[PATCH 1/2] libext2fs: use statement-expression for container_of only on GNU-compatible compilers

[PATCH 1/2] libext2fs: use statement-expression for container_of only on GNU-compatible compilers

[Michael Forney]

Functionally, the statement-expression is not necessary here; it
just gives a bit of type-safety to make sure the pointer really
does have a compatible type with the specified member of the struct.

When statement expressions are not available, we can just use a
portable fallback macro that skips this member type check.

Signed-off-by: Michael Forney <mforney@mforney.org>
---
 lib/ext2fs/compiler.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/lib/ext2fs/compiler.h b/lib/ext2fs/compiler.h
index 9aa9b4ec..03c35ab8 100644
--- a/lib/ext2fs/compiler.h
+++ b/lib/ext2fs/compiler.h
@@ -14,9 +14,14 @@
 #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
 #endif
 
+#ifdef __GNUC__
 #define container_of(ptr, type, member) ({			\
 	const __typeof__( ((type *)0)->member ) *__mptr = (ptr);	\
 	(type *)( (char *)__mptr - offsetof(type,member) );})
+#else
+#define container_of(ptr, type, member)				\
+	((type *)((char *)(ptr) - offsetof(type, member)))
+#endif
 
 
 #endif /* _EXT2FS_COMPILER_H */
-- 
2.30.1

[PATCH 2/2] libext2fs: use offsetof() from stddef.h

[Michael Forney]

offsetof is a standard C feature available from stddef.h, going
back all the way to ANSI C.

Signed-off-by: Michael Forney <mforney@mforney.org>
---
Perhaps there is some reason to prefer compiler builtins over libc
that I'm not seeing?

 lib/ext2fs/compiler.h | 13 +------------
 1 file changed, 1 insertion(+), 12 deletions(-)

diff --git a/lib/ext2fs/compiler.h b/lib/ext2fs/compiler.h
index 03c35ab8..42faa61c 100644
--- a/lib/ext2fs/compiler.h
+++ b/lib/ext2fs/compiler.h
@@ -1,18 +1,7 @@
 #ifndef _EXT2FS_COMPILER_H
 #define _EXT2FS_COMPILER_H
 
-#ifndef __has_builtin
-#define __has_builtin(x) 0
-#endif
-
-#undef offsetof
-#if __has_builtin(__builtin_offsetof)
-#define offsetof(TYPE, MEMBER) __builtin_offsetof(TYPE, MEMBER)
-#elif defined(__compiler_offsetof)
-#define offsetof(TYPE,MEMBER) __compiler_offsetof(TYPE,MEMBER)
-#else
-#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
-#endif
+#include <stddef.h>
 
 #ifdef __GNUC__
 #define container_of(ptr, type, member) ({			\
-- 
2.30.1

Re: [PATCH 1/2] libext2fs: use statement-expression for container_of only on GNU-compatible compilers

[Theodore Ts'o]

On Wed, Apr 14, 2021 at 12:41:27AM -0700, Michael Forney wrote:
> Functionally, the statement-expression is not necessary here; it
> just gives a bit of type-safety to make sure the pointer really
> does have a compatible type with the specified member of the struct.
> 
> When statement expressions are not available, we can just use a
> portable fallback macro that skips this member type check.
> 
> Signed-off-by: Michael Forney <mforney@mforney.org>

Applied, thanks.

						- Ted

Re: [PATCH 2/2] libext2fs: use offsetof() from stddef.h

[Theodore Ts'o]

On Wed, Apr 14, 2021 at 12:41:28AM -0700, Michael Forney wrote:
> offsetof is a standard C feature available from stddef.h, going
> back all the way to ANSI C.
> 
> Signed-off-by: Michael Forney <mforney@mforney.org>

Thanks, applied.

> Perhaps there is some reason to prefer compiler builtins over libc
> that I'm not seeing?

It's because I pulled container_of from the kernel, and the kernel
header files has to provide offsetof since we don't use the standard
header files --- and it has to work across a bunch of compilers and
architectures.

						- Ted