Think of the file data on the disk as a sequential stream of bits (this is an approximation- but a good thought model here). Or perhaps a sequence of numbers: 123789, if we want the sequence 1234789, that means that the data 789 has to move down to make room! We have to carve a "hole" for the new "4" data and that is a complex thing to do. If say, the new "4" data is exactly the same size as the "7" data, and I mean *EXACTLY*, we could use seek() to move to that position and wind up with 123489 (replace 7 data with 4).

Think of cassette tapes, if you record 3 songs and you want to add a new song at the beginning of the tape, you have to record the new song and then all of the next 3 songs over again. The disk works like that.

Far and away your best bet if these files aren't huge, is to open the file for read, slurp it into a memory array, do what want to that data, open a new file for write, write the data, close the file and then delete the old file and rename the new file to the old file's name.

Tie::File would do some grunt work for you, but this is what it essentially does and its not portable to Windows if that matters to you.

To be frank, you are in way over your head if you intend to write something that can insert things into an existing file by yourself. That is definitely not beginner stuff!

If the idea of reading the file into memory, working on it and writing it back out doesn't work, then this is time for using a Perl DB module or Tie::File.

Anyway it works this way because the hardware works that way.

In reply to Re^3: Read and write files concurrently in Perl by Marshall
in thread Read and write files concurrently in Perl by sselva

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