3 ms·

The %.* example is so close to hitting its single most useful application: char something; / no null termination */ size_t something_length; printf("

The %.* example is so close to hitting its single most useful application: char *something; /* no null termination */ size_t something_length; printf("%.*s", (int)something_length, something); Unfortunately, the .* argument has type (int), not size_t, and it's signed… but if that's not a problem this is a great way to format non-\0-terminated strings. (And of course you can also use it to print a substring without copying it around first.)

jrpelkonen 3y ago
In this simple case, if the int cast is a problem, fwrite would be an adequate alternative, don’t you think?
- PaulDavisThe1st 3y ago
  not for s(n)printf ...
  - LegionMammal978 3y ago
    For that, the analogue would be memcpy; but both alternatives lose the ease of surrounding the string with other text, since you either have to do the length calculations or define helper functions.
- eqvinox 3y ago
  The point of this is that you can use it in format strings where you may be printing a whole bunch of other things. In particular there are some APIs where you need to submit the whole format/string in one go, an example from libc is syslog(), but other libraries do this as well occasionally.
thaliaarchi 3y ago
Somewhat related to this, printf alone in a loop is Turing-complete, by using %-directives like that. It was introduced in “Control-Flow Bending: On the Effectiveness of Control-Flow Integrity” (Carlini, et al. 2015) and the authors have implemented Brainfuck and an obfuscated tic-tac-toe with it. [0]: https://nebelwelt.net/publications/files/15SEC.pdf https://nebelwelt.net/publications/files/15SEC.pdf [1]: https://github.com/HexHive/printbf https://github.com/HexHive/printbf [2]: https://github.com/carlini/printf-tac-toe https://github.com/carlini/printf-tac-toe
- isatty 3y ago
  > While its primary purpose is to serve as The One True Debugger, printf also happens to be Turing complete. Excellent.

The %.* example is so close to hitting its single most useful application: char *something; /* no null termination */ size_t something_length; printf("

jrpelkonen 3y ago

In this simple case, if the int cast is a problem, fwrite would be an adequate alternative, don’t you think?

PaulDavisThe1st 3y ago
not for s(n)printf ...
- LegionMammal978 3y ago
  For that, the analogue would be memcpy; but both alternatives lose the ease of surrounding the string with other text, since you either have to do the length calculations or define helper functions.
eqvinox 3y ago
The point of this is that you can use it in format strings where you may be printing a whole bunch of other things. In particular there are some APIs where you need to submit the whole format/string in one go, an example from libc is syslog(), but other libraries do this as well occasionally.

thaliaarchi 3y ago

Somewhat related to this, printf alone in a loop is Turing-complete, by using %-directives like that. It was introduced in “Control-Flow Bending: On the Effectiveness of Control-Flow Integrity” (Carlini, et al. 2015) and the authors have implemented Brainfuck and an obfuscated tic-tac-toe with it. [0]: https://nebelwelt.net/publications/files/15SEC.pdf https://nebelwelt.net/publications/files/15SEC.pdf [1]: https://github.com/HexHive/printbf https://github.com/HexHive/printbf [2]: https://github.com/carlini/printf-tac-toe https://github.com/carlini/printf-tac-toe

isatty 3y ago
> While its primary purpose is to serve as The One True Debugger, printf also happens to be Turing complete. Excellent.