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       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 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
       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)


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