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Re: format string is not a string literal
- From: Simon Marchi <simon dot marchi at polymtl dot ca>
- To: Paul_Koning at dell dot com
- Cc: howarth dot mailing dot lists at gmail dot com, gdb at sourceware dot org
- Date: Wed, 25 Feb 2015 19:05:27 -0500
- Subject: Re: format string is not a string literal
- Authentication-results: sourceware.org; auth=none
- References: <CADtEn-1ZT1uznxPzQxr_nBQya1AVrNLyE+ZSDmm2x_ux8qyzUQ at mail dot gmail dot com> <0AB56024-875B-4724-8ED2-A9DDB237CBFF at dell dot com> <CADtEn-0txdtq6x6dAZZ5wew-VOvYU28fueT_kJ2cx7=H0=vdPg at mail dot gmail dot com> <23CC7871-C616-436C-920C-4A635DC87189 at dell dot com>
> I didnât say itâs a bug, Andrew did. But I agree with him.
> My comment (âthe code is legitâ) simply meant that GDB uses variable formats for obvious valid reasons (so the format can vary, being user-supplied). Given that itâs intentional, the warning is not wanted.
> But that point is really applicable to printf, not vprintf. Andrewâs point is that checking formats for vprintf is not possible because you canât know the argument list; only in printf do you see the arguments so you can match the types. So the bug is that format checking and complaining for non-literal formats should not be enabled at all for vprintf. That may be a header issue rather than a compiler issue, but either way, itâs not the right thing to do.
I think the warning is relevant. If you instruct the compiler that
inferior_debug takes a format string and format arguments (using a
format attribute, as mentioned by Richard in the bug report), then it
can check if the callers are doing something wrong.
In the case of inferior_debug, the attribute should be
__attribute__((format (printf, 2, 3)))
By adding the attribute, you get nice warnings of this kind:
test.c: In function âmainâ:
test.c:17:2: warning: too many arguments for format [-Wformat-extra-args]
inferior_debug (1, "pouet %d", 2, "hello");
If the function is vprintf-style, it's similar but the last argument
should be 0. It will push the argument check a level higher, where
eventually they are explicitely defined printf-style. The doc is
somewhere here  in the middle.
The warning also has some value because it will tell you if the string
originally comes from a non-literal, which should be avoided .