| .\" Copyright 2003 Abhijit Menon-Sen <ams@wiw.org> |
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| .\" 2005-04-08 mtk, noted kernel version and added BUGS |
| .\" 2010-10-09, mtk, document arm_fadvise64_64() |
| .\" |
| .TH POSIX_FADVISE 2 2017-09-15 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| posix_fadvise \- predeclare an access pattern for file data |
| .SH SYNOPSIS |
| .nf |
| .B #include <fcntl.h> |
| .PP |
| .BI "int posix_fadvise(int " fd ", off_t " offset ", off_t " len \ |
| ", int " advice ");" |
| .fi |
| .PP |
| .ad l |
| .in -4n |
| Feature Test Macro Requirements for glibc (see |
| .BR feature_test_macros (7)): |
| .in |
| .PP |
| .BR posix_fadvise (): |
| .RS 4 |
| _POSIX_C_SOURCE\ >=\ 200112L |
| .RE |
| .ad |
| .SH DESCRIPTION |
| Programs can use |
| .BR posix_fadvise () |
| to announce an intention to access |
| file data in a specific pattern in the future, thus allowing the kernel |
| to perform appropriate optimizations. |
| .PP |
| The \fIadvice\fP applies to a (not necessarily existent) region starting |
| at \fIoffset\fP and extending for \fIlen\fP bytes (or until the end of |
| the file if \fIlen\fP is 0) within the file referred to by \fIfd\fP. |
| The \fIadvice\fP is not binding; |
| it merely constitutes an expectation on behalf of |
| the application. |
| .PP |
| Permissible values for \fIadvice\fP include: |
| .TP |
| .B POSIX_FADV_NORMAL |
| Indicates that the application has no advice to give about its access |
| pattern for the specified data. |
| If no advice is given for an open file, |
| this is the default assumption. |
| .TP |
| .B POSIX_FADV_SEQUENTIAL |
| The application expects to access the specified data sequentially (with |
| lower offsets read before higher ones). |
| .TP |
| .B POSIX_FADV_RANDOM |
| The specified data will be accessed in random order. |
| .TP |
| .B POSIX_FADV_NOREUSE |
| The specified data will be accessed only once. |
| .IP |
| In kernels before 2.6.18, \fBPOSIX_FADV_NOREUSE\fP had the |
| same semantics as \fBPOSIX_FADV_WILLNEED\fP. |
| This was probably a bug; since kernel 2.6.18, this flag is a no-op. |
| .TP |
| .B POSIX_FADV_WILLNEED |
| The specified data will be accessed in the near future. |
| .IP |
| \fBPOSIX_FADV_WILLNEED\fP initiates a |
| nonblocking read of the specified region into the page cache. |
| The amount of data read may be decreased by the kernel depending |
| on virtual memory load. |
| (A few megabytes will usually be fully satisfied, |
| and more is rarely useful.) |
| .TP |
| .B POSIX_FADV_DONTNEED |
| The specified data will not be accessed in the near future. |
| .IP |
| \fBPOSIX_FADV_DONTNEED\fP attempts to free cached pages associated with |
| the specified region. |
| This is useful, for example, while streaming large |
| files. |
| A program may periodically request the kernel to free cached data |
| that has already been used, so that more useful cached pages are not |
| discarded instead. |
| .IP |
| Requests to discard partial pages are ignored. |
| It is preferable to preserve needed data than discard unneeded data. |
| If the application requires that data be considered for discarding, then |
| .I offset |
| and |
| .I len |
| must be page-aligned. |
| .IP |
| The implementation |
| .I may |
| attempt to write back dirty pages in the specified region, |
| but this is not guaranteed. |
| Any unwritten dirty pages will not be freed. |
| If the application wishes to ensure that dirty pages will be released, |
| it should call |
| .BR fsync (2) |
| or |
| .BR fdatasync (2) |
| first. |
| .SH RETURN VALUE |
| On success, zero is returned. |
| On error, an error number is returned. |
| .SH ERRORS |
| .TP |
| .B EBADF |
| The \fIfd\fP argument was not a valid file descriptor. |
| .TP |
| .B EINVAL |
| An invalid value was specified for \fIadvice\fP. |
| .TP |
| .B ESPIPE |
| The specified file descriptor refers to a pipe or FIFO. |
| .RB ( ESPIPE |
| is the error specified by POSIX, |
| but before kernel version 2.6.16, |
| .\" commit 87ba81dba431232548ce29d5d224115d0c2355ac |
| Linux returned |
| .B EINVAL |
| in this case.) |
| .SH VERSIONS |
| Kernel support first appeared in Linux 2.5.60; |
| the underlying system call is called |
| .BR fadvise64 (). |
| .\" of fadvise64_64() |
| Library support has been provided since glibc version 2.2, |
| via the wrapper function |
| .BR posix_fadvise (). |
| .PP |
| Since Linux 3.18, |
| .\" commit d3ac21cacc24790eb45d735769f35753f5b56ceb |
| support for the underlying system call is optional, |
| depending on the setting of the |
| .B CONFIG_ADVISE_SYSCALLS |
| configuration option. |
| .SH CONFORMING TO |
| POSIX.1-2001, POSIX.1-2008. |
| Note that the type of the |
| .I len |
| argument was changed from |
| .I size_t |
| to |
| .I off_t |
| in POSIX.1-2003 TC1. |
| .SH NOTES |
| Under Linux, \fBPOSIX_FADV_NORMAL\fP sets the readahead window to the |
| default size for the backing device; \fBPOSIX_FADV_SEQUENTIAL\fP doubles |
| this size, and \fBPOSIX_FADV_RANDOM\fP disables file readahead entirely. |
| These changes affect the entire file, not just the specified region |
| (but other open file handles to the same file are unaffected). |
| .PP |
| The contents of the kernel buffer cache can be cleared via the |
| .IR /proc/sys/vm/drop_caches |
| interface described in |
| .BR proc (5). |
| .PP |
| One can obtain a snapshot of which pages of a file are resident |
| in the buffer cache by opening a file, mapping it with |
| .BR mmap (2), |
| and then applying |
| .BR mincore (2) |
| to the mapping. |
| .SS C library/kernel differences |
| The name of the wrapper function in the C library is |
| .BR posix_fadvise (). |
| The underlying system call is called |
| .BR fadvise64 () |
| (or, on some architectures, |
| .BR fadvise64_64 ()). |
| .SS Architecture-specific variants |
| Some architectures require |
| 64-bit arguments to be aligned in a suitable pair of registers (see |
| .BR syscall (2) |
| for further detail). |
| On such architectures, the call signature of |
| .BR posix_fadvise () |
| shown in the SYNOPSIS would force |
| a register to be wasted as padding between the |
| .I fd |
| and |
| .I offset |
| arguments. |
| Therefore, these architectures define a version of the |
| system call that orders the arguments suitably, |
| but is otherwise exactly the same as |
| .BR posix_fadvise (). |
| .PP |
| For example, since Linux 2.6.14, ARM has the following system call: |
| .PP |
| .in +4n |
| .EX |
| .BI "long arm_fadvise64_64(int " fd ", int " advice , |
| .BI " loff_t " offset ", loff_t " len ); |
| .EE |
| .in |
| .PP |
| These architecture-specific details are generally |
| hidden from applications by the glibc |
| .BR posix_fadvise () |
| wrapper function, |
| which invokes the appropriate architecture-specific system call. |
| .SH BUGS |
| In kernels before 2.6.6, if |
| .I len |
| was specified as 0, then this was interpreted literally as "zero bytes", |
| rather than as meaning "all bytes through to the end of the file". |
| .SH SEE ALSO |
| .BR fincore (1), |
| .BR mincore (2), |
| .BR readahead (2), |
| .BR sync_file_range (2), |
| .BR posix_fallocate (3), |
| .BR posix_madvise (3) |