Manpage of READAHEAD
READAHEADSection: Linux Programmer's Manual (2)
NAMEreadahead - initiate file readahead into page cache
#define _GNU_SOURCE /* See feature_test_macros(7) */ #include <fcntl.h>ssize_t readahead(int fd, off64_t offset, size_t count);
DESCRIPTIONreadahead() initiates readahead on a file so that subsequent reads from that file will be satisfied from the cache, and not block on disk I/O (assuming the readahead was initiated early enough and that other activity on the system did not in the meantime flush pages from the cache).
The fdargument is a file descriptor identifying the file which is to be read. The offsetargument specifies the starting point from which data is to be read and countspecifies the number of bytes to be read. I/O is performed in whole pages, so that offsetis effectively rounded down to a page boundary and bytes are read up to thepage boundary greater than or equal to (offset+count). readahead() does not read beyond the end of the file. The file offset of the open file description referred to by fdis left unchanged.
RETURN VALUEOn success, readahead() returns 0; on failure, -1 is returned, with errnoset to indicate the cause of the error.
- fdis not a valid file descriptor or is not open for reading.
- fddoes not refer to a file type to which readahead() can be applied.
VERSIONSThe readahead() system call appeared in Linux 2.4.13; glibc support has been provided since version 2.3.
CONFORMING TOThe readahead() system call is Linux-specific, and its use should be avoided in portable applications.
NOTESOn some 32-bit architectures, the calling signature for this system call differs, for the reasons described in syscall(2).
BUGSreadahead() attempts to schedule the reads in the background and return immediately. However, it may block while it reads the filesystem metadata needed to locate the requested blocks. This occurs frequently with ext on large files using indirect blocks instead of extents, giving the appearance that the call blocks until the requested data has been read.
SEE ALSOlseek(2), madvise(2), mmap(2), posix_fadvise(2), read(2)
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