git-compat-util: add UNUSED macro
In preparation for compiling with -Wunused-parameter, we'd like to be
able to annotate some function parameters as false positives (e.g.,
parameters which must exist to conform to a callback interface).
Ideally our annotation will:
- be portable, turning into nothing on platforms which don't support
it
- be easy to read, without looking too syntactically odd or taking
attention away from the rest of the parameters
- help us notice when a parameter marked as unused is actually used,
which keeps our annotations accurate. In theory a compiler could
tell us this easily, but gcc has no such warning. Clang has
-Wused-but-marked-unused, but it triggers false positives with our
MAYBE_UNUSED annotation (e.g., for commit-slab functions)
This patch introduces an UNUSED() macro which takes the parameter name
as an argument. That lets us tweak the name in such a way that we'll
notice if somebody tries to use it. It looks like this in use:
int some_ref_cb(const char *refname,
const struct object_id *UNUSED(oid),
int UNUSED(flags),
void *UNUSED(data))
{
printf("got refname %s", refname);
return 0;
}
Because the unused parameter names are rewritten behind the scenes to
UNUSED_oid, etc, adding code like:
printf("oid is %s", oid_to_hex(oid));
will fail compilation with "oid undeclared". Sadly, the "did you mean"
feature of modern compilers is not generally smart enough to suggest the
"unused" name. If we used a very short prefix like U_oid, that does
convince gcc to say "did you mean", but since the "U_" in the suggestion
isn't much of a hint, it doesn't really help. In practice, a look at the
function definition usually makes the problem pretty obvious.
Note that we have to put the definition of UNUSED early in
git-compat-util.h, because it will eventually be used for some compat
functions themselves (both directly here and in mingw.h).
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git-compat-util.h b/git-compat-util.h
index 36a25ae..c6669db 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -189,6 +189,12 @@ struct strbuf;
#define _NETBSD_SOURCE 1
#define _SGI_SOURCE 1
+#if defined(__GNUC__)
+#define UNUSED(var) UNUSED_##var __attribute__((unused))
+#else
+#define UNUSED(var) UNUSED_##var
+#endif
+
#if defined(WIN32) && !defined(__CYGWIN__) /* Both MinGW and MSVC */
# if !defined(_WIN32_WINNT)
# define _WIN32_WINNT 0x0600
