To: gapa83 AT udcf DOT gla DOT ac DOT uk
Subject: Re: strcpy() bug?
From: pfulmek AT email DOT tuwien DOT ac DOT at (Paul FULMEK)
Reply-To: pfulmek AT email DOT tuwien DOT ac DOT at
Cc: djgpp AT sun DOT soe DOT clarkson DOT edu (djgpp)
Date: Wed, 9 Mar 1994 08:52:27 +0000


>>>From: Paul Harness <gapa83 AT udcf DOT gla DOT ac DOT uk>
>>>Subject: strcpy() bug?
>>>
>>>The following program:
>>>
>>>#include<stdio.h>
>>>#include<string.h>
>>>int main(void)
>>>{
>>>        char *element1[]={"length","length"};
You don't ask the Compiler (with capital 'C') to allocate MEMORY for Your
element1[]-array. The Compiler only reserves RAM for the 2 POINTERS:
element1[0] and element1[1] - That's exactly what You asked it to do.
Then the Compiler allocates memory for the constant string "length" only
once, as the two strings are identical - the Compiler is a clever
one -, and sets both element1[]-pointers to the starting address of this
unique string.
(Besides, string constants shouldn't be writeable!)
If you check the addresses (In common it's a problem with virtual memory, I
know) by
  printf("%X %X\n",&element1[0][0],&element1[1][0]);
You'll find out, that in this case both element1[]-pointers point to the
same address.
That's is Your problem.
strcopy() acts correctly: it copies the string "text" to
the address, which BOTH element1[]-pointers point at!
Initializing with {"length","short"} forces the Compiler to
allocate RAM for "length" AND "short", and therefore to set Your
element1[]-pointers to different addresses.
As You obviously want the Compiler to allocate RAM and set two
pointers to addresses of initialized RAM-areas, You
should better tell him explicitly to do so.
For example

  char element1[2][7] = {"length","length"};

or

  char *element1[];
  element1[0] = (char *)malloc( sizeof(char)*7 );
  element1[1] = (char *)malloc( sizeof(char)*7 );
  strcpy( &element1[0][0], "length" );
  strcpy( &element1[1][0], "length" );

will work in the desired way.

Hopefully this helps,

  Paul

--

Technische Universitaet Wien
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Dipl.-Ing. Paul FULMEK
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