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

       readahead - 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);

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

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

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

       This page is part of release 5.11 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                              2021-03-22                       READAHEAD(2)