max strlen at compile time?

[Gernot Frisch]

Hi,

can I use the preprocessor, using sizeof(a)/sizeof(a[0]) to yield an
error for too long strings?

Like:

#define CRYPT(a) \
#if sizeof(a)/sizeof(a[0]) 31 \
xCRYPT(a) \
#else\
#error xy\
#endif

(which of course doe not work)
--
-Gernot
int main(int argc, char** argv) {printf
("%silto%c%cf%c gl%ssic%ccom%c" , "ma", 58, 'g', 64, "ba", 46, 10);}

_______________ _______________ __________
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www.GLBasic.com

[Laurent Deniau]

Gernot Frisch wrote:

>>What we want, then, is a compile-time error if the size of Array
exceeds 31.
Here's how:

char Array[SUSPECT_LENGTH] = {0};
char Error_ArrayIsTo oLong[((sizeof Array / sizeof Array[0] <=
OKAY_LENGTH ) *
2) - 1] = {0};

>>foo.c: In function `main':
foo.c:9: size of array `Error_ArrayIsT ooLong' is negative
foo.c:9: warning: unused variable `Error_ArrayIsT ooLong'

Very clever! Thank you.

Your problem can be generalized to "how to program an elegant and robust
static assert" (compile time assert). Here is what I use (open to any
improvement):

#define cos_STATIC_ASSE RT(cond) \
struct cos_PP_CAT(STAT IC_ASSERT_,__LI NE__) { \
enum { cos_PP_CAT(STAT IC_ASSERT_,__LI NE__) = !(cond) } _; \
int STATIC_ASSERT[(cond) ? 1 : -1]; \
}

where

#define cos_PP_CAT( a,b) cos_PP_CAT_(a,b )
#define cos_PP_CAT_(a,b ) a##b

This should give a meaningfull error message with most compilers.
The enum is there to forbid the use of sizeof at runtime (c99) and to
ensure consistent behavior between c89 and c99.

a+, ld.

[Gernot Frisch]

One matter I didn't notice at first is that your and Richard's
variants
generate data, while mine doesn't (it just makes a typedef).

That's a point!

[Frederick Gotham]

Laurent Deniau posted:

int STATIC_ASSERT[(cond) ? 1 : -1];

Would you not prefer a simple typedef?

(COMPASS = Compile-time assert)

Something like:

typedef COMPASS(expr) char[(expr) ? 2 : -2 ];

--

Frederick Gotham

[Ben Pfaff]

Frederick Gotham <fg*******@SPAM .comwrites:

Laurent Deniau posted:

> int STATIC_ASSERT[(cond) ? 1 : -1];

Would you not prefer a simple typedef?

(COMPASS = Compile-time assert)

Something like:

typedef COMPASS(expr) char[(expr) ? 2 : -2 ];

verify.h from gnulib is a refined version of the idea of a
compile-time assert. I won't post it here, because it's about
150 lines long (mostly comments), but I'd recommend taking a look
at on the web:
http://cvs.savannah.gnu.org/viewcvs/...in&root=gnulib

I found it very educational.
--
"The way I see it, an intelligent person who disagrees with me is
probably the most important person I'll interact with on any given
day."
--Billy Chambless

[Laurent Deniau]

Frederick Gotham wrote:

Laurent Deniau posted:

> int STATIC_ASSERT[(cond) ? 1 : -1];

Would you not prefer a simple typedef?

why?

(COMPASS = Compile-time assert)

Something like:

typedef COMPASS(expr) char[(expr) ? 2 : -2 ];

This may not detect some problems at compile time but at runtime in c99.

a+, ld.