[libc-commits] [libc] [libc] Keep headers compatible with -std=c89 mode (PR #121981)

[Roland McGrath via libc-commits]

https://github.com/frobtech created https://github.com/llvm/llvm-project/pull/121981

C89 doesn't have the `restrict` keyword.  When in `-std=c89`
mode, GNU-compatible compilers require the `__restrict` spelling.


>From 2d71b6a001ae28881b5c3a0039312ac515a20e06 Mon Sep 17 00:00:00 2001
From: Roland McGrath <mcgrathr at google.com>
Date: Tue, 7 Jan 2025 11:01:06 -0800
Subject: [PATCH] [libc] Keep headers compatible with -std=c89 mode

C89 doesn't have the `restrict` keyword.  When in `-std=c89`
mode, GNU-compatible compilers require the `__restrict` spelling.
---
 libc/include/__llvm-libc-common.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/libc/include/__llvm-libc-common.h b/libc/include/__llvm-libc-common.h
index d9d70aff771c08..c63eb134a5e5d7 100644
--- a/libc/include/__llvm-libc-common.h
+++ b/libc/include/__llvm-libc-common.h
@@ -50,7 +50,14 @@
 #define __END_C_DECLS
 
 #undef __restrict
-#define __restrict restrict // C99 and above support the restrict keyword.
+#if __STDC_VERSION__ >= 199901L
+// C99 and above support the restrict keyword.
+#define __restrict restrict
+#elif !defined(__GNUC__)
+// GNU-compatible compilers accept the __ spelling in all modes.
+// Otherwise, omit the qualifier for pure C89 compatibility.
+#define __restrict
+#endif
 
 #undef _Noreturn
 #if __STDC_VERSION__ >= 201112L