On 2 March 2018 at 14:27, Mason wrote:
> Hello,
>
> Consider the following testcase:
>
> #include <stdio.h>
>
> struct foo { int val; char txt[60]; };
>
> void bar(struct foo *p)
> {
> char buf[80];
> snprintf(buf, sizeof buf, "%s%s", p->txt, p->txt);
> }
>
>
> Compiled at different optimization levels:
>
>
> $ gcc-7 -O0 -Wall -S bug.c
> bug.c: In function 'bar':
> bug.c:8:31: warning: '%s' directive output may be truncated writing up to 59 bytes into a region of size between 21 and 80 [-Wformat-truncation=]
> snprintf(buf, sizeof buf, "%s%s", p->txt, p->txt);
> ^~
> bug.c:8:2: note: 'snprintf' output between 1 and 119 bytes into a destination of size 80
> snprintf(buf, sizeof buf, "%s%s", p->txt, p->txt);
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>
>
> $ gcc-7 -O1 -Wall -S bug.c
> bug.c: In function 'bar':
> bug.c:8:31: warning: '%s' directive output may be truncated writing up to 59 bytes into a region of size between 21 and 80 [-Wformat-truncation=]
> snprintf(buf, sizeof buf, "%s%s", p->txt, p->txt);
> ^~
> In file included from /usr/include/stdio.h:936:0,
> from bug.c:1:
> /usr/include/x86_64-linux-gnu/bits/stdio2.h:64:10: note: '__builtin___snprintf_chk' output between 1 and 119 bytes into a destination of size 80
> return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> __bos (__s), __fmt, __va_arg_pack ());
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>
> Note: Using godbolt CE, GCC trunk prints the first warning for all optimization levels.
>
>
> I am confused by both of these warnings.
> I am aware that adding two 60-char strings might overflow an 80-char buffer,
> which is why I'm using snprintf.
In which case the output would be truncated instead of overflowing,
and that's what the warning says.
> I'm not sure these warnings are very helpful. What am I missing?
> (I may have oversimplified the testcase.)
I think the point is that although you avoid undefined behaviour, you
might still not get the output you expected.
