Macro to verify that an expression has no side effects at compile time

I'd like to implement a macro strnul():

#define strnul(s)  (s + strlen(s))

But that expands and evaluates s twice. I'd like to avoid that.

I could do (using GNU extensions)

#define strnul(s)   \
({                  \
    auto  s_ = s;   \
    s_ + strlen(s_);\
})

However, this adds some other problem: a local variable named s_. This local variable must have a name different from whatever users of this function may pass as argument, as otherwise we'd have auto s_ = s_;, which can't work.

But I have reasons to pass an argument called s_, so I'd like to find a way to entirely avoid the variable. If possible, an alternative would be to do

#define strnul(s)                         \
({                                        \
    static_assert(!has_side_effects(s));  \
    s + strlen(s);                        \
})

Is it possible to somehow do this in C? Even if the solution would require using compiler extensions, I'd be interested in finding a solution.

posted 4 days ago

CC BY-SA 4.0

4d ago

alx‭

728 reputation 20 10 108 138