Re: [Libguestfs] [PATCH nbdkit 1/2] common/include: Change unique-name macros to use __COUNTER__
On Tue, Feb 22, 2022 at 09:35:03PM +0000, Richard W.M. Jones wrote:
Previously the macros used __LINE__ which meant they created a
unique
name specific to the line on which the macro was expanded. This
worked to a limited degree for cases like:
#define FOO \
({ int NBDKIT_UNIQUE_NAME(foo) = 42; \
NBDKIT_UNIQUE_NAME(foo) * 2 })
Missing a ; after '* 2'.
since the “FOO” macro is usually expanded at one place so both uses of
NBDKIT_UNIQUE_NAME expanded to the same unique name. It didn't work
if FOO was used twice on the same line, eg:
int i = FOO * FOO;
This didn't actually fail. The failure we saw was more subtle:
MIN (MIN (1, 2), 3)
compiled (the inner MIN() creates an _x evaluated in the context of
the initializer of the outer MIN's _x, which is not in scope until the
initializer completes), but:
MIN (1, MIN (2, 3))
failed to compile with -Werror -Wshadow, because now the inner MIN's
_x is declared inside the scope of the initializer of the outer MIN's
_y, when an outer _x is already in scope.
would fail, but this would work:
int i = FOO *
FOO;
Or, back to the example we actually hit,
MIN (1,
MIN (2, 3))
worked.
Use __COUNTER__ instead of __LINE__, but NBDKIT_UNIQUE_NAME must now
be used differently. The FOO macro above must be rewritten as:
#define FOO FOO_1(NBDKIT_UNIQUE_NAME(foo))
#define FOO_1(foo) \
({ int foo = 42; \
foo * 2 })
Thanks: Eric Blake
---
+++ b/common/include/test-checked-overflow.c
@@ -39,29 +39,25 @@
#define TEST_MUL(a, b, result, expected_overflow, expected_result) \
do { \
- bool NBDKIT_UNIQUE_NAME(_actual_overflow); \
+ bool actual_overflow; \
\
- NBDKIT_UNIQUE_NAME(_actual_overflow) = \
- MUL_OVERFLOW_FALLBACK ((a), (b), (result)); \
- assert (NBDKIT_UNIQUE_NAME(_actual_overflow) == (expected_overflow)); \
+ actual_overflow = MUL_OVERFLOW_FALLBACK ((a), (b), (result)); \
Extra spacing after =
The commit message may need touching up, but ACK to the change itself.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org