Re: problem with malloc/realloc. Pls help.

[Omololu <prayerz at yahoo dot com>]

Thanks a lot, Igor & Mark. i was able to rectify the
code to fix the problem. Now it works. (i had to use a
pointer to a pointer to int, rather than just a
pointer to int in the subroutine)

thanks again. 

o.

--- Igor Peshansky <pechtcha@cs.nyu.edu> 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.
> HTH,
> 	Igor
> -- 
> 				http://cs.nyu.edu/~pechtcha/
>       |\      _,,,---,,_	    pechtcha@cs.nyu.edu |
> igor@watson.ibm.com
> ZZZzz /,`.-'`'    -.  ;-;;,_		Igor Peshansky, Ph.D.
> (name changed!)
>      |,4-  ) )-,_. ,\ (  `'-'		old name: Igor
> Pechtchanski
>     '---''(_/--'  `-'\_) fL	a.k.a
> JaguaR-R-R-r-r-r-.-.-.  Meow!
> 
> "Las! je suis sot... -Mais non, tu ne l'es pas,
> puisque tu t'en rends compte."
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> fool, there's proof enough in
> that!" -- Rostand, "Cyrano de Bergerac"
> 


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