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 *start, size_t length, int advice);
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DESCRIPTION

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

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

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

88       The current Linux implementation (2.4.0) views this system call more as
89       a  command  than as advice and hence may return an error when it cannot
90       do what it usually would do in response to this advice. (See the ERRORS
91       description above.)  This is nonstandard behaviour.
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93       The  Linux  implementation  requires  that  the  address start be page-
94       aligned, and allows length to be zero. If there are some parts  of  the
95       specified  address range that are not mapped, the Linux version of mad‐
96       vise() ignores them and applies the  call  to  the  rest  (but  returns
97       ENOMEM from the system call, as it should).
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HISTORY

100       The madvise() function first appeared in 4.4BSD.
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CONFORMING TO

103       POSIX.1b.    POSIX.1-2001   describes  posix_madvise()  with  constants
104       POSIX_MADV_NORMAL, etc., with a behaviour close to that described here.
105       There is a similar posix_fadvise() for file access.
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107       MADV_REMOVE, MADV_DONTFORK, and MADV_DOFORK are Linux specific.
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