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Having something Turing-complete is surprisingly easy, and it hides everywhere. The repository have a small document that explains how you can use printf() as a

Having something Turing-complete is surprisingly easy, and it hides everywhere. The repository have a small document that explains how you can use printf() as a computer : it can performs additions, logical union and negation, which is enough. It was unintentional, but Ken Thompson being Ken Thompson, can't be 100% sure.

danbruc 7mo ago
So there was no extension of the functionality over time, all the formats have been supported from day one?
- st_goliath 7mo ago
  The key features that is used here is the '%n' format specifier, that fetches a pointer as the next argument, and writes a character count back. There is actually an interesting question here: was '%n' always in printf, or was it added at one point? I took a cursory look at some old Unix source archives at TUHS: https://www.tuhs.org/cgi-bin/utree.pl https://www.tuhs.org/cgi-bin/utree.pl As far as I can tell from the PDP-11 assembly, Version 7 research Unix (relevant file: /usr/src/libc/stdio/doprnt.s) does not appear to implement it. The 4.1BSD version of that file even explicitly throws an error, treating it as an invalid format specifier. The implementation in a System III archive looks suspiciously similar to the BSD one, also throwing an error. Only in a System V R4 archive (relevant file: svr4/ucblib/libc/port/stdio/doprnt.c) I found an implementation of "%n" that works as expected. I guess it was added at some point to System V and through that eventually made it into POSIX?
  - sltkr 7mo ago
    I think it was first introduced in 4.3 BSD Tahoe (released June 15, 1988): https://www.tuhs.org/cgi-bin/utree.pl?file=4.3BSD-Tahoe/usr/src/lib/libc/stdio/doprnt.c https://www.tuhs.org/cgi-bin/utree.pl?file=4.3BSD-Tahoe/usr/... This was an update to the earlier 4.3 BSD (1986) which still implemented printf() in VAX assembly instead, and doesn't support the %n feature. So %n may have originally been implemented in 4.3 BSD Tahoe and made its way into SVR4 subsequently.
- deleted 7mo ago
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gwern 7mo ago
List of examples: https://gwern.net/turing-complete https://gwern.net/turing-complete It was probably unintentional, yeah, I don't recall any mentions of early printf being overloaded to do stuff, nor is it clear why you would do that since you're using it in a much more convenient Turing-complete language already (C).

danbruc 7mo ago

So there was no extension of the functionality over time, all the formats have been supported from day one?

st_goliath 7mo ago
The key features that is used here is the '%n' format specifier, that fetches a pointer as the next argument, and writes a character count back. There is actually an interesting question here: was '%n' always in printf, or was it added at one point? I took a cursory look at some old Unix source archives at TUHS: https://www.tuhs.org/cgi-bin/utree.pl https://www.tuhs.org/cgi-bin/utree.pl As far as I can tell from the PDP-11 assembly, Version 7 research Unix (relevant file: /usr/src/libc/stdio/doprnt.s) does not appear to implement it. The 4.1BSD version of that file even explicitly throws an error, treating it as an invalid format specifier. The implementation in a System III archive looks suspiciously similar to the BSD one, also throwing an error. Only in a System V R4 archive (relevant file: svr4/ucblib/libc/port/stdio/doprnt.c) I found an implementation of "%n" that works as expected. I guess it was added at some point to System V and through that eventually made it into POSIX?
- sltkr 7mo ago
  I think it was first introduced in 4.3 BSD Tahoe (released June 15, 1988): https://www.tuhs.org/cgi-bin/utree.pl?file=4.3BSD-Tahoe/usr/src/lib/libc/stdio/doprnt.c https://www.tuhs.org/cgi-bin/utree.pl?file=4.3BSD-Tahoe/usr/... This was an update to the earlier 4.3 BSD (1986) which still implemented printf() in VAX assembly instead, and doesn't support the %n feature. So %n may have originally been implemented in 4.3 BSD Tahoe and made its way into SVR4 subsequently.
deleted 7mo ago
[deleted]

gwern 7mo ago

List of examples: https://gwern.net/turing-complete https://gwern.net/turing-complete It was probably unintentional, yeah, I don't recall any mentions of early printf being overloaded to do stuff, nor is it clear why you would do that since you're using it in a much more convenient Turing-complete language already (C).