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From: morrison@uhhacb.tmc.edu (Jay Morrison)
Newsgroups: comp.lang.c
Subject: Re: Simple ptr passing question
Message-ID: <13012@uhccux.uhcc.Hawaii.Edu>
Date: 14 May 91 17:31:46 GMT
Article-I.D.: uhccux.13012
References: <24268@unix.SRI.COM>
Sender: news@uhccux.uhcc.Hawaii.Edu
Organization: UHH Computer Science Department
Lines: 59

In article <24268@unix.SRI.COM> ric@ace.sri.com (Richard Steinberger) writes:
>
>	I am a bit rusty with C.  Could someone help me with this simple
>pointer passing problem.  I wanted to pass a ptr to a function and
>have the function allocate some space and pass back the address in the ptr.
>Here's what I tried:
>  [etc...]
>


You have to pass a pointer to a pointer in this situation.  I had the
same problem in a lab for an operating systems class.  Programming queue
operations in C you need pointers to pointers also.

Try this:


#include <stdio.h>
main()
{
  extern void zip();
  int *ip;

  zip(&ip);

  printf("**ip is %d\n",*ip);
}

void zip (iptr)
int **iptr;
{
  int * jptr;
  jptr = (int *) malloc(sizeof (int) );
  *jptr = 12;
  *iptr = jptr;
}


The problem is that in C you always pass by reference, even when you
pass a pointer.  In other words, it is going pass a copy of the pointer,
and you are changing that copy.  Upon return to your main function, the
value it had there is restored.  So you need to pass a pointer to the
pointer, so that when you change the pointer (via *iptr = jptr), the
value will be there upon return to main.  Totally confused?  Think
about it for a while, its totally logical actually.

Another common problem you may encounter is a returning a FILE*
structure from a routine which opens a file.  In this situation you must
also use pointers to pointers.  This even had my professor confused as
to what was happening!! (temporarily).



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/   Jay Morrison                    \ "C programming:  all the power 
\   morrison@uhhacb.uhh.hawaii.edu  /   of assembly language with the 
/===================================    ease of assembly language..." 
\  *********** and God said:  "Let there be SURF!!" ***************          
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