$ date ; ./insert_space /media/rodolico/backup/testfile 1048576 ; date
Wed Oct  9 00:08:50 CDT 2019
%sizeof fstat.off_t is 8
%Sought size is 107374182400
%Adding head-end 1048576 bytes
Wed Oct  9 01:43:41 CDT 2019



virt 100G
mem 31.4
res 27.3G
shr 27.3G
cpu 2%

iotop -o -p 29937
19M/s in and out

never touched swap, so did it within

Wed Oct  9 00:08:50 CDT 2019


fallocate -l 10G /media/rodolico/backup/testfile
mkfs.ext4 -m0 -L testdisk /media/rodolico/backup/testfile
mount -o loop /media/rodolico/backup/testfile /mnt
rsync -av Downloads /mnt
umount /mnt
insert_space /media/rodolico/backup/testfile 1048576
fdisk /media/rodolico/backup/testfile
# create first partition at offset 2048, which is 1M assuming 512 byte sectors
# partition is 82? Linux ext
save
kpartx -lv /media/rodolico/backup/testfile
kpartx -av /media/rodolico/backup/testfile
mount /dev/mapper/testfilep1 /mnt
# should be no files copied on following command
rsync -av Downloads /mnt


man 3 getopt

would actually be more flexible if it is assumed the space was already added to the end of the file. The program would move from 0 to filesize-extendsize. In this way, a device like a zfs volume or a lv could be used, simply by appending the correct space to the end, then calling this.
question? How does gpartd do it. it moves partitions.