In a C language library I wrote, I found most of the operations are too verbose, so I tried to make it terse – through use of macros and by reserving the _ identifier.

As I understand, identifiers beginning with _ are reserved for the C standard, the implementation(s), and libraries; but _ alone is not mentioned. I sensibly guess that it can be used by the program (i.e. the application).

This practice isn’t new, $_ is used in Perl quite often, and has lots of idioms. In Python and some shell/scripting languages, _ also refer to the value of the last expression.

Of course, I won’t be interfacing scripting languages directly. What need to be taken care and note of when using a lexically-local _ identifier in C? and in some other system programming languages (e.g. Rust, C++)? What kind of interoperability problems can arise from this?