#define _GNU_SOURCE /* See feature_test_macros(7) */
ssize_t readahead(int fd, off64_t offset, size_t count);
readahead() 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
The fd argument is a file descriptor identifying the file which is to
be read. The offset argument specifies the starting point from which
data is to be read and count specifies the number of bytes to be read.
I/O is performed in whole pages, so that offset is effectively rounded
down to a page boundary and bytes are read up to the next page boundary
greater than or equal to (offset+count). readahead() does not read
beyond the end of the file. The current file offset of the open file
referred to by fd is left unchanged.
On success, readahead() returns 0; on failure, -1 is returned, with
errno set to indicate the cause of the error.
EBADF fd is not a valid file descriptor or is not open for reading.
EINVAL fd does not refer to a file type to which readahead() can be
The readahead() system call appeared in Linux 2.4.13; glibc support has
been provided since version 2.3.
The readahead() system call is Linux-specific, and its use should be
avoided in portable applications.
On some 32-bit architectures, the calling signature for this system
call differs, for the reasons described in syscall(2).
readahead() attempts to schedule the reads in the background and return
immediately. However, it may block while it reads the filesystem meta-
data 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
lseek(2), madvise(2), mmap(2), posix_fadvise(2), read(2)
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