Skip to content

HTTPS clone URL

Subversion checkout URL

You can clone with HTTPS or Subversion.

Download ZIP
Browse files

make error()'s constant return value more visible

When git is compiled with "gcc -Wuninitialized -O3", some
inlined calls provide an additional opportunity for the
compiler to do static analysis on variable initialization.
For example, with two functions like this:

  int get_foo(int *foo)
  {
	if (something_that_might_fail() < 0)
		return error("unable to get foo");
	*foo = 0;
	return 0;
  }

  void some_fun(void)
  {
	  int foo;
	  if (get_foo(&foo) < 0)
		  return -1;
	  printf("foo is %d\n", foo);
  }

If get_foo() is not inlined, then when compiling some_fun,
gcc sees only that a pointer to the local variable is
passed, and must assume that it is an out parameter that
is initialized after get_foo returns.

However, when get_foo() is inlined, the compiler may look at
all of the code together and see that some code paths in
get_foo() do not initialize the variable. As a result, it
prints a warning. But what the compiler can't see is that
error() always returns -1, and therefore we know that either
we return early from some_fun, or foo ends up initialized,
and the code is safe.  The warning is a false positive.

If we can make the compiler aware that error() will always
return -1, it can do a better job of analysis. The simplest
method would be to inline the error() function. However,
this doesn't work, because gcc will not inline a variadc
function. We can work around this by defining a macro. This
relies on two gcc extensions:

  1. Variadic macros (these are present in C99, but we do
     not rely on that).

  2. Gcc treats the "##" paste operator specially between a
     comma and __VA_ARGS__, which lets our variadic macro
     work even if no format parameters are passed to
     error().

Since we are using these extra features, we hide the macro
behind an #ifdef. This is OK, though, because our goal was
just to help gcc.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
  • Loading branch information...
commit e208f9cc7574f5980faba498d0aa30b4defeb34f 1 parent bfae342
Jeff King peff authored committed
Showing with 12 additions and 0 deletions.
  1. +11 −0 git-compat-util.h
  2. +1 −0  usage.c
11 git-compat-util.h
View
@@ -288,6 +288,17 @@ extern NORETURN void die_errno(const char *err, ...) __attribute__((format (prin
extern int error(const char *err, ...) __attribute__((format (printf, 1, 2)));
extern void warning(const char *err, ...) __attribute__((format (printf, 1, 2)));
+/*
+ * Let callers be aware of the constant return value; this can help
+ * gcc with -Wuninitialized analysis. We have to restrict this trick to
+ * gcc, though, because of the variadic macro and the magic ## comma pasting
+ * behavior. But since we're only trying to help gcc, anyway, it's OK; other
+ * compilers will fall back to using the function as usual.
+ */
+#ifdef __GNUC__
+#define error(fmt, ...) (error((fmt), ##__VA_ARGS__), -1)
+#endif
+
extern void set_die_routine(NORETURN_PTR void (*routine)(const char *err, va_list params));
extern void set_error_routine(void (*routine)(const char *err, va_list params));
1  usage.c
View
@@ -130,6 +130,7 @@ void NORETURN die_errno(const char *fmt, ...)
va_end(params);
}
+#undef error
int error(const char *err, ...)
{
va_list params;
Please sign in to comment.
Something went wrong with that request. Please try again.