readahead(2) — Linux manual page


READAHEAD(2)              Linux Programmer's Manual             READAHEAD(2)

NAME         top

       readahead - initiate file readahead into page cache

SYNOPSIS         top

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <fcntl.h>

       ssize_t readahead(int fd, off64_t offset, size_t count);

DESCRIPTION         top

       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 cache).

       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 file offset of the
       open file description referred to by the file descriptor fd is left

RETURN VALUE         top

       On success, readahead() returns 0; on failure, -1 is returned, with
       errno set to indicate the cause of the error.

ERRORS         top

       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

VERSIONS         top

       The readahead() system call appeared in Linux 2.4.13; glibc support
       has been provided since version 2.3.

CONFORMING TO         top

       The readahead() system call is Linux-specific, and its use should be
       avoided in portable applications.

NOTES         top

       On some 32-bit architectures, the calling signature for this system
       call differs, for the reasons described in syscall(2).

BUGS         top

       readahead() 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[234] on large files using indirect blocks
       instead of extents, giving the appearance that the call blocks until
       the requested data has been read.

SEE ALSO         top

       lseek(2), madvise(2), mmap(2), posix_fadvise(2), read(2)

COLOPHON         top

       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2019-03-06                     READAHEAD(2)

Pages that refer to this page: arm_fadvise(2)arm_fadvise64_64(2)fadvise64(2)fadvise64_64(2)posix_fadvise(2)syscall(2)syscalls(2)feature_test_macros(7)