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

6       posix_fadvise - predeclare an access pattern for file data
7

SYNOPSIS

9       #include <fcntl.h>
10
11       int posix_fadvise(int fd, off_t offset, off_t len, int advice);
12
13   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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15       posix_fadvise():
16           _POSIX_C_SOURCE >= 200112L
17

DESCRIPTION

19       Programs  can  use  posix_fadvise()  to announce an intention to access
20       file data in a specific pattern in the future, thus allowing the kernel
21       to perform appropriate optimizations.
22
23       The  advice  applies to a (not necessarily existent) region starting at
24       offset and extending for len bytes (or until the end of the file if len
25       is 0) within the file referred to by fd.  The advice is not binding; it
26       merely constitutes an expectation on behalf of the application.
27
28       Permissible values for advice include:
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30       POSIX_FADV_NORMAL
31              Indicates that the application has no advice to give  about  its
32              access  pattern  for  the specified data.  If no advice is given
33              for an open file, this is the default assumption.
34
35       POSIX_FADV_SEQUENTIAL
36              The application expects to access  the  specified  data  sequen‐
37              tially (with lower offsets read before higher ones).
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39       POSIX_FADV_RANDOM
40              The specified data will be accessed in random order.
41
42       POSIX_FADV_NOREUSE
43              The specified data will be accessed only once.
44
45              In kernels before 2.6.18, POSIX_FADV_NOREUSE had the same seman‐
46              tics as POSIX_FADV_WILLNEED.  This was  probably  a  bug;  since
47              kernel 2.6.18, this flag is a no-op.
48
49       POSIX_FADV_WILLNEED
50              The specified data will be accessed in the near future.
51
52              POSIX_FADV_WILLNEED  initiates  a nonblocking read of the speci‐
53              fied region into the page cache.  The amount of data read may be
54              decreased  by  the  kernel depending on virtual memory load.  (A
55              few megabytes will usually  be  fully  satisfied,  and  more  is
56              rarely useful.)
57
58       POSIX_FADV_DONTNEED
59              The specified data will not be accessed in the near future.
60
61              POSIX_FADV_DONTNEED  attempts  to  free  cached pages associated
62              with the specified region.  This is useful, for  example,  while
63              streaming  large  files.  A program may periodically request the
64              kernel to free cached data that has already been used,  so  that
65              more useful cached pages are not discarded instead.
66
67              Requests to discard partial pages are ignored.  It is preferable
68              to preserve needed data than  discard  unneeded  data.   If  the
69              application  requires  that  data  be considered for discarding,
70              then offset and len must be page-aligned.
71
72              The implementation may attempt to write back dirty pages in  the
73              specified  region,  but  this  is not guaranteed.  Any unwritten
74              dirty pages will not be freed.  If  the  application  wishes  to
75              ensure  that  dirty  pages  will  be  released,  it  should call
76              fsync(2) or fdatasync(2) first.
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RETURN VALUE

79       On success, zero is returned.  On error, an error number is returned.
80

ERRORS

82       EBADF  The fd argument was not a valid file descriptor.
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84       EINVAL An invalid value was specified for advice.
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86       ESPIPE The specified file descriptor refers to a pipe or FIFO.  (ESPIPE
87              is  the  error  specified  by  POSIX,  but before kernel version
88              2.6.16, Linux returned EINVAL in this case.)
89

VERSIONS

91       Kernel support first appeared in Linux 2.5.60;  the  underlying  system
92       call  is  called  fadvise64().  Library support has been provided since
93       glibc version 2.2, via the wrapper function posix_fadvise().
94
95       Since Linux 3.18, support for the underlying system call  is  optional,
96       depending  on  the  setting of the CONFIG_ADVISE_SYSCALLS configuration
97       option.
98

CONFORMING TO

100       POSIX.1-2001, POSIX.1-2008.  Note that the type of the len argument was
101       changed from size_t to off_t in POSIX.1-2003 TC1.
102

NOTES

104       Under Linux, POSIX_FADV_NORMAL sets the readahead window to the default
105       size for the backing device; POSIX_FADV_SEQUENTIAL doubles  this  size,
106       and  POSIX_FADV_RANDOM disables file readahead entirely.  These changes
107       affect the entire file, not just the specified region (but  other  open
108       file handles to the same file are unaffected).
109
110       The  contents  of  the  kernel  buffer  cache  can  be  cleared via the
111       /proc/sys/vm/drop_caches interface described in proc(5).
112
113       One can obtain a snapshot of which pages of a file are resident in  the
114       buffer  cache  by  opening  a  file,  mapping it with mmap(2), and then
115       applying mincore(2) to the mapping.
116
117   C library/kernel differences
118       The name of the wrapper function in the C library  is  posix_fadvise().
119       The underlying system call is called fadvise64() (or, on some architec‐
120       tures, fadvise64_64()); the difference between the two is that the for‐
121       mer  system  call  assumes that the type of the len argument is size_t,
122       while the latter expects loff_t there.
123
124   Architecture-specific variants
125       Some architectures require 64-bit arguments to be aligned in a suitable
126       pair  of registers (see syscall(2) for further detail).  On such archi‐
127       tectures, the call signature of posix_fadvise() shown in  the  SYNOPSIS
128       would  force a register to be wasted as padding between the fd and off‐
129       set arguments.  Therefore, these architectures define a version of  the
130       system  call  that  orders  the  arguments  suitably,  but is otherwise
131       exactly the same as posix_fadvise().
132
133       For example, since Linux 2.6.14, ARM has the following system call:
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135           long arm_fadvise64_64(int fd, int advice,
136                                 loff_t offset, loff_t len);
137
138       These architecture-specific details are generally hidden from  applica‐
139       tions  by the glibc posix_fadvise() wrapper function, which invokes the
140       appropriate architecture-specific system call.
141

BUGS

143       In kernels before 2.6.6, if len was  specified  as  0,  then  this  was
144       interpreted  literally  as  "zero  bytes",  rather than as meaning "all
145       bytes through to the end of the file".
146

SEE ALSO

148       fincore(1), mincore(2), readahead(2), sync_file_range(2),  posix_fallo‐
149       cate(3), posix_madvise(3)
150

COLOPHON

152       This  page  is  part of release 5.02 of the Linux man-pages project.  A
153       description of the project, information about reporting bugs,  and  the
154       latest     version     of     this    page,    can    be    found    at
155       https://www.kernel.org/doc/man-pages/.
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159Linux                             2019-03-06                  POSIX_FADVISE(2)
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