|Linux Programmer's Manual
posix_fadvise - predeclare an access pattern for file data
int posix_fadvise(int fd, off_t offset, off_t len, int advice);
_POSIX_C_SOURCE >= 200112L
Programs can use 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.
The advice applies to a (not necessarily existent) region starting at offset and extending for len bytes (or until the end of the file if len is 0) within the file referred to by fd. The advice is not binding; it merely constitutes an expectation on behalf of the application.
Permissible values for advice include:
- 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.
- The application expects to access the specified data sequentially (with lower offsets read before higher ones).
- The specified data will be accessed in random order.
- The specified data will be accessed only once.
- In kernels before 2.6.18, POSIX_FADV_NOREUSE had the same semantics as POSIX_FADV_WILLNEED. This was probably a bug; since kernel 2.6.18, this flag is a no-op.
- The specified data will be accessed in the near future.
- POSIX_FADV_WILLNEED 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.)
- The specified data will not be accessed in the near future.
- POSIX_FADV_DONTNEED 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.
- 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 offset and len must be page-aligned.
- The implementation 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 fsync(2) or fdatasync(2) first.
On success, zero is returned. On error, an error number is returned.
Kernel support first appeared in Linux 2.5.60; the underlying system call is called fadvise64(). Library support has been provided since glibc version 2.2, via the wrapper function posix_fadvise().
Since Linux 3.18, support for the underlying system call is optional, depending on the setting of the CONFIG_ADVISE_SYSCALLS configuration option.
POSIX.1-2001, POSIX.1-2008. Note that the type of the len argument was changed from size_t to off_t in POSIX.1-2001 TC1.
Under Linux, POSIX_FADV_NORMAL sets the readahead window to the default size for the backing device; POSIX_FADV_SEQUENTIAL doubles this size, and POSIX_FADV_RANDOM 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).
The contents of the kernel buffer cache can be cleared via the /proc/sys/vm/drop_caches interface described in proc(5).
C library/kernel differences¶
The name of the wrapper function in the C library is posix_fadvise(). The underlying system call is called fadvise64() (or, on some architectures, fadvise64_64()); the difference between the two is that the former system call assumes that the type of the len argument is size_t, while the latter expects loff_t there.
Some architectures require 64-bit arguments to be aligned in a suitable pair of registers (see syscall(2) for further detail). On such architectures, the call signature of posix_fadvise() shown in the SYNOPSIS would force a register to be wasted as padding between the fd and offset arguments. Therefore, these architectures define a version of the system call that orders the arguments suitably, but is otherwise exactly the same as posix_fadvise().
For example, since Linux 2.6.14, ARM has the following system call:
long arm_fadvise64_64(int fd, int advice, loff_t offset, loff_t len);
These architecture-specific details are generally hidden from applications by the glibc posix_fadvise() wrapper function, which invokes the appropriate architecture-specific system call.
In kernels before 2.6.6, if 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".
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