[Libguestfs] [PATCH] build: Reduce -Wformat-overflow=2 to =1.

[Richard W.M. Jones]

On Mon, Jan 21, 2019 at 04:02:22PM +0000, Richard W.M. Jones wrote:
> GCC 9 gives this error which I believe is bogus:

It turns out it's not bogus (sort of).  C99 says the minimum length of
strings supported by %s has to be at least 4095:

https://stackoverflow.com/questions/8119914/printf-fprintf-maximum-size-according-to-c99?rq=1

Of course that's in some extreme minimal case and I hope it doesn't
apply to our actual glibc printf.  It would be nice if the limit could
reflect what glibc can do.

I think turned down or off the warning is still the right thing here,
but the commit message could be better.

Rich.

> qemuopts.c: In function 'qemuopts_to_config_channel':
> qemuopts.c:987:29: error: '%.*s' directive output between 0 and 2147483647 bytes may exceed minimum required size of 4095 [-Werror=format-overflow=]
>   987 |             fprintf (fp, "  %.*s = ", (int) k, values[j]);
>       |                             ^~~~
> qemuopts.c:987:26: note: assuming directive output of 1 byte
>   987 |             fprintf (fp, "  %.*s = ", (int) k, values[j]);
>       |                          ^~~~~~~~~~~
> ---
>  m4/guestfs-c.m4 | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/m4/guestfs-c.m4 b/m4/guestfs-c.m4
> index 3fd44ed6b..e20721f5f 100644
> --- a/m4/guestfs-c.m4
> +++ b/m4/guestfs-c.m4
> @@ -105,6 +105,9 @@ gl_WARN_ADD([-Wimplicit-fallthrough=4])
>  dnl GCC level 2 gives incorrect warnings, so use level 1.
>  gl_WARN_ADD([-Wformat-truncation=1])
>  
> +dnl GCC 9 at level 2 gives apparently bogus errors when %.*s is used.
> +gl_WARN_ADD([-Wformat-overflow=1])
> +
>  AC_SUBST([WARN_CFLAGS])
>  
>  NO_SNV_CFLAGS=
> -- 
> 2.20.1
> 
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