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

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

[Andy Parkins]

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(-)

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

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

[Junio C Hamano]

Andy Parkins <andyparkins@gmail.com> writes:

> 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?

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

[Linus Torvalds]

On Sat, 29 Sep 2007, Junio C Hamano wrote:
> 
> 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