READAHEAD(2)              (2019-03-06)               READAHEAD(2)

     NAME
          readahead - initiate file readahead into page cache

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

          ssize_t readahead(int fd, off64_t offset, size_t count

     DESCRIPTION
          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 spec-
          ifies 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 bound-
          ary 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.

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

     ERRORS
          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 applied.

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

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

     NOTES
          On some 32-bit architectures, the calling signature for this

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     READAHEAD(2)              (2019-03-06)               READAHEAD(2)

          system call differs, for the reasons described in
          syscall(2).

     BUGS
          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 appear-
          ance that the call blocks until the requested data has been
          read.

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

     COLOPHON
          This page is part of release 5.10 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 https://www.kernel.org/doc/man-pages/.

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