Re: rawio usage

From: Douglas Gilbert (
Date: Tue Feb 06 2001 - 14:09:54 EST

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"Mayank Vasa" <> wrote:
> I am quite new to rawio and am experimenting with with its usage. My test
> environment is Redhat 7.0, kernel version 2.2.16-22 having an external fibre
> channel drive having 2 disks (/dev/sda1 and /dev/sdb1)
> All I am trying to do is to write and read to & from the disk using a raw
> device. Externally I did a "raw /dev/raw/raw1 /dev/sdb1" and then I wrote a
> small program to do the read/write.


Raw devices need to meet the alignment requirements of the
device they are bound to; in the case of most disk this
will be 512 bytes. You need to take this into account for:
  - the buffer you give to the read() and write() calls
  - the 'size' given to read() and write() should be a
    multiple of 512
  - the SEEK_SET 'offset' given to lseek() should be a
    multiple of 512. Note you have a 2 G limit here.
    You can use _llseek() to get around this.

A small program that just reads from a raw device (or the
corresponding block device which should give the same
result) attached.

If you were binding a raw device to a cdrom device then
the BLKSIZE would need to be 2048 bytes (in most cases).

Doug Gilbert
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#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>

#define BLKSIZE 512
#define BLKS2READ 1

int main(int argc, char *argv[])
    int fd, k;
    unsigned char buff[BLKSIZE * (BLKS2READ + 1)]; // allow extra for alignment
    unsigned char * arbp; // aligned read buffer ptr
    long block_addr = 0;

    arbp = (char *)(((unsigned long)buff + (BLKSIZE - 1)) & (~(BLKSIZE - 1)));

    fd = open(argv[1], O_RDONLY);
    if (fd < 0) {
        exit (1);
    if ((lseek(fd, block_addr * BLKSIZE, SEEK_SET)) < 0){
        perror("lseek"); // problem if 2nd arg > 2G
        exit (1);
    if ((read(fd, arbp, BLKSIZE * BLKS2READ)) < 0) {

    printf("First 16 bytes of the readbuf (in hex) are:\n ");
    for (k = 0; k < 16; ++k)
        printf("%x ", (int)arbp[k]);
    return 0;


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