[cfe-dev] The Conditional Operator and Unused Results

[Justin Bogner]

Chris Lattner <clattner at apple.com> writes:
> What is the real-world code you're seeing this on, and what diagnostic are you getting?

There are two cases in the code base I was compiling. The first is
setting up a threading domain, of which the relevent bits are simply:

    #define _GNU_SOURCE
    #include <sched.h>
    extern cpu_set_t cpuMask;
    
    void setCpuAffinity(const int cpu)
    {
        CPU_ZERO( &cpuMask );
        CPU_SET( cpu, &cpuMask );  
    }

This gives me:

    $ ./Debug+Asserts/bin/clang -fsyntax-only ~/tmp/cpuset.c 
    /home/bogner/tmp/cpuset.c:8:5: warning: expression result unused [-Wunused-value]
        CPU_SET( cpu, &cpuMask );  
        ^~~~~~~~~~~~~~~~~~~~~~~~
    In file included from /home/bogner/tmp/cpuset.c:2:
    /usr/include/sched.h:72:33: note: instantiated from:
    # define CPU_SET(cpu, cpusetp)   __CPU_SET_S (cpu, sizeof (cpu_set_t), cpusetp)
                                     ^
    In file included from /home/bogner/tmp/cpuset.c:2:
    In file included from /usr/include/sched.h:35:
    /usr/include/bits/sched.h:145:9: note: instantiated from:
          : 0; }))
            ^
    1 warning generated.

This is the more serious case, because It's not easy to suppress
warnings from standard headers. The other case involved an iterator
foreach macro, which has something like:

    for (...; ...; doNextIteration ? ++i : i)

This one's not so bad, as I can change it to:

    for (...; ...; (void)(doNextIteration ? ++i : i))