How to Create sparse file with alternate data and hole.

mango7 · 10-13-2011, 04:12 AM

Hi All,

I have to write a program in c where I have to create a file with known pattern data and hole in it, and it should be such that a 4k data and then it will skip 12K and then again it will write the same 4k data .

It should be such that

data hole data hole data
(4k) (12K) (4k) (12K) (4k)

Please help me .....

syg00 · 10-13-2011, 04:14 AM

Well, after you show us some code that you wrote, maybe we can offer some commentary.

mango7 · 10-13-2011, 04:25 AM

I have this code which i have to modify

#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#define BLOCK_SIZE 4096

int main(int argc, char **argv)
{
int fd=0;
int i;
void *buf;
int sector_per_block=BLOCK_SIZE/512;
int block_count=4;

if(argc !=2 ){
printf("Wrong usage\n USAGE: program absolute_path_to_write\n");
_exit(-1);
}
fd=open(argv[1],O_WRONLY|O_TRUNC|O_CREAT,0666);
if(fd <= 0){
printf("file open failed\n");
_exit(0);
}

while(block_count > 0){
lseek(fd,BLOCK_SIZE,SEEK_CUR);
block_count--;
for(i=0;i<sector_per_block;i++)
write(fd,buf,512);
block_count--;
}
close(fd);
return 0;
}

right now it is writing it from buffer I want a known pattern should be written.
thanks in advance

16pide · 10-13-2011, 07:30 AM

This is on Mac OS, but probably would be just the same on Linux:

http://stackoverflow.com/questions/1...-c-on-mac-os-x

Or you can use the dd command in Linux. Option to use is seek=

16pide · 10-13-2011, 08:02 AM

here is the solution for 400 chars of data and 1200 holes.
you need to turn this into a loop though
Say hello to your teacher for me (guessing it's homework)

Code:

#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>

int main()
{
    char buf[400];
    int fd = open("file.hole", O_WRONLY|O_TRUNC|O_CREAT, 0600);
    memset(buf, 'a', 400);
    write(fd, buf, 400);
    lseek(fd, 1200, SEEK_CUR); // Make a hole
    write(fd, buf, 400);
    lseek(fd, 1200, SEEK_CUR); // Make a hole
    write(fd, buf, 400);
    close(fd);

    return 0;
}