From b6a161833289d402986164912aeb9d005857675d Mon Sep 17 00:00:00 2001
From: Michal Nazarewicz <mina86@mina86.com>
Date: Mon, 13 Feb 2017 02:09:24 +0100
Subject: [PATCH] =?UTF-8?q?book:=20don=E2=80=99t=20use=20GNU=20extensions?=
=?UTF-8?q?=20in=20the=20example=20unnecessarily?=
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The use of a GNU C extension for bloc expressions is immaterial to the
actual problem with C macros that the section tries to show so don’t
use it and instead use a plain C way of writing the macro.
---
src/doc/book/src/macros.md | 14 ++++++--------
1 file changed, 6 insertions(+), 8 deletions(-)
diff --git a/src/doc/book/src/macros.md b/src/doc/book/src/macros.md
index 3ccbeb05f01d..ae1e1c65dd22 100644
--- a/src/doc/book/src/macros.md
+++ b/src/doc/book/src/macros.md
@@ -261,36 +261,34 @@ The metavariable `$x` is parsed as a single expression node, and keeps its
place in the syntax tree even after substitution.
Another common problem in macro systems is ‘variable capture’. Here’s a C
-macro, using [a GNU C extension] to emulate Rust’s expression blocks.
-
-[a GNU C extension]: https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html
+macro using a block with multiple statements.
```text
-#define LOG(msg) ({ \
+#define LOG(msg) do { \
int state = get_log_state(); \
if (state > 0) { \
printf("log(%d): %s\n", state, msg); \
} \
-})
+} while (0)
```
Here’s a simple use case that goes terribly wrong:
```text
const char *state = "reticulating splines";
-LOG(state)
+LOG(state);
```
This expands to
```text
const char *state = "reticulating splines";
-{
+do {
int state = get_log_state();
if (state > 0) {
printf("log(%d): %s\n", state, state);
}
-}
+} while (0);
```
The second variable named `state` shadows the first one. This is a problem