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From: Junio C Hamano <gitster@pobox.com>
To: "Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>
Cc: git@vger.kernel.org,
	"Sibi Siddharthan" <sibisiddharthan.github@gmail.com>,
	"Đoàn Trần Công Danh" <congdanhqx@gmail.com>,
	"SZEDER Gábor" <szeder.dev@gmail.com>,
	"Johannes Schindelin" <johannes.schindelin@gmx.de>
Subject: Re: [PATCH 10/10] hashmap_for_each_entry(): work around MSVC's run-time check failure #3
Date: Fri, 25 Sep 2020 11:38:35 -0700	[thread overview]
Message-ID: <xmqq8scxln10.fsf@gitster.c.googlers.com> (raw)
In-Reply-To: <dc46d39611df4ebd90d9308364d887e638c1bc30.1601044119.git.gitgitgadget@gmail.com> (Johannes Schindelin via GitGitGadget's message of "Fri, 25 Sep 2020 14:28:38 +0000")

"Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>
writes:

> From: Johannes Schindelin <johannes.schindelin@gmx.de>
>
> When compiling Git in Visual C, we do not have the luxury of
> support for `typeof()`, and therefore `OFFSETOF_VAR()` unfortunately
> has to fall back to pointer arithmetic.

Sigh.

Short of changing the signature of hashmap_put_entry() and friends
to also take the type of these variables, I do not see any kosher
way to reimplement the users of OFFSETOF_VAR() to help compilers
without typeof() offhand.  As a one-time annotation, the unfortunate
noise we see in this patch may be tolerable, but what may make this
approach unsustainable is that average programmers would not know,
without compiling with that particular compiler, if their new
variable that points at a hash_entry needs to have an oterwise
unnecessary initialization.  Also the variables that are left
uninitialized by this patch may later require such an initialization.

Of course, it does not help that this workaround relies on an
undefined behaviour, as you pointed out.

> When compiling code using the `hashmap_for_each_entry()` macro in Debug
> mode, this leads to the "run-time check failure #3" because the variable
> passed as `var` are not initialized, yet we calculate the pointer
> difference `&(var->member)-var`.

Whoa, wait.  If it is just that macro, can we perhaps do something
like the attached patch?

>
> This "run-time check failure" causes a scary dialog to pop up.
>
> Work around this by initializing the respective variables.
>
> Note: according to the C standard, performing pointer arithmetic
> with `NULL` is not exactly well-defined, but it seems to work
> here, and it is at least better than performing pointer arithmetic
> with an uninitialized pointer.

 hashmap.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hashmap.h b/hashmap.h
index ef220de4c6..49cd8a8e92 100644
--- a/hashmap.h
+++ b/hashmap.h
@@ -449,7 +449,7 @@ static inline struct hashmap_entry *hashmap_iter_first(struct hashmap *map,
  * containing a @member which is a "struct hashmap_entry"
  */
 #define hashmap_for_each_entry(map, iter, var, member) \
-	for (var = hashmap_iter_first_entry_offset(map, iter, \
+	for (var = NULL, var = hashmap_iter_first_entry_offset(map, iter, \
 						OFFSETOF_VAR(var, member)); \
 		var; \
 		var = hashmap_iter_next_entry_offset(iter, \

  reply	other threads:[~2020-09-25 18:38 UTC|newest]

Thread overview: 70+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-25 14:28 [PATCH 00/10] CMake and Visual Studio Johannes Schindelin via GitGitGadget
2020-09-25 14:28 ` [PATCH 01/10] cmake: ignore files generated by CMake as run in " Johannes Schindelin via GitGitGadget
2020-09-25 14:28 ` [PATCH 02/10] cmake: do find Git for Windows' shell interpreter Johannes Schindelin via GitGitGadget
2020-09-25 16:25   ` Sibi Siddharthan
2020-09-26 20:32     ` Johannes Schindelin
2020-09-27  2:25       ` Đoàn Trần Công Danh
2020-09-28 13:56         ` Johannes Schindelin
2020-09-29 14:04           ` Đoàn Trần Công Danh
2020-09-29 18:42             ` Johannes Schindelin
2020-09-25 14:28 ` [PATCH 03/10] cmake: ensure that the `vcpkg` packages are found on Windows Johannes Schindelin via GitGitGadget
2020-09-25 14:28 ` [PATCH 04/10] cmake: fall back to using `vcpkg`'s `msgfmt.exe` " Johannes Schindelin via GitGitGadget
2020-09-25 14:28 ` [PATCH 05/10] cmake: quote the path accurately when editing `test-lib.sh` Johannes Schindelin via GitGitGadget
2020-09-25 14:28 ` [PATCH 06/10] cmake (Windows): let the `.dll` files are found when running the tests Johannes Schindelin via GitGitGadget
2020-09-25 19:48   ` Eric Sunshine
2020-09-26 21:00     ` Johannes Schindelin
2020-09-25 14:28 ` [PATCH 07/10] cmake (Windows): complain when encountering an unknown compiler Johannes Schindelin via GitGitGadget
2020-09-25 17:29   ` Sibi Siddharthan
2020-09-26 20:33     ` Johannes Schindelin
2020-09-25 14:28 ` [PATCH 08/10] cmake (Windows): initialize vcpkg/build dependencies automatically Johannes Schindelin via GitGitGadget
2020-09-30  5:05   ` Sibi Siddharthan
2020-09-30 15:25     ` Johannes Schindelin
2020-09-25 14:28 ` [PATCH 09/10] cmake (Windows): recommend using Visual Studio's built-in CMake support Johannes Schindelin via GitGitGadget
2020-09-25 18:22   ` Junio C Hamano
2020-09-26 20:45     ` Johannes Schindelin
2020-09-25 14:28 ` [PATCH 10/10] hashmap_for_each_entry(): work around MSVC's run-time check failure #3 Johannes Schindelin via GitGitGadget
2020-09-25 18:38   ` Junio C Hamano [this message]
2020-09-26 16:54     ` Junio C Hamano
2020-09-26 20:57       ` Johannes Schindelin
2020-09-26 21:32 ` [PATCH v2 00/10] CMake and Visual Studio Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 01/10] cmake: ignore files generated by CMake as run in " Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 02/10] cmake: do find Git for Windows' shell interpreter Johannes Schindelin via GitGitGadget
2020-09-28 11:17     ` Øystein Walle
2020-09-28 19:39       ` Johannes Schindelin
2020-09-26 21:32   ` [PATCH v2 03/10] cmake: ensure that the `vcpkg` packages are found on Windows Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 04/10] cmake: fall back to using `vcpkg`'s `msgfmt.exe` " Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 05/10] cmake: quote the path accurately when editing `test-lib.sh` Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 06/10] cmake (Windows): let the `.dll` files be found when running the tests Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 07/10] cmake (Windows): complain when encountering an unknown compiler Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 08/10] cmake (Windows): initialize vcpkg/build dependencies automatically Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 09/10] cmake (Windows): recommend using Visual Studio's built-in CMake support Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 10/10] hashmap_for_each_entry(): workaround MSVC's runtime check failure #3 Junio C Hamano via GitGitGadget
2020-09-28 21:09   ` [PATCH v3 00/11] CMake and Visual Studio Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 01/11] cmake: ignore files generated by CMake as run in " Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 02/11] cmake: do find Git for Windows' shell interpreter Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 03/11] cmake: ensure that the `vcpkg` packages are found on Windows Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 04/11] cmake: fall back to using `vcpkg`'s `msgfmt.exe` " Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 05/11] cmake: quote the path accurately when editing `test-lib.sh` Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 06/11] cmake (Windows): let the `.dll` files be found when running the tests Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 07/11] cmake (Windows): complain when encountering an unknown compiler Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 08/11] cmake (Windows): initialize vcpkg/build dependencies automatically Johannes Schindelin via GitGitGadget
2020-09-29  6:51       ` Sibi Siddharthan
2020-09-29 12:07         ` Johannes Schindelin
2020-09-28 21:09     ` [PATCH v3 09/11] cmake (Windows): recommend using Visual Studio's built-in CMake support Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 10/11] hashmap_for_each_entry(): workaround MSVC's runtime check failure #3 Junio C Hamano via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 11/11] cmake: fix typo in message when `msgfmt` was not found Johannes Schindelin via GitGitGadget
2020-09-28 22:11       ` Junio C Hamano
2020-09-29 12:07         ` Johannes Schindelin
2020-09-30 15:26     ` [PATCH v4 00/10] CMake and Visual Studio Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` [PATCH v4 01/10] cmake: ignore files generated by CMake as run in " Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` [PATCH v4 02/10] cmake: do find Git for Windows' shell interpreter Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` [PATCH v4 03/10] cmake: ensure that the `vcpkg` packages are found on Windows Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` [PATCH v4 04/10] cmake: fall back to using `vcpkg`'s `msgfmt.exe` " Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` [PATCH v4 05/10] cmake: quote the path accurately when editing `test-lib.sh` Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` [PATCH v4 06/10] cmake (Windows): let the `.dll` files be found when running the tests Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` [PATCH v4 07/10] cmake (Windows): complain when encountering an unknown compiler Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` [PATCH v4 08/10] cmake (Windows): initialize vcpkg/build dependencies automatically Johannes Schindelin via GitGitGadget
2020-09-30 19:17         ` Johannes Schindelin
2020-09-30 23:08           ` Junio C Hamano
2020-09-30 15:26       ` [PATCH v4 09/10] cmake (Windows): recommend using Visual Studio's built-in CMake support Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` [PATCH v4 10/10] hashmap_for_each_entry(): workaround MSVC's runtime check failure #3 Junio C Hamano via GitGitGadget

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