Re: [PATCH] selftests/nolibc: fix libc-test with musl libc

[Willy Tarreau <w@1wt.eu>]

On Thu, Mar 19, 2026 at 10:44:36PM +0000, David Laight wrote:
> On Thu, 19 Mar 2026 17:18:33 +0100
> Thomas Weißschuh <linux@weissschuh.net> wrote:
> 
> > On 2026-03-19 10:52:07+0000, David Laight wrote:
> > > On Wed, 18 Mar 2026 22:52:31 +0000
> > > David Laight <david.laight.linux@gmail.com> wrote:
> > >   
> > > > On Wed, 18 Mar 2026 18:20:46 +0100
> > > > Thomas Weißschuh <linux@weissschuh.net> wrote:
> > > >   
> > > > > Some of the nolibc testcases fail on musl. In these cases nolibc mirrors
> > > > > the non-standard behavior of glibc.
> > > > > 
> > > > > Avoid the failures by only running these testcases on nolibc itself.
> > > > > 
> > > > > Fixes: a5f00be9b3b0 ("tools/nolibc: Add a simple test for writing to a FILE and reading it back")
> > > > > Fixes: d94393e48c09 ("tools/nolibc/printf: Add support for length modifiers tzqL and formats iX")
> > > > > Fixes: c2d234d3dc56 ("tools/nolibc/printf: Special case 0 and add support for %#x")
> > > > > Fixes: 63befd993da4 ("tools/nolibc/printf: Add support for zero padding and field precision")
> > > > > Fixes: d94393e48c09 ("tools/nolibc/printf: Add support for length modifiers tzqL and formats iX")
> > > > > Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> > > > > ---
> > > > > I am thinking about folding the printf fixups directly into the original
> > > > > patches. Any objections?
> > > > > ---
> > > > >  tools/testing/selftests/nolibc/nolibc-test.c | 12 ++++++------
> > > > >  1 file changed, 6 insertions(+), 6 deletions(-)
> > > > > 
> > > > > diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
> > > > > index 180611aabbfb..b03fb9aeb54e 100644
> > > > > --- a/tools/testing/selftests/nolibc/nolibc-test.c
> > > > > +++ b/tools/testing/selftests/nolibc/nolibc-test.c
> > > > > @@ -867,7 +867,7 @@ int test_file_stream(void)
> > > > >  
> > > > >  	errno = 0;
> > > > >  	r = fwrite("foo", 1, 3, f);
> > > > > -	if (r != 0 || errno != EBADF) {
> > > > > +	if (r != 0 || (is_nolibc && errno != EBADF)) {
> > > > >  		fclose(f);
> > > > >  		return -1;
> > > > >  	}
> > > > > @@ -1824,13 +1824,13 @@ static int run_printf(int min, int max)
> > > > >  		CASE_TEST(hex_alt_prec); EXPECT_VFPRINTF(1, "| 0x02|0x03| 0x123|", "|%#5.2x|%#04x|%#6.2x|", 2, 3, 0x123); break;
> > > > >  		CASE_TEST(hex_0_alt);    EXPECT_VFPRINTF(1, "|0|0000|   00|", "|%#x|%#04x|%#5.2x|", 0, 0, 0); break;
> > > > >  		CASE_TEST(pointer);      EXPECT_VFPRINTF(1, "0x1", "%p", (void *) 0x1); break;
> > > > > -		CASE_TEST(pointer_NULL); EXPECT_VFPRINTF(1, "|(nil)|(nil)|", "|%p|%.4p|", (void *)0, (void *)0); break;
> > > > > -		CASE_TEST(string_NULL);  EXPECT_VFPRINTF(1, "|(null)||(null)|", "|%s|%.5s|%.6s|", (void *)0, (void *)0, (void *)0); break;
> > > > > +		CASE_TEST(pointer_NULL); EXPECT_VFPRINTF(is_nolibc, "|(nil)|(nil)|", "|%p|%.4p|", (void *)0, (void *)0); break;
> > > > > +		CASE_TEST(string_NULL);  EXPECT_VFPRINTF(is_nolibc, "|(null)||(null)|", "|%s|%.5s|%.6s|", (void *)0, (void *)0, (void *)0); break;
> > > > >  		CASE_TEST(percent);      EXPECT_VFPRINTF(1, "a%d42%69%", "a%%d%d%%%d%%", 42, 69); break;
> > > > > -		CASE_TEST(perc_qual);    EXPECT_VFPRINTF(1, "a%d2", "a%-14l%d%d", 2); break;
> > > > > -		CASE_TEST(invalid);      EXPECT_VFPRINTF(1, "a%12yx3%y42%P", "a%12yx%d%y%d%P", 3, 42); break;
> > > > > +		CASE_TEST(perc_qual);    EXPECT_VFPRINTF(is_nolibc, "a%d2", "a%-14l%d%d", 2); break;
> > > > > +		CASE_TEST(invalid);      EXPECT_VFPRINTF(is_nolibc, "a%12yx3%y42%P", "a%12yx%d%y%d%P", 3, 42); break;
> > > > >  		CASE_TEST(intmax_max);   EXPECT_VFPRINTF(1, "9223372036854775807", "%lld", ~0ULL >> 1); break;
> > > > > -		CASE_TEST(intmax_min);   EXPECT_VFPRINTF(1, "-9223372036854775808", "%Li", (~0ULL >> 1) + 1); break;
> > > > > +		CASE_TEST(intmax_min);   EXPECT_VFPRINTF(is_nolibc, "-9223372036854775808", "%Li", (~0ULL >> 1) + 1); break;
> > > > >  		CASE_TEST(uintmax_max);  EXPECT_VFPRINTF(1, "18446744073709551615", "%ju", ~0ULL); break;
> > > > >  		CASE_TEST(truncation);   EXPECT_VFPRINTF(1, "012345678901234567890123456789", "%s", "012345678901234567890123456789"); break;
> > > > >  		CASE_TEST(string_width); EXPECT_VFPRINTF(1, "         1", "%10s", "1"); break;    
> > > > 
> > > > Hmmm....
> > > > Those are deliberately matching glibc behaviour, at least annotating that
> > > > might be useful.
> > > > Can you think of a way of detecting whether glibc is being used?
> > > > Even !is_musl might be better than is_nolibc - even even they are the same.  
> > > 
> > > Thinking more, the nil/null tests want is_nolibc || is_glibc.
> > > Setting is_glibc might be hard (unless done from the command line).  
> > 
> > We already have a test for __GLIBC__.  We can also use that for
> > is_glibc.
> > 
> > > The musl people think you don't need to know because their library is conformant.
> > > I found something from someone trying to detect pthread_setname_np(),
> > > glibc's features.h will set __USE_GNU (and similar) that musl doesn't set.
> > > There might also be issues with bionic and uClibc.  
> > 
> > Why would __GLIBC__ not work?
> 
> It wasn't mentioned in the stack overflow article I found :-(

I confirm, I'm using it all the time as well, the macro contains the lib's
major version (2).

David you can check for such macros this way if you're interested:

  $ printf "#include <stdio.h>\n" | gcc -dM -E -xc -|grep GLIBC

Willy