Is there a switch to disable trigraphs with clang?

Question

I've got some (legacy) code that I'm building with clang for the first time. The code is something like:

sprintf(buf, "%s <%s ????>", p1, p2);

Clang gives the following warning (error with -Werror):

test.c:6:33: error: trigraph converted to '}' character [-Werror,-Wtrigraphs]
    sprintf(buf, "%s <%s ????>", p1, p2);
                           ^

Clearly the ??> is not intended as a trigraph, so I want to disable trigraphs entirely (the source does not intentionally use them anywhere).

I have tried -no-trigraphs but that's not really an option:

clang: warning: argument unused during compilation: '-no-trigraphs'

I can turn off the trigraphs warning with -Wno-trigraphs but I don't want the trigraph conversion to actually take place at all.

NOTE: Trigraphs were enabled as an unintended side effect of using -std=c89.

Whitehouse

Ouch! GCC has a sensible policy of ignoring trigraphs, even in strict compliance mode, unless you hold its hands to the fire with -trigraphs. Time to break out sed? Or an equivalent. – Extenuation

Actually GCC's strict compliance modes (-std=c89, -std=c99, etc) do turn on trigraph conversion. – Ruffianism

And if anyone is wondering WTH is a trigraph. Here's the pony... – Monogyny

They do? [...pause...conduct experiment...] Oh, yes, apparently they do. Oh, that's good; it saves me doing a search in my source code, since the stuff works. I'll add -Wtrigraphs to my uber-fussy compilation options for a while, or go grepping, I suppose, but I've not seen any problems so it is unlikely to afflict me. – Extenuation

@JonathanLeffler: -Wtrigraphs is on by default as long as you don't request trigraph conversion (explicitly or implicitly). – Ruffianism

Answer 1

Try using gnu* mode - "Trigraphs default to being off in gnu* modes; they can be enabled by the -trigraphs option." (see http://clang.llvm.org/docs/UsersManual.html#c_modes for other differences and command line switch)

Loomis

Ah, that was it! I was using -std=c89, but with -std=gnu89 I get the message error: trigraph ignored [-Werror,-Wtrigraphs] which I can disable with -Wno-trigraphs. – Whitehouse

Is there a way to disable trigraphs without also enabling gnu extensions? – Ecclesiolatry

Answer 2

I couldn't see an obvious way to disable trigraphs (rather than the trigraphs warning). Probably the easiest way to fix this code is to change it to:

sprintf(buf, "%s <%s ????"">", p1, p2);

Ranunculus

That seems like a reasonable option. Out of millions of lines of source, ??> only occurs four times. – Whitehouse

"...???\?>" should also work. Put the backslash between the two question marks of the trigraph itself. – Ruffianism

Since trigraphs can appear anywhere, maybe sed "s/??\([-<>()\/=#']\)/?\\?\1/g" to convert any trigraph into a safe sequence. On the other hand, outside of a string, maybe the trigraph would be best converted to its standard form? It gets a bit tricky to decide what's best. (Just for once, my sed script uses double quotes around the pattern, because there is a single quote in the pattern.) – Extenuation