Date: Mon, 13 Oct 1997 12:24:19 -0400 (EDT)
From: "Art S. Kagel" <kagel AT ns1 DOT bloomberg DOT com>
Reply-To: kagel AT ns1 DOT bloomberg DOT com
To: Guan Foo Wah <djgpp AT rocketmail DOT com>
Cc: djgpp AT delorie DOT com
Subject: Re: Is this a bug ??? or feature ??
In-Reply-To: <19971013092045.160.rocketmail@send1.rocketmail.com>
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On Mon, 13 Oct 1997, Guan Foo Wah wrote:

> Check the program out below
> 
> 
> //starts here-------------------------------------------------------
> #include <stdio.h>
> 
> char *num[] = {
>     "one",      //line 4
>     "one",      //line 5 :- this string exactly the same as line 4
>     "three",
>     "four"
> };
> 
> char *id[] = {
>     "ten",
>     "two",
>     "one",      //line 13 :- exactly same as line 4 and 5
>     "F16",
>     "DJGPP"
> };
> 
> void main (void)
> {
>     //prints the address of the 3 variables 
>     printf ("Address : %p %p %p %p\n", num[0], num[1], id[2]);
> 
>     //prints the value of the 3 variables before modifying
>     printf ("Value before modifying : %s %s %s\n", num[0], num[1],
> id[2]);
> 
>     //modify one of the 3 variables
>     num[0][0] = 'f';
> 
>     //prints the value of the 3 variables after modifying
>     printf ("Value after modifying  : %s %s %s\n", num[0], num[1],
> id[2]);
> }
> 
> //ends here-----------------------------------------------------------

Your results are as one would expect and simply show the difference
between two implementations.  GCC places all literal strings together
in memory and only creates a particular string once.  By assigning the
same literal string to several element of array of char you get the
same address as the string only exists once.  You then modify the
literal which changes it in memory and all of the array elements
pointing to it change also.  Borland simply does not perform this
space optimization so the results differ.  First you should not be
modifying literal strings, it is bad form and bad code.  If the
strings may me modified then malloc space for the elements of the
array to point to and strcpy() the literals into that space.

Art S. Kagel, kagel AT bloomberg DOT com