From 9b240347543f240dbf7e541173ac45c34155ca1b Mon Sep 17 00:00:00 2001 From: Jeff King Date: Fri, 19 Aug 2022 06:08:27 -0400 Subject: [PATCH] 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 Signed-off-by: Junio C Hamano --- git-compat-util.h | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/git-compat-util.h b/git-compat-util.h index 36a25ae252..c6669db07d 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