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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 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 effec‐
       tively  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 applied.

       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

       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.  How‐
       ever, 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

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

       This  page  is  part of release 3.74 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2014-03-15                               READAHEAD(2)

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