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Date: Sun, 11 Dec 2022 00:53:35 -0500
From: Rich Felker <>
To: Yuriy Chernyshov <>
Subject: Re: Various conflicts with linux system headers (ioctl.h)

On Tue, Dec 06, 2022 at 11:36:24AM +0100, Yuriy Chernyshov wrote:
> Hello,
> I am trying to build DBMS software (ydb from
> with musl-libc.
> So far, I have run into certain problems.
> As YDB uses ioctl.h, it has to include both ioctl.h and certain headers
> from linux/ itself. This gives me a bunch of conflicts around _IOC macros
> family being defined by both:
> The error looks as follows:
> contrib/libs/musl/arch/generic/bits/ioctl.h:7:9: error: '_IOW' macro
> > redefined [-Werror,-Wmacro-redefined]
> > #define _IOW(a,b,c) _IOC(_IOC_WRITE,(a),(b),sizeof(c))
> >         ^
> > contrib/libs/linux-headers/asm-generic/ioctl.h:90:9: note: previous
> > definition is here
> > #define _IOW(type,nr,size)
> >  _IOC(_IOC_WRITE,(type),(nr),(_IOC_TYPECHECK(size)))
> >

The pathnames in the above error message make it look like you might
be using musl in a dubious way, that's likely causing the problem.
Particularly, trying to use the headers from a fork of the source tree
rather than the installed headers processed by the compiler driver as
being the "system include" path where this kind of warning would be

That's not to say the conflicts aren't a problem, but to warn you that
you might hit other places where there's real breakage. Particularly,
if you're running your own build of musl not using the build process,
it's possible you'll be missing CFLAGS (e.g. -ffreestanding and
related things) necessary to suppress transformations that aren't
valid when compiling part of the implementation, and other issues like

> The following workaround helps, but looks quite ugly:
> --- arch/generic/bits/ioctl.h (b4624b83eafbdd5f2e2c37374d62426c27687f35)
> > +++ arch/generic/bits/ioctl.h (d545cbc1ae3f5c9132eb26b176bef3638c9d8063)
> > @@ -1,3 +1,9 @@
> > +#undef _IO
> > +#undef _IOC
> > +#undef _IOR
> > +#undef _IOW
> > +#undef _IOWR
> > +
> >  #define _IOC(a,b,c,d) ( ((a)<<30) | ((b)<<8) | (c) | ((d)<<16) )
> >  #define _IOC_NONE  0U
> >  #define _IOC_WRITE 1U
> >
> Is it possible to get official solution for the macro conflict?

It's explicitly unsupported to include linux/* headers that might
produce conflicting definitions *before* the libc headers they might
conflict with. Does the same problem happen if you put the linux/*
headers after?

> NB: we have to use linux/fs.h in order to get BLKGETSIZE64 constant defined
> which is missing in sys/ioctl.h.
> Another conflict is in NGROUPS_MAX value: musl sets it to 32, while Linux
> itself (starting from 2.6.4, which is below the minimal version recommended
> by musl) sets it to 65536.
> It would be nice to have this value increased in musl itself.

This has been an open issue for a while. At least the initgroups()
function is not prepared to handle large values of NGROUPS_MAX.
There's been some discussion in the past on how this could be changed.
I'll need to dig it up. I think the general leaning was that it should
be changed, once we work out a good way to do it.


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