Re: (NULL == 0) ?

[Zefram <A.Main@dcs.warwick.ac.uk>]

>Does anything guarantees that on those (theoretical) systems where NULL is
>not really zero, all static and external pointers are initialized to this
>NULL?  There are many many places where this assumption is used.

Yes.  All variables with static duration are initialised as if they had
been explicitly initialised with `= 0', so integers get 0, floating
point variables get a floating point 0.0, and pointers get NULL.

>Does anyone heard about such a Unix system?  I bet that if such a system
>exists memset would be just the easiest among many other problems.

The only significant problems I am aware of are use of memset/calloc,
and assumptions about the result of treating a pointer as an integer.
I'm pretty sure zsh never casts a pointer to an integral type, so
memset/calloc is the only thing we need to worry about.

>A couple of other questions about Unix and C standards: does anything
>guarantee that a variable is always 8 * sizeof(var) bits long?

Only if a byte is 8 bits long and there are no holes in the integral
types.  This type of holes are very rare, and we assume 8 bit bytes
elsewhere.  I don't think we actually assume this directly anyway.

>                                                                Does any
>Unix or C standard guarantee that ASCII is used (zsh assumes ASCII in many
>places)?

No.  That is certainly an issue, but it's going to be very difficult to
resolve.

>          Does any Unix or C standard guarantees that an int always has at
>least 32 bits?

No.  But C guarantees that long is at least 32 bits, so if there's
anywhere we need 32 bits we should change it to use long.  I don't know
of anywhere in zsh tht needs fixing in this respect.

The C standard guarantees the following:

CHAR_BIT >= 8

SCHAR_MAX >=  0x7f
SCHAR_MIN <= -0x7f
UCHAR_MAX >=  0xff

There are no holes in char.

SHRT_MAX  >=  0x7fff
SHRT_MIN  <= -0x7fff
USHRT_MAX >=  0xffff

I.e. short is at least 16 bits.

INT_MAX   >=  0x7fff
INT_MIN   <= -0x7fff
UINT_MAX  >=  0xffff

I.e. int is at least 16 bits.

LONG_MAX  >=  0x7fffffff
LONG_MIN  <= -0x7fffffff
ULONG_MAX >=  0xffffffff

I.e. long is at least 32 bits.

It is further guaranteed that SCHAR_MAX SHRT_MAX INT_MAX LONG_MAX is a
nondecreasing sequence, as is UCHAR_MAX USHRT_MAX UINT_MAX ULONG_MAX,
and SCHAR_MIN SHRT_MIN INT_MIN LONG_MIN is a nonincreasing sequence.
The definition of the standard library indirectly requires SHRT_MAX >=
UCHAR_MAX.

The maximum values of the unsigned types are required to be of the form
2^n-1, where n is an integer.  There are also some requirements
relating each unsigned type to its signed counterpart -- basically
Ufoo_MAX > foo_MAX.

sizeof(char) sizeof(short) sizeof(int) sizeof(long) is also required to
be a nondecreasing sequence, and for each integral type sizeof(unsigned
foo) == sizeof(signed foo).

-zefram