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Re: Writing to files

by turnstep (Parson)
on Apr 07, 2000 at 22:52 UTC ( #7157=note: print w/replies, xml ) Need Help??


in reply to Writing to files

The short answer is, no, not as easily as you might think. Unlike say, word processing, you can't just go to the start of the file, hit the 'insert' key, and add a string: file editing is 'overwrite' mode only. There are two basic ways to do it, and both require you to read in all the data from the file at some point.
  • Open a new temporary file, add your string, then open the old file. While reading from the old file, write everything to the new file. Close both when done, then turn the new file into the old file through renaming.

    This is the "general solution" mentioned in the above link. It has the advantage of not using much memory, as you only have to store one line at a time in memory, instead of the whole file. It has the disadvantages of using a temporary file, making the renaming system calls, not working well with file locking, and not being able to work with the data as easily, although you can always read in the whole file (@myfile = <ODLFILE>) at the cost of the memory advantage mentioned above.

  • A second way is to not use a temporary file, but just open the file for reading and writing, slurping in the whole file, make changes/add strings, rewind the file via seek, then write the changed data back into the file. One truncate and one close later, and you are done.

    This method has the disadvantages of reading the whole file into memory, and perhaps being harder for those not familiar with seek and truncate. The advantages are not using a temporary file, not having to rename, and a better ability to make non line-by-line changes. It's also the best way to do it if you are file locking.

I prefer the second way myself, but whatever floats your boat - as long as it gets the job done!

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