Re: bitwise comparators

[gisle@kondor.ii.uib.no (Gisle S�lensminde)]

In article <yecya9mpsv8.fsf@king.cts.com>, Keith Thompson wrote:
>Jeff Carter <jrcarter010@earthlink.net> writes:
>[...]
>> > With true arrays, do you mean out of bound checking, etc?  This can be done
>> > with _proper_ programming in C!
>> 
>> C does not have arrays; it only has different notations for address
>> arithmetic.
>
>That's a slight exaggeration.  C does have array types and array
>objects.  For example this:
>    int a[10];
>declares a as an array of 10 ints, very much like Ada's
>    A: array(0 .. 9) of Integer;
>It does not, contrary to popular misconception, declare a as a
>pointer.
>
>What often causes confusion is that, in most expression contexts, a
>reference to the name of an array object "decays" to a pointer to the
>array's first element.
>
>C arrays are not first-class types, but they do exist.


The semantics of C array is confusing for a lot of people, and 
they are frequently incorrectly used. C arrays are passed by 
reference to functions, but not if they are placed inside a struct.
I have frequently seen code like this one:

(Which not even gives warning on several compilers, like sunpro CC)

char* dig2hex(char digest[20]){
  int i;
  char hexstr[40];
  /* insert calculation here */

  return hexstr; /* returns a pointer to a local 
		    - may corrupt data later*/
}


This code returns a pointer to a local variable, and since these
data is placed on the stack, they will typically be overwritten
later. Instead you can place the array inside a struct, like this:


typedef struct hexstruct {
  char hexstr[40];
} hex;

hex dig2hex(char digest[20]){
  int i;
  hex tmphex;
  /* insert calculation here */

  return tmphex; /* return the struct including array by copy - ok*/
}

As long as the array is placed inside a struct you get by copy 
semantics, while an array on it's own behaves like a different
notation for a pointer.

A strange behavior here, is that the struct must be typedefed.
Otherwise it won't compile.


--
Gisle S�lensminde ( gisle@ii.uib.no )   

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