Title: Compile Time Assertions in C Revisited

#ifndef BITSIZE_H
#define BITSIZE_H

/*
In response to Jon Jagger's article "Compile Time Assertions in C" (C Vu 11.3), 
I wonder if it is really necessary to have a general-purpose assertion in order 
to achieve what he wants?  Besides the problems Francis mentions, it would be 
nice if the error messages were more meaningful.  But CHAR_BIT can be tested 
directly:
*/

#include <limits.h>
#if CHAR_BIT<8
#error This program assumes a compiler that supports 8-bit chars
#endif

/*
Unfortunately limits.h does not define the number of bits in the other types, 
only the maximum values, and the preprocessor can wrap around values that are 
larger than these.  So to check 'int', once you have established that there 
are 8-bit chars you have to do a painstaking byte-for-byte comparison:
*/

#define byte1(c) ((c)&0xFF)
#define byte2(c) byte1((c)>>8)
#define byte3up(c) ((c)>>16)
#define TwoByteFail(c,min2,min1) (!byte3up(c) && (byte2(c)<min2 || byte1(c)<min1))

#if TwoByteFail(USHRT_MAX,0xFF,0xFF)
#error This program assumes a compiler that supports 16-bit shorts
#endif
#if TwoByteFail(UINT_MAX,0xFF,0xFF)
#error Strange - it seems your 'int' is shorter than your 'short'
#endif

/*
but once a minimum 16-bit word length is established, you can take advantage 
of it when checking 'long':
*/

#define word1(c) ((c)&0xFFFF)
#define word2(c) word1((c)>>16)
#define word3up(c) ((c)>>32)
#define TwoWordFail(c,min2,min1) (!word3up(c) && (word2(c)<min2 || word1(c)<min1))

#if TwoWordFail(ULONG_MAX,0xFFFF,0xFFFF)
#error This program assumes a compiler that supports 32-bit longs
#endif

/*
Of course, this is still no good for checking the sizes of compiler-provided 
structs and typedefs.  If you are concerned about them then another approach 
is to write an entirely separate conformance test program, that can give 
meaningful error messages when things are wrong, and insist that the user 
compile and run it before proceeding with installing your program.  You could 
get it to return an error code through exit(), so any batch install script 
can stop if necessary.
*/
#endif

I am leaving it to the readership to comment.