Re: difference between casting and atol,atoi functions

From: Peter Nilsson (airia_at_acay.com.au)
Date: 02/15/05 

Date: 14 Feb 2005 21:56:13 -0800

[There's no need to cross post this to two different groups.]

Janice wrote:
> unsigned char buf[255];
> signed long* x;
> signed long y;
> buf[0]=255;
> buf[1]=255;
> buf[2]=255;
> buf[3]=255;
> //statment 1
> x=(signed long*)buf;
> //statement 2
> y=atol(buf,4);
>
> What is the difference between statement 1 and 2?

None. They both invoke undefined behaviour.

The first is ill-formed since buf may not be suitably
aligned for a signed long, and it may represent a trap
representation in anycase. [Casts are usually an error
and should be avoided.]

The second is ill-formed since buf may not be null
byte terminated, as a string should be. Consequently,
atol() may not be able to convert it. Secondly, the
character code 255 need not represent a digit
character. [In fact, it's unlikely to.]

What you're doing is viewing a byte sequence as one
of two forms. Either a long integer (1), or a text string
description (2). The difference is analogous to trying
to read '1234' as a series of English 'words', or trying
to read 'one thousand...' as a number.

A better example might be...

  #include <stdio.h>
  #include <stdlib.h>

  void dump(const void *p, size_t n)
  {
    const unsigned char *uc = p;
    while (n--) printf(" 0x%02X", 0u + *uc++);
  }

  int main(void)
  {
    long x = 1234;
    long y, z;
    char buf[42]; /* 42 is known to be big enough here */
    unsigned char *mem = malloc(sizeof x);

    printf("long: x = %ld\n", x);

    printf("\nbyte sequence at &x: ");
    dump(&x, sizeof x);
    putchar('\n');

    if (mem)
    {
      memcpy(mem, &x, sizeof x);
      y = * (long *) mem;
      printf("interpreted byte sequence: y = %ld\n", y);
    }

    sprintf(buf, "%ld", x);
    printf("\nx as a string: buf = \"%s\"\n", buf);

    z = atol(buf);
    printf("string back to long: z = %ld\n", z);

    return 0;
  }

Note that the size and representation of a signed long can
(and does) differ from machine to machine. Also note that
a non null return value from malloc will be suitably
aligned for any type. 

-- 
Peter

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