1posix_fadvise(2)              System Calls Manual             posix_fadvise(2)
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NAME

6       posix_fadvise - predeclare an access pattern for file data
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <fcntl.h>
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14       int posix_fadvise(int fd, off_t offset, off_t len, int advice);
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16   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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18       posix_fadvise():
19           _POSIX_C_SOURCE >= 200112L
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DESCRIPTION

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

82       On success, zero is returned.  On error, an error number is returned.
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ERRORS

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

94       Under Linux, POSIX_FADV_NORMAL sets the readahead window to the default
95       size for the backing device; POSIX_FADV_SEQUENTIAL doubles this size,
96       and POSIX_FADV_RANDOM disables file readahead entirely.  These changes
97       affect the entire file, not just the specified region (but other open
98       file handles to the same file are unaffected).
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100   C library/kernel differences
101       The name of the wrapper function in the C library is posix_fadvise().
102       The underlying system call is called fadvise64() (or, on some architec‐
103       tures, fadvise64_64()); the difference between the two is that the for‐
104       mer system call assumes that the type of the len argument is size_t,
105       while the latter expects loff_t there.
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107   Architecture-specific variants
108       Some architectures require 64-bit arguments to be aligned in a suitable
109       pair of registers (see syscall(2) for further detail).  On such archi‐
110       tectures, the call signature of posix_fadvise() shown in the SYNOPSIS
111       would force a register to be wasted as padding between the fd and off‐
112       set arguments.  Therefore, these architectures define a version of the
113       system call that orders the arguments suitably, but is otherwise ex‐
114       actly the same as posix_fadvise().
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116       For example, since Linux 2.6.14, ARM has the following system call:
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118           long arm_fadvise64_64(int fd, int advice,
119                                 loff_t offset, loff_t len);
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121       These architecture-specific details are generally hidden from applica‐
122       tions by the glibc posix_fadvise() wrapper function, which invokes the
123       appropriate architecture-specific system call.
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STANDARDS

126       POSIX.1-2008.
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HISTORY

129       POSIX.1-2001.
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131       Kernel support first appeared in Linux 2.5.60; the underlying system
132       call is called fadvise64().  Library support has been provided since
133       glibc 2.2, via the wrapper function posix_fadvise().
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135       Since Linux 3.18, support for the underlying system call is optional,
136       depending on the setting of the CONFIG_ADVISE_SYSCALLS configuration
137       option.
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139       The type of the len argument was changed from size_t to off_t in
140       POSIX.1-2001 TC1.
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NOTES

143       The contents of the kernel buffer cache can be cleared via the
144       /proc/sys/vm/drop_caches interface described in proc(5).
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146       One can obtain a snapshot of which pages of a file are resident in the
147       buffer cache by opening a file, mapping it with mmap(2), and then ap‐
148       plying mincore(2) to the mapping.
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BUGS

151       Before Linux 2.6.6, if len was specified as 0, then this was inter‐
152       preted literally as "zero bytes", rather than as meaning "all bytes
153       through to the end of the file".
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SEE ALSO

156       fincore(1), mincore(2), readahead(2), sync_file_range(2), posix_fallo‐
157       cate(3), posix_madvise(3)
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161Linux man-pages 6.04              2023-03-30                  posix_fadvise(2)
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