How best can I programmatically apply `__attribute__ ((unused))` to these auto-generated objects?

Question

In my makefile I have the following target, which "compiles" text/HTML resources into unsigned char arrays using xxd -i.

I wrap the result in an anonymous namespace and header guards for multiple-inclusion safety both inside and between translation units.

templates.h:
    @echo "#ifndef TEMPLATES_H" > templates.h
    @echo "#define TEMPLATES_H" >> templates.h
    @echo "// Auto-generated file! Do not modify!" >> templates.h
    @echo "// NB: arrays are not null-terminated" >> templates.h
    @echo "// (anonymous namespace used to force internal linkage)" >> templates.h
    @echo "namespace {" >> templates.h
    @echo "namespace templates {" >> templates.h
    @cd templates;\
    for i in * ;\
    do \
        echo "Compiling $$i...";\
        xxd -i $$i >> ../templates.h;\
    done;\
    cd ..
    @echo "}" >> templates.h
    @echo "}" >> templates.h
    @echo "#endif" >> templates.h

The output, if I had just one such resource, looks like this (actual content redacted):

#ifndef TEMPLATES_H
#define TEMPLATES_H
// Auto-generated file! Do not modify!
// NB: arrays are not null-terminated
// (anonymous namespace used to force internal linkage)
namespace {
namespace templates {
unsigned char alert_email_finished_events_html[] = {
  0x3c, 0x74, 0x61, 0x62, 0x6c, 0x0d, 0x0a
};
unsigned int alert_email_finished_events_html_len = 7;
}
}
#endif

What would be the best way to programmaticaly apply GCC's __attribute__ ((unused)) to these character arrays? I don't want GCC warning about any resources that I end up not using in any given TU, but I also don't want to turn "unused variable" warnings off altogether.

Heavierthanair

Any reason why you don't compile using -fvisibility=hidden and only export those symbols you do want to have external linkage ? This way you could avoid including the templates everywhere. – Anticosti

@MatthieuM. Seems cumbersome since I want external linkage by default everywhere else. I want an approach that affects just the templates, and nothing else. I'm not too worried about duplicating the templates everywhere for the time being as I'm only using them in one TU; this approach simply provides error mitigation for when someone comes and #includes template.h in another TU for the first time. If it eventually ends up in lots of TUs then, sure, something cleverer may be worth devising. – Heavierthanair

@MatthieuM. (The other answer is that I don't yet completely "get" -fvisibility="hidden". :P) – Heavierthanair

Another solution would be to selectively apply __attribute__ ((visibility ("hidden")) to those symbols then, instead of marking them as unused. The issue I have with unused is that a symbol used but marked as unused may cause warnings (if not immediately, perhaps in the future). – Anticosti

@MatthieuM.: Really? The doc says "This attribute, attached to a function, means that the function is meant to be possibly unused. GCC will not produce a warning for this function." – Heavierthanair

@MatthieuM.: So I'd use "hidden" visibility and get rid of the anonymous namespace, and I gain sharing of identical objects across modules? – Heavierthanair

I think it was suggested on Clang, may have been abandonned though if GCC never warns. It is still misleading, of course, but then for generated code it does not matter as much. – Anticosti

@MatthieuM.: Indeed, it's more like maybe-unused :D – Heavierthanair

Answer 1

I think a quick sed should work, given that the output of xxd -i is very regular:

xxd -i $$i | sed -e 's/ =/ __attribute__((unused)) =/' >> ../templates.h

Mishnah