea to read some pages ahead.)
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45       MADV_DONTNEED
46              Do not expect access in the near future.  (For the  time  being,
47              the  application is finished with the given range, so the kernel
48              can free resources associated with it.)  Subsequent accesses  of
49              pages  in this range will succeed, but will result either in re-
50              loading of the memory contents from the underlying  mapped  file
51              (see  mmap(2)) or zero-fill-on-demand pages for mappings without
52              an underlying file.
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54       MADV_REMOVE (Since Linux 2.6.16)
55              Free up a given range of pages and its associated backing store.
56              Currently,  only  shmfs/tmpfs  supports this; other file systems
57              return with the error ENOSYS.
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59       MADV_DONTFORK (Since Linux 2.6.16)
60              Do not make the pages in this range available to the child after
61              a  fork(2).   This  is useful to prevent copy-on-write semantics
62              from changing the physical location of a page(s) if  the  parent
63              writes  to  it  after  a  fork(2).  (Such page relocations cause
64              problems for hardware that DMAs into the page(s).)
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66       MADV_DOFORK (Since Linux 2.6.16)
67              Undo the effect of MADV_DONTFORK, restoring the  default  behav‐
68              ior, whereby a mapping is inherited across fork(2).
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RETURN VALUE

71       On  success  madvise() returns zero.  On error, it returns -1 and errno
72       is set appropriately.
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ERRORS

75       EAGAIN A kernel resource was temporarily unavailable.
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77       EBADF  The map exists, but the area maps something that isn't a file.
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79       EINVAL The value len is negative, addr is not page-aligned,  advice  is
80              not  a  valid value, or the application is attempting to release
81              locked or shared pages (with MADV_DONTNEED).
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83       EIO    (for  MADV_WILLNEED)  Paging  in  this  area  would  exceed  the
84              process's maximum resident set size.
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86       ENOMEM (for MADV_WILLNEED) Not enough memory: paging in failed.
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88       ENOMEM Addresses  in  the  specified range are not currently mapped, or
89              are outside the address space of the process.
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CONFORMING TO

92       POSIX.1b.   POSIX.1-2001  describes  posix_madvise(3)  with   constants
93       POSIX_MADV_NORMAL,  etc., with a behavior close to that described here.
94       There is a similar posix_fadvise(2) for file access.
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96       MADV_REMOVE, MADV_DONTFORK, and MADV_DOFORK are Linux-specific.
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NOTES

99   Linux Notes
100       The current Linux implementation (2.4.0) views this system call more as
101       a  command  than as advice and hence may return an error when it cannot
102       do what it usually would do in  response  to  this  advice.   (See  the
103       ERRORS description above.)  This is non-standard behavior.
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105       The  Linux  implementation  requires  that  the  address  addr be page-
106       aligned, and allows length to be zero.  If there are some parts of  the
107       specified  address range that are not mapped, the Linux version of mad‐
108       vise() ignores them and applies the  call  to  the  rest  (but  returns
109       ENOMEM from the system call, as it should).
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COLOPHON

115       This  page  is  part of release 3.22 of the Linux man-pages project.  A
116       description of the project, and information about reporting  bugs,  can
117       be found at http://www.kernel.org/doc/man-pages/.
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121Linux                             2008-04-22                        MADVISE(2)