#define _GNU_SOURCE             /* See feature_test_macros(7) */
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <linux/falloc.h>
#include <unistd.h>

#define BLOCKSIZE 4096

int main(int argc, char** argv) {
    ssize_t rsize;
    int fh, result;
    char buff[BLOCKSIZE], ebuff[BLOCKSIZE];
    off_t curpos, pstart, psize;
    int freed;

    if (argc < 2) {
    printf("%s [file...]\n", argv[0]);
    return 0;
    }

    memset(ebuff, 0, BLOCKSIZE);    // prepare a block of 0's

    for (int i = 1; i < argc; i++) {
    char* file=argv[i];
    curpos = 0;
    pstart = 0;
    psize = 0;
    freed = 0;

    fh = open(file, O_RDWR, O_NOATIME);
    if (fh == -1) {
        perror("open()");
        return -1;
    }

    printf("sparseifying %s ", file);
    fflush(stdout);
    while ((rsize = read(fh, buff, BLOCKSIZE)) > 0) {
        result = memcmp(buff, ebuff,rsize);
        if (result == 0) { // block is empty
        if (pstart == 0) { // previous block as not empty?
            pstart = curpos;
            psize = rsize; // save for later punching
        } else {
            psize += rsize; // previous block was empty too, add size
        }

        freed += rsize;
        } else if (pstart) { // block is not empty and we have a block that we still need to punch
        result = fallocate(fh, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, pstart, psize);
        if (result == -1) {
            perror("fallocate()");
            return -1;
        }
        pstart = 0;
        psize = 0;
        }
        curpos += rsize;
    }

    if (pstart) { // still a block to do ?
        result = fallocate(fh, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, pstart, psize);
        if (result == -1) {
        perror("fallocate()");
        return -1;
        }
    }

    if (rsize == 0) {
        printf("done. freed %i bytes\n", freed);
        continue;
    }
    if (rsize == -1) {
        perror("read()");
        return -1;
    }
    }
    return -1;
}