|POSIX_FADVISE(2)||Linux Programmer's Manual||POSIX_FADVISE(2)|
NAMEposix_fadvise - predeclare an access pattern for file data
int posix_fadvise(int fd , off_t offset , off_t len , int advice );
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
DESCRIPTIONPrograms 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.
- 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.
- The specified data will be accessed in the near future.
- The specified data will not be accessed in the near future.
RETURN VALUEOn success, zero is returned. On error, an error number is returned.
- The fd argument was not a valid file descriptor.
- An invalid value was specified for advice.
- The specified file descriptor refers to a pipe or FIFO. (Linux actually returns EINVAL in this case.)
VERSIONSKernel 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().
CONFORMING TOPOSIX.1-2001. Note that the type of the len argument was changed from size_t to off_t in POSIX.1-2003 TC1.
NOTESUnder 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).
Architecture-specific variantsSome 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 len arguments. Therefore, these architectures define a version of the system call that orders the arguments suitably, but otherwise is otherwise exactly the same as posix_fadvise().
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.
BUGSIn 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".
SEE ALSOreadahead(2), sync_file_range(2), posix_fallocate(3), posix_madvise(3)
COLOPHONThis page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.