error appending file to itself

[Sponge Belly]

Hello All!

I recently noticed some unexplained behaviour when appending a file to itself using type (from the command line):

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<nul set /p "=###" >hashes.tmp
for /l %I in (1 1 8) do type hashes.tmp >>hashes.tmp

So far, so good. The file size is 768 bytes, as expected. But when I do this:

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type hashes.tmp >>hashes.tmp

The file size jumps to 1792 bytes instead of the expected 1536.

Where are those additional 256 bytes coming from? Only happens on the 9th iteration. And doesn’t happen if the file size is a power of 2 to begin with.

- SB

[aGerman]

I wouldn't have expected that it works. You append to the file while TYPE still reads. So in my opinion this should rather end up in an infinite loop. Seems that some kind of internal buffering protects you from terribly crashing. Depending on the buffer size you may get a file size that isn't a multiple of the former content.

Steffen

[Squashman]

I wouldn't have expected that it works. You append to the file while TYPE still reads. So in my opinion this should rather end up in an infinite loop. Seems that some kind of internal buffering protects you from terribly crashing. Depending on the buffer size you may get a file size that isn't a multiple of the former content.

Steffen

Dave introduced me to that technique in one of my threads.
viewtopic.php?f=3&t=2727&start=15#p12603

[Squashman]

I don't seem to have the problem when starting with a single character.

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C:\Users\Squashman\hashes><nul set /p "=A">hashes.tmp

C:\Users\Squashman\hashes>for /l %I in (1 1 9) do @type hashes.tmp >>hashes.tmp

C:\Users\Squashman\hashes>for %G in (hashes.tmp) do @echo %~zG
512

C:\Users\Squashman\hashes><nul set /p "=A">hashes.tmp

C:\Users\Squashman\hashes>for /l %I in (1 1 10) do @type hashes.tmp >>hashes.tmp

C:\Users\Squashman\hashes>for %G in (hashes.tmp) do @echo %~zG
1024

C:\Users\Squashman\hashes>@type hashes.tmp >>hashes.tmp

C:\Users\Squashman\hashes>for %G in (hashes.tmp) do @echo %~zG
2048

Seems like there is a 1024 byte buffer.

Code: Select all

C:\Users\Squashman\hashes><nul set /p "=A">hashes.tmp

C:\Users\Squashman\hashes>for /l %I in (1 1 10) do @type hashes.tmp >>hashes.tmp

C:\Users\Squashman\hashes>for %G in (hashes.tmp) do @echo %~zG
1024

C:\Users\Squashman\hashes><nul set /p "=A">>hashes.tmp

C:\Users\Squashman\hashes>for %G in (hashes.tmp) do @echo %~zG
1025

C:\Users\Squashman\hashes>@type hashes.tmp>>hashes.tmp

C:\Users\Squashman\hashes>for %G in (hashes.tmp) do @echo %~zG
2561