1POSIX_FADVISE(2)           Linux Programmer's Manual          POSIX_FADVISE(2)
2
3
4

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)):
14
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:
29
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).
38
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 ap‐
69              plication requires that data be considered for discarding, then
70              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 en‐
75              sure that dirty pages will be released, it should call fsync(2)
76              or fdatasync(2) first.
77

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.
83
84       EINVAL An invalid value was specified for advice.
85
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.1-2001 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 ap‐
115       plying 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 ex‐
131       actly the same as posix_fadvise().
132
133       For example, since Linux 2.6.14, ARM has the following system call:
134
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 in‐
144       terpreted literally as "zero bytes", rather than as meaning "all bytes
145       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 5.13 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/.
156
157
158
159Linux                             2021-03-22                  POSIX_FADVISE(2)
Impressum