readahead(2) — Linux manual page

readahead(2)               System Calls Manual              readahead(2)

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)

COLOPHON

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Linux man-pages 6.9.1          2024-05-02                   readahead(2)

Pages that refer to this page: posix_fadvise(2), syscall(2), syscalls(2), off_t(3type), feature_test_macros(7)

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