Re: E 0.9.3 release?

On 1/3/11 at 7:23 AM, [email protected] (Ben Laurie) wrote:

>Sorry I missed this before - following up on David's post: an int cast
>is _not_ a safe fix, despite it making the warning go away.
>
>FWIW, since you asked me in IM if C++ also suffers from this issue the
>answer is that it could easily avoid it, but in practice probably
>doesn't.
>
>How to avoid it:
>
>inline bool isprint(char c) { return isprint(static_cast<unsigned char>(c)); }
>inline bool isprint(unsigned char c) { ... }
>
>Why it is probably not avoided: in practice C++ code is likely to use
>the C implementation, which cannot be fixed in this way.

Ben brings out the language lawyer in me, and my copy of the 
ANSI C standard.

isprint is defined in the standard as:

   #include <ctype.h>
   int isprint(int c);

It also defines a bunch of constants with the note, "Their 
implementation-defined values shall be equal or greater in 
magnitude (absolute value) ot those shown with the same sign.

   CHAR_BIT 8 -- number of bits for smallest object that is not 
a bit-field
   SCHAR_MIN -127
   SCHAR_MAX +127
   UCHAR_MAX 255

There are also CHAR_MIN and CHAR_MAX with the definition of, "If 
the value of an object of type char is treated as a signed 
integer when used in an expression, the value of CHAR_MIN shall 
be the same as that of SCHAR_MIN and the value of CHAR_MAX shall 
be the same as that of SCHAR_MAX. Otherwise the value of 
CHAR_MIN shall be 0 and the value of CHAR_MAX shall be the same 
as that of UCHAR_MAX.

Using the above values, it seems that we can define a macro for isprint:

#define isprint(c) (__isprint_look_up_table[(unsigned int)(c) & 0xff)

If the compiler "knows" that c is only 8 bits (and has been 
loaded into a register with the rest of the bits zero), it can 
optimize the & operation away.

If other values are chosen (for supporting 16 bit characters for 
example), then the mask value will change or a construct such as 
could be used:

#define isprint(c) ((c) < CHAR_MIN || (c) > CHAR_MAX ? 0 : 
__iplut[(c) - CHAR_MIN])

Cheers - Bill

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