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Subject: [Bug libc/29313] MIN/MAX macros can cause double evaluation of
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Date: Fri, 01 Jul 2022 17:00:14 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D29313

Noah Goldstein <goldstein.w.n at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |goldstein.w.n at gmail dot=
 com

--- Comment #3 from Noah Goldstein <goldstein.w.n at gmail dot com> ---
(In reply to Vasilis Gkoumplias from comment #0)
> * MIN, MAX macros as defined in misc/sys/param.h
> https://sourceware.org/git/?p=3Dglibc.git;a=3Dblob_plain;f=3Dmisc/sys/par=
am.h;
> hb=3DHEAD
> can cause the double evaluation of one of the passed in arguments.
> Currently MAX is defined as:
> #define MAX(a,b) (((a)>(b))?(a):(b))
> This will evaluate the maximum argument twice.
>=20
> * A test case to reproduce this would go like this:
> int a =3D 1000;
> int b =3D 10;
> MAX(a++, b++) ;
> // This will fail as a will be 1002 due to double evaluation by the MAX m=
acro
> EXPECT_EQ(a, 1001)
>=20
> * solution
> #define max(a,b) \
>    ({ __typeof__ (a) _a =3D (a); \
>        __typeof__ (b) _b =3D (b); \
>      _a > _b ? _a : _b; })
>=20
> * A discussion (and a solution) for the matter can be found here
> https://stackoverflow.com/questions/3437404/min-and-max-in-c
>=20
I don't think we can change the existing macros to that because
they will no longer be usable outside of functions.

#define MAX(x, y) ((x) > (y) ? (x) : (y))

#define max(a, b)=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
\
    ({=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
\
        __typeof__(a) _a =3D (a);=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
\
        __typeof__(b) _b =3D (b);=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
\
        _a > _b ? _a : _b;=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20
\
    })


enum { A =3D 1, B =3D 2 };

// Okay
enum { C =3D MAX(A, B) };

// Compile error
enum { D =3D max(A, B) };


>=20
> Thanks

--=20
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