1POSIX_FADVISE(2) Linux Programmer's Manual POSIX_FADVISE(2)
2
6 posix_fadvise - predeclare an access pattern for file data
7
9 #define _XOPEN_SOURCE 600
10 #include <fcntl.h>
11 int posix_fadvise(int fd, off_t offset, off_t len, int advice);
13
15 Programs can use posix_fadvise() to announce an intention to access
16 file data in a specific pattern in the future, thus allowing the kernel
17 to perform appropriate optimizations.
18 The advice applies to a (not necessarily existent) region starting at
20 offset and extending for len bytes (or until the end of the file if len
21 is 0) within the file referred to by fd. The advice is not binding; it
22 merely constitutes an expectation on behalf of the application.
23 Permissible values for advice include:
25 POSIX_FADV_NORMAL
27 Indicates that the application has no advice to give about its
28 access pattern for the specified data. If no advice is given
29 for an open file, this is the default assumption.
30 POSIX_FADV_SEQUENTIAL
32 The application expects to access the specified data sequen‐
33 tially (with lower offsets read before higher ones).
34 POSIX_FADV_RANDOM
36 The specified data will be accessed in random order.
37 POSIX_FADV_NOREUSE
39 The specified data will be accessed only once.
40 POSIX_FADV_WILLNEED
42 The specified data will be accessed in the near future.
43 POSIX_FADV_DONTNEED
45 The specified data will not be accessed in the near future.
46
48 On success, zero is returned. On error, an error number is returned.
49
51 EBADF The fd argument was not a valid file descriptor.
52 EINVAL An invalid value was specified for advice.
54 ESPIPE The specified file descriptor refers to a pipe or FIFO. (Linux
56 actually returns EINVAL in this case.)
57
59 posix_fadvise() appeared in kernel 2.5.60. Glibc support has been pro‐
60 vided since version 2.2.
61
63 POSIX.1-2001. Note that the type of the len argument was changed from
64 size_t to off_t in POSIX.1-2003 TC1.
65
67 Under Linux, POSIX_FADV_NORMAL sets the readahead window to the default
68 size for the backing device; POSIX_FADV_SEQUENTIAL doubles this size,
69 and POSIX_FADV_RANDOM disables file readahead entirely. These changes
70 affect the entire file, not just the specified region (but other open
71 file handles to the same file are unaffected).
72 POSIX_FADV_WILLNEED initiates a nonblocking read of the specified
74 region into the page cache. The amount of data read may be decreased
75 by the kernel depending on virtual memory load. (A few megabytes will
76 usually be fully satisfied, and more is rarely useful.)
77 In kernels before 2.6.18, POSIX_FADV_NOREUSE had the same semantics as
79 POSIX_FADV_WILLNEED. This was probably a bug; since kernel 2.6.18,
80 this flag is a no-op.
81 POSIX_FADV_DONTNEED attempts to free cached pages associated with the
83 specified region. This is useful, for example, while streaming large
84 files. A program may periodically request the kernel to free cached
85 data that has already been used, so that more useful cached pages are
86 not discarded instead.
87 Pages that have not yet been written out will be unaffected, so if the
89 application wishes to guarantee that pages will be released, it should
90 call fsync(2) or fdatasync(2) first.
91
93 In kernels before 2.6.6, if len was specified as 0, then this was
94 interpreted literally as "zero bytes", rather than as meaning "all
95 bytes through to the end of the file".
96
98 readahead(2), sync_file_range(2), posix_fallocate(3), posix_madvise(3),
99 feature_test_macros(7)
100
102 This page is part of release 3.25 of the Linux man-pages project. A
103 description of the project, and information about reporting bugs, can
104 be found at http://www.kernel.org/doc/man-pages/.
105Linux 2010-06-14 POSIX_FADVISE(2)