NAME

       readahead - initiate file readahead into page cache

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #define _FILE_OFFSET_BITS 64
       #include <fcntl.h>

       ssize_t readahead(int fd, off_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 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.

RETURN VALUE

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

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

STANDARDS

       Linux.

HISTORY

       Linux 2.4.13, glibc 2.3.

NOTES

       _FILE_OFFSET_BITS should be defined to be 64 in code that uses a pointer to readahead,  if  the  code  is
       intended  to  be  portable to traditional 32-bit x86 and ARM platforms where off_t's width defaults to 32
       bits.

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  appearance
       that the call blocks until the requested data has been read.

SEE ALSO

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

Linux man-pages 6.7                                2023-10-31                                       readahead(2)