Re: conf: flush stdout before early exit

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 29 May 2025 13:08:58 -0400
Jon Maloy <jmaloy@redhat.com> wrote:

> Before doing an early exit any contents of stdout is normally flushed.
> This doesn't happen when the output goes into a pipe and we return with
> _exit(). We now add an explicit flush in such cases.
> 
> Fixes: d0006fa784a7 ("treewide: use _exit() over exit()")
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
> ---
>  conf.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/conf.c b/conf.c
> index a6d7e22..a1d55a3 100644
> --- a/conf.c
> +++ b/conf.c
> @@ -975,6 +975,7 @@ static void usage(const char *name, FILE *f, int status)
>  		"    SPEC is as described for TCP above\n"
>  		"    default: none\n");
>  
> +	fflush(f);
>  	_exit(status);
>  
>  pasta_opts:
> @@ -1029,6 +1030,7 @@ pasta_opts:
>  		"  --ns-mac-addr ADDR	Set MAC address on tap interface\n"
>  		"  --no-splice		Disable inbound socket splicing\n");
>  
> +	fflush(f);
>  	_exit(status);
>  }
>  
> @@ -1594,6 +1596,7 @@ void conf(struct ctx *c, int argc, char **argv)
>  			FPRINTF(stdout,
>  				c->mode == MODE_PASTA ? "pasta " : "passt ");
>  			FPRINTF(stdout, VERSION_BLOB);
> +			fflush(stdout);

I was about to apply this, but clang-tidy now complains:

/home/sbrivio/passt/conf.c:978:2: error: the value returned by this function should not be disregarded; neglecting it may lead to errors [cert-err33-c,-warnings-as-errors]
  978 |         fflush(f);
      |         ^~~~~~~~~
/home/sbrivio/passt/conf.c:978:2: note: cast the expression to void to silence this warning
/home/sbrivio/passt/conf.c:1033:2: error: the value returned by this function should not be disregarded; neglecting it may lead to errors [cert-err33-c,-warnings-as-errors]
 1033 |         fflush(f);
      |         ^~~~~~~~~
/home/sbrivio/passt/conf.c:1033:2: note: cast the expression to void to silence this warning
/home/sbrivio/passt/conf.c:1599:4: error: the value returned by this function should not be disregarded; neglecting it may lead to errors [cert-err33-c,-warnings-as-errors]
 1599 |                         fflush(stdout);
      |                         ^~~~~~~~~~~~~~
/home/sbrivio/passt/conf.c:1599:4: note: cast the expression to void to silence this warning

I don't see any sensible way to use the return value from fflush(),
because, of course, if 'f' or stdout is not a valid stream or isn't open
for writing (that's the only defined error for fflush()), it doesn't
make sense to even try and print a warning to stdout or stderr.

So I would rather suppress the warning as suggested by clang-tidy
(casting to void instead of a specific clang-tidy suppression, just in
case other static checkers start complaining as well). I can do this on
merge if you're fine with it.

-- 
Stefano