Re: Detecting libc in perf (was Re: perf tools build broken after v5.1-rc1)

[Rich Felker]

On Tue, Apr 30, 2019 at 03:53:18PM +0000, Vineet Gupta wrote:
> On 4/29/19 6:18 PM, Arnaldo Carvalho de Melo wrote:
> >>> Auto-detecting system features:
> >>> ...                         dwarf: [ OFF ]
> >>> ...            dwarf_getlocations: [ OFF ]
> >>> ...                         glibc: [ on  ]
> >> Not related to current issue, this run uses a uClibc toolchain and yet it is
> >> detecting glibc - doesn't seem right to me.
> > Ok, I'll improve that, I think it just tries to detect a libc, yeah,
> > see:
> >
> > [acme@quaco linux]$ cat tools/build/feature/test-glibc.c
> > // SPDX-License-Identifier: GPL-2.0
> > #include <stdlib.h>
> >
> > #if !defined(__UCLIBC__)
> > #include <gnu/libc-version.h>
> > #else
> > #define XSTR(s) STR(s)
> > #define STR(s) #s
> > #endif
> >
> > int main(void)
> > {
> > #if !defined(__UCLIBC__)
> > 	const char *version = gnu_get_libc_version();
> > #else
> > 	const char *version = XSTR(__GLIBC__) "." XSTR(__GLIBC_MINOR__);
> > #endif
> >
> > 	return (long)version;
> > }
> > [acme@quaco linux]$
> >
> > [perfbuilder@59ca4b424ded /]$ grep __GLIBC__ /arc_gnu_2017.09-rc2_prebuilt_uclibc_le_arc700_linux_install/arc-snps-linux-uclibc/sysroot/usr/include/*.h
> > /arc_gnu_2017.09-rc2_prebuilt_uclibc_le_arc700_linux_install/arc-snps-linux-uclibc/sysroot/usr/include/features.h:   The macros `__GNU_LIBRARY__', `__GLIBC__', and `__GLIBC_MINOR__' are
> > /arc_gnu_2017.09-rc2_prebuilt_uclibc_le_arc700_linux_install/arc-snps-linux-uclibc/sysroot/usr/include/features.h:#define	__GLIBC__	2
> > /arc_gnu_2017.09-rc2_prebuilt_uclibc_le_arc700_linux_install/arc-snps-linux-uclibc/sysroot/usr/include/features.h:	((__GLIBC__ << 16) + __GLIBC_MINOR__ >= ((maj) << 16) + (min))
> > [perfbuilder@59ca4b424ded /]$
> >
> > Isn't that part of uClibc?
> 
> Right you are. Per the big fat comment right above that code, this gross hack in
> uclibc is unavoidable as applications tend to rely on that define.
> So a better fix would be to check for various !GLIBC libs explicitly.
> 
> #ifdef __UCLIBC__
> 
> #elseif defined __MUSL__
> 
> ....
> 
> Not pretty from app usage pov, but that seems to be the only sane way of doing it.

What are you trying to achieve? I was just CC'd and I'm missing the
context.

Rich