Trim Null Bytes from a Sparse File

I was running a server that didn’t do log rotation, so when the log grew too big I had to truncate it. This left me with a sparse file with a bunch of 0s (null bytes) at the beginning.

To trim the null bytes, first find the offset of the first non-null byte. This can be done with some C:

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#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <fcntl.h>
#define BUF_SIZE 4 * 1024
int main(int argc, char *argv[]) {
    char buf[BUF_SIZE];
    unsigned long long count;
    int n, i, fd;
    if (argc != 2) {
        fprintf(stderr, "Usage: %s FILE\n", argv[0]);
        exit(1);
    }
    fd = open(argv[1], O_RDONLY);
    if (fd < 0) {
        fprintf(stderr, "Error opening file: %s: %s\n", argv[1], strerror(errno));
        exit(1);
    }
    count = 0;
    while ((n = read(fd, buf, BUF_SIZE)) > 0) {
        for (i = 0; i < n; i++) if (buf[i] != 0) break;
        count += i;
        if (i != n) break; 
    }
    if (n < 0) {
        fprintf(stderr, "Error reading file: %s: %s\n", argv[1], strerror(errno));
        exit(1);
    }
    printf("%ld\n", count);
    return 0;
}

Find the offset of the first non-null byte:

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gcc -o nulllength nulllength.c
./nulllength log.bin
7981307584

Trim the first 7981307584 bytes from the file:

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dd if=log.bin of=trimmed.bin bs=7981307584 skip=1

Specifying the bs as offset and skipping 1 is faster than doing it the other way, because it would copy 1 byte at a time – slow!