readahead - perform file readahead into page cache


#define _GNU_SOURCE #include <fcntl.h>

ssize_t readahead(int fd, off64_t offset, size_t count);


readahead() populates the page cache with data from a file so that subsequent reads from that file will not block on disk I/O. 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. readahead() blocks until the specified data has been read. 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 applications.


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


This page is part of release 3.23 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at

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