Truncating the first 100MB of a file in linux

Question

I am referring to How can you concatenate two huge files with very little spare disk space?

I\'m in the midst of implementing the following:

1. Allocate a

Answer 1

Answer, now this is reality with Linux kernel v3.15 (ext4/xfs)

Read here http://man7.org/linux/man-pages/man2/fallocate.2.html

Testing code

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

#ifndef FALLOC_FL_COLLAPSE_RANGE
#define FALLOC_FL_COLLAPSE_RANGE        0x08
#endif

int main(int argc, const char * argv[])
{
    int ret;
    char * page = malloc(4096);
    int fd = open("test.txt", O_CREAT | O_TRUNC | O_RDWR, 0644);

    if (fd == -1) {
        free(page);
        return (-1);
    }

    // Page A
    printf("Write page A\n");
    memset(page, 'A', 4096);
    write(fd, page, 4096);

    // Page B
    printf("Write page B\n");
    memset(page, 'B', 4096);
    write(fd, page, 4096);

    // Remove page A
    ret = fallocate(fd, FALLOC_FL_COLLAPSE_RANGE, 0, 4096);
    printf("Page A should be removed, ret = %d\n", ret);

    close(fd);
    free(page);

    return (0);
}

Answer 2

If you can work with ASCII lines and not bytes, then removing the first n lines of a file is easy. For example to remove the first 100 lines:

sed -i 1,100d /path/to/file