[Bug c/61081] excessive warnings: right-hand operand of comma expression has no effect

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61081

--- Comment #8 from Manuel López-Ibáñez  ---
(In reply to Peter Eisentraut from comment #2)
> No, these "functions" need to have a usable return value, because someone
> could write
> 
> if (!sigemptyset(...))
> weirderror();

Note that just using:

(void) sigemptyset();

silences the warning in those cases where you want to ignore the return value,
which could be useful while someone comes up with a patch for a more permanent
fix.

Or you can use -Wno-unused-value for now.

[Bug c/61081] excessive warnings: right-hand operand of comma expression has no effect

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61081

--- Comment #7 from Marek Polacek  ---
Can we settle on just disabling the warning for system headers?  Those are kind
of out of control, but otherwise if you really want it, use ({ }) or make it a
static inline function (as glibc/kernel).  For static inlines there is no
performance penalty.
Note that the C++ FE has this warning for much more longer than the C FE,
completely removing it doesn't seem desirable to me.

[Bug c/61081] excessive warnings: right-hand operand of comma expression has no effect

[peter_e at gmx dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61081

--- Comment #6 from Peter Eisentraut  ---
This particular case is in system headers, but there are other cases that are
not, so this isn't going to help in general.

I think this is a legitimate way to write a function-like macro that has a
side-effect and a return value, and the warning is bogus.

[Bug c/61081] excessive warnings: right-hand operand of comma expression has no effect

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61081

Manuel López-Ibáñez  changed:

   What|Removed |Added

 CC||manu at gcc dot gnu.org

--- Comment #5 from Manuel López-Ibáñez  ---
Apple should mark their system headers as such using:

#pragma GCC system_header

Other compilers will ignore this pragma.

You could do that simply by adding a dummy header that includes the real
header. Or you can just disable the warning in your code with #pragma GCC
diagnostic ignore "Wwhatever".

We could also not warn if the right-hand operand comes from the macro
definition, but I am not sure how difficult is to do that and whether it is
really worth the effort.

[Bug c/61081] excessive warnings: right-hand operand of comma expression has no effect

[peter_e at gmx dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61081

--- Comment #4 from Peter Eisentraut  ---
Not really, because this code is in header files not under my control, and
those header files should presumably work with a variety of C compilers and
shouldn't need to rely on GCC extensions.

[Bug c/61081] excessive warnings: right-hand operand of comma expression has no effect

[polacek at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61081

--- Comment #3 from Marek Polacek  ---
On Tue, May 06, 2014 at 06:33:03PM +, peter_e at gmx dot net wrote:
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61081
> 
> --- Comment #2 from Peter Eisentraut  ---
> No, these "functions" need to have a usable return value, because someone 
> could
> write
> 
> if (!sigemptyset(...))
> weirderror();

So would the following work for you?
#define sigemptyset(set) (__extension__ ({ *(set) = 0; 0; }))

Re: [Bug c/61081] excessive warnings: right-hand operand of comma expression has no effect

[Marek Polacek]

On Tue, May 06, 2014 at 06:33:03PM +, peter_e at gmx dot net wrote:
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61081
> 
> --- Comment #2 from Peter Eisentraut  ---
> No, these "functions" need to have a usable return value, because someone 
> could
> write
> 
> if (!sigemptyset(...))
> weirderror();

So would the following work for you?
#define sigemptyset(set) (__extension__ ({ *(set) = 0; 0; }))

[Bug c/61081] excessive warnings: right-hand operand of comma expression has no effect

[peter_e at gmx dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61081

--- Comment #2 from Peter Eisentraut  ---
No, these "functions" need to have a usable return value, because someone could
write

if (!sigemptyset(...))
weirderror();

[Bug c/61081] excessive warnings: right-hand operand of comma expression has no effect

[mpolacek at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61081

Marek Polacek  changed:

   What|Removed |Added

 CC||mpolacek at gcc dot gnu.org

--- Comment #1 from Marek Polacek  ---
Can't you just cast the RHS to void?