Bug #20438: String format "%\n" and "%\0" does not raise format error

• Status: Open
• Assignee: -
• Target version: -
• ruby -v: ruby 3.3.0 (2023-12-25 revision 5124f9ac75) +YJIT +MN [arm64-darwin22]
• Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN

[tompng (tomoya ishida)]

"%" % 1 raises incomplete format specifier; use %% (double %) instead
"%=" % 1 raises malformed format string - %=.
But "%\n" % 1 "%\0" % 1 does not raise error.
In sprintf.c, \n and \0 are explicitly accepted. Is this expected?

Some other language are:
Perl: Warns Invalid conversion in printf. Just prints. "%d% " → "1% "
Python: Error ValueError: unsupported format character '?' (0xa) with print("%\n" % 123)
PHP: Error Unknown format specifier with sprintf("%\n", 3)
C, C++: Warns incomplete format specifier. "%\n" → "\n", "%" → "", "% " → ""

sprintf("%f%\n",x) and "%f%\n" % x is used in some codes https://github.com/search?q=language%3ARuby+%22f%25%5Cn%22&type=code

[nobu (Nobuyoshi Nakada)]

• Has duplicate Bug #20439: Invalid string format with n$ flag is ignored in some case added

[nobu (Nobuyoshi Nakada)]

"%\n" has been treated as "%%" since 554b989ba162, probably Tue Aug 6 01:12:32 1996 according to the commit log.
I don't know the reason.

[nobu (Nobuyoshi Nakada)]

• Related to Bug #13315: Single "%" at the end of `printf` format string appears in the result added