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Date: Mon, 21 Aug 2006 14:04:27 -0400 (EDT)
From: Igor Peshansky <pechtcha AT cs DOT nyu DOT edu>
Reply-To: cygwin AT cygwin DOT com
To: cygwin AT cygwin DOT com
cc: Omololu <prayerz DOT Omo AT gmail DOT com>
Subject: Re: problem with malloc/realloc. Pls help.
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On Mon, 21 Aug 2006, Igor Peshansky wrote:

> On Mon, 21 Aug 2006, Omololu wrote:
>
> > Hi,
> >  i have the following code. it compiles with gcc in Cygwin but the
> > contents of the array ifitQ that I get after the call to the subroutine
> > readCharges2 is gibberish.
>
> This is expected behavior.  Read on.
>
> > The code compiles with gcc under Linux and it runs correctly. it also
> > compiles and runs correctly with windows visual studio. Pls help. The
> > resuls i get with Cygwin is:
> >
> > isumNatms = 5
> > rrr 13
> > rrr 14
> > rrr 15
> > rrr 16
> > *** 1628693268
> > *** 1628693268
> > *** 16
> > *** 1034
> >
> > instead of:
> > isumNatms = 5
> > rrr 13
> > rrr 14
> > rrr 15
> > rrr 16
> > *** 13
> > *** 14
> > *** 15
> > *** 16
> >
> > the code is:
>
> Ouch.  Indentation issues aside, you have a bug in this program.
>
> > #include <stdio.h>
> > #include <stdlib.h>
> >
> >         void readCharges2(int *, int *, int *);
> > int main()
> > {
> >         static int *ifitQ;
> >         int *ipUniqAtms, *ipindexToFit;
> >                 int j;
> > int x,y;
> > ipUniqAtms =&x;
> > ipindexToFit=&y;
> >                ifitQ = (int *) malloc(sizeof(int));
> >        if(ifitQ==NULL){printf("Unable to allocate matrix ifitQ\n");
> >                exit(EXIT_FAILURE);}
> >                 readCharges2(ifitQ,ipUniqAtms,ipindexToFit);
>
> Note that ifitQ is passed *by value*, and any changes you make to it
> inside readCharges2() will be lost when you come back to main.  So, you
> realloc it inside readCharges2(), and store values in the new array, but
> in main(), you print the values in the *old* array (which are, as you
> said, gibberish, as the memory has been released to the system).
>
> >        for(j=0; j< *ipUniqAtms ; j++)
> >        {
> >               printf("*** %d\n",ifitQ[j]);
> >        }
> >
> >         return 0;
> > }
> >
> > void readCharges2(int *ifitQ, int * ipUniqAtms, int * ipindexToFit)
> > {
> >        int  j, isumNatms=0;
> >        isumNatms=5;
> >        printf("isumNatms = %d \n",isumNatms);
> >        ifitQ = (int *) realloc(ifitQ,isumNatms*sizeof(int));
> >        if(ifitQ==NULL){printf("Unable to allocate matrix ifitQ\n");
> >                exit(EXIT_FAILURE);}
> >
> >        ifitQ[0]=13;
> >        ifitQ[1]=14;
> >        ifitQ[2]=15;
> >        ifitQ[3]=16;
> > *ipUniqAtms =  4;
> > *ipindexToFit =  3;
> >        for(j=0; j< *ipUniqAtms ; j++)
> >        {
> >               printf("rrr %d\n",ifitQ[j]);
> >        }
> > }
>
> The reason it probably worked on Linux is because in Linux, realloc will
> try hard to keep the array at the same address if it can simply bump the
> size of the pointer.  Cygwin's realloc doesn't.

s/pointer/memory block/.  Sorry for the typo.
	Igor
-- 
				http://cs.nyu.edu/~pechtcha/
      |\      _,,,---,,_	    pechtcha AT cs DOT nyu DOT edu | igor AT watson DOT ibm DOT com
ZZZzz /,`.-'`'    -.  ;-;;,_		Igor Peshansky, Ph.D. (name changed!)
     |,4-  ) )-,_. ,\ (  `'-'		old name: Igor Pechtchanski
    '---''(_/--'  `-'\_) fL	a.k.a JaguaR-R-R-r-r-r-.-.-.  Meow!

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