Question

I have this program which reads data from LBA (logical block address), but everytime whatever the LBA number i provide, it gives the same output.

How do i validate it?

#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <linux/fs.h>
//#include "common.h"

typedef unsigned long long int var64;


int getSectorSize(int handle)
{
        int sectorSize = 0;

        //get the physical sector size of the disk
        if (ioctl(handle, BLKSSZGET, &sectorSize)) {

                printf("getSectorSize: Reading physical sector size failed.\n");

                sectorSize = 512;
        }

        return sectorSize;
}



var64 readLBA(int handle, var64 lba, void* buf, var64 bytes)
{
        int ret = 0;
        int sectorSize = getSectorSize(handle);
        var64 offset = lba * sectorSize;

        printf("readFromLBA: entered.\n");

        lseek64(handle, offset, SEEK_SET);
        ret = read(handle, buf, bytes);
        if(ret != bytes) {

              printf("read LBA: read failed.\n");

                return -1;
        }

        printf("read LBA: retval: %lld.\n", ret);
        return ret;
}

int main()
{
  int sectorSize, fd;
  char buff[100];
  printf("Calling getSectorSize\n");

  fd = open("/dev/sda1", O_RDONLY);

  if(fd == -1)
  {
    printf("open /dev/sda1 failed");
    exit(1);
  }
  sectorSize = getSectorSize(fd);
  printf("Sector size = %u\n", sectorSize);
  memset(buff, 0, sizeof(buff)); 
  readLBA(fd, 1, buff, 2); // if i put the 2nd arg as -12378 gives same answer
}

Here is the output:

sles10-sp3:~ # gcc getSectorSizeMain.c
getSectorSizeMain.c: In function ‘main’:
getSectorSizeMain.c:75: warning: incompatible implicit declaration of built-in function ‘memset’
sles10-sp3:~ # ./a.out
Calling getSectorSize
Sector size = 512
read LBA: entered.
read LBA: retval: 8589934594. // This is always constant, how to validate? If i tell to read an invalid LBA number like -123456 the answer remains same. How to validate?

Answer 1

retval doesn’t contain the data you are interested in, but the count of bytes read() has stored into your buffer, so it’s natural it always contains the same value. But in your test output you try to print it using “%lld” (long long int), even when it’s just a plain int, so printf will combine its value with whatever it finds next to it on the stack (notice that 8589934594==0x200000002 – the last digit is your value, the first one propably garbage).

The data you want to check/use/whatever are inside the array buff.