First of all, php readdir,perl readdir and c readdir are mixed.
Secondly, I don't think this function is so special to have its own tag and We need not tag for any function of c (or any other) language.
Besides, there is no questions with only this tag.
meta tags, tags that cannot stand alone as the only tag on a question, are not allowed.
If I understand you correctly, string would need to be burninated too – it cannot stand alone on a question (on Stack Overflow), and it's a concept present in a whole bunch of languages. Same for arrays in a way. But these tags do help, in my view, to get a general idea of what questions are about.
The tag complements questions about readdir pretty well, whether they are C, Perl or some other language altogether. And it is a tricky function (in C anyway).
It makes searching for issues with that function more efficient (compared to searching for just "readdir", which will hit all questions that use that function in related code but aren't about readdir itself).
I don't see how this tag (or opendir/closedir for that matter) hurts.
