threads / patch / 10075

patch, 2 partsReplace literal STRLEN_ #defines in refs.h with compiler evaluated expressions

Subject: [PATCH 2/2] Replace literal STRLEN_ #defines in refs.h with compiler evaluated expressions

## tl;dr

3 messages between Sep 29, 2007 and Sep 29, 2007. Diffs are folded; open one to read it.

replies: 2people: 3as markdown or json

Andy Parkins· Sep 29, 2007, 13:00 UTC · lore
Bill Lear pointed out that the following:
 #define PATH_REMOTES             "remotes/"
 #define STRLEN_PATH_REMOTES      8
Could be replaced by the less error-prone
 #define PATH_REMOTES "remotes/"
 #define LIT_STRLEN(S) ((sizeof(S) / sizeof(S[0])) -1)
 #define STRLEN_PATH_REMOTES LIT_STRLEN(PATH_REMOTES)
which is what this patch does.
Signed-off-by: Andy Parkins <andyparkins@gmail.com>
---
 refs.h |   11 ++++++-----
 1 files changed, 6 insertions(+), 5 deletions(-)
Show changes to refs.h +6 −5
diff --git a/refs.h b/refs.h
index 1025d04..fb58889 100644
--- a/refs.h
+++ b/refs.h
@@ -13,16 +13,17 @@ struct ref_lock {
 #define REF_ISSYMREF 01
 #define REF_ISPACKED 02
 
+#define LIT_STRLEN(S)            ((sizeof(S) / sizeof(S[0])) -1)
 #define PATH_OBJECTS             "objects/"
-#define STRLEN_PATH_OBJECTS      8
+#define STRLEN_PATH_OBJECTS      LIT_STRLEN(PATH_OBJECTS)
 #define PATH_REFS                "refs/"
-#define STRLEN_PATH_REFS         5
+#define STRLEN_PATH_REFS         LIT_STRLEN(PATH_REFS)
 #define PATH_HEADS               "heads/"
-#define STRLEN_PATH_HEADS        6
+#define STRLEN_PATH_HEADS        LIT_STRLEN(PATH_HEADS)
 #define PATH_TAGS                "tags/"
-#define STRLEN_PATH_TAGS         5
+#define STRLEN_PATH_TAGS         LIT_STRLEN(PATH_TAGS)
 #define PATH_REMOTES             "remotes/"
-#define STRLEN_PATH_REMOTES      8
+#define STRLEN_PATH_REMOTES      LIT_STRLEN(PATH_REMOTES)
 #define PATH_REFS_HEADS          PATH_REFS PATH_HEADS
 #define STRLEN_PATH_REFS_HEADS   (STRLEN_PATH_REFS+STRLEN_PATH_HEADS)
 #define PATH_REFS_TAGS           PATH_REFS PATH_TAGS

-- 
1.5.3.rc5.11.g312e
Junio C Hamano· Sep 29, 2007, 16:58 UTC · re: Andy Parkins · lore

Re: [PATCH 2/2] Replace literal STRLEN_ #defines in refs.h with compiler evaluated expressions

Andy Parkins <andyparkins@gmail.com> writes:
Show 12 quoted lines
> Bill Lear pointed out that the following:
>
>  #define PATH_REMOTES             "remotes/"
>  #define STRLEN_PATH_REMOTES      8
>
> Could be replaced by the less error-prone
>
>  #define PATH_REMOTES "remotes/"
>  #define LIT_STRLEN(S) ((sizeof(S) / sizeof(S[0])) -1)
>  #define STRLEN_PATH_REMOTES LIT_STRLEN(PATH_REMOTES)
>
> which is what this patch does.

I do not think the above is wrong per se, but doesn't a good compiler optimize

	#define PATH_REMOTES	"remotes/"
	#define STRLEN_PATH_REMOTES strlen(PATH_REMOTES)
	... and much later in some *.c file that includes the
	... above and <string.h>
        foo = strlen(PATH_REMOTES)
anyway?
Linus Torvalds· Sep 29, 2007, 18:15 UTC · re: Junio C Hamano · lore

Re: [PATCH 2/2] Replace literal STRLEN_ #defines in refs.h with compiler evaluated expressions

On Sat, 29 Sep 2007, Junio C Hamano wrote:
Show 6 quoted lines
> 
> I do not think the above is wrong per se, but doesn't a good
> compiler optimize
> 
> 	#define PATH_REMOTES	"remotes/"
> 	#define STRLEN_PATH_REMOTES strlen(PATH_REMOTES)
Gcc does, yes.
HOWEVER.

Using "strlen()" may be optimized at compile-time, but it still ends up resulting in illegal C code if the constant needs to be a constant in the semantic sense.

IOW, the "sizeof()" trick can be portably used for things like array declarations etc. But strlen() cannot. Ie

	char array[sizeof("hello")-1];
is legal in non-function scope, but doing the same with "strlen()" is not.

(That said, gcc has been known to accept bad C code, and will in fact accept this one too!)

		Linus

← back to recent threads