Path: utzoo!attcan!uunet!cs.utexas.edu!rutgers!cunixf.cc.columbia.edu!cubmol!ping
From: ping@cubmol.bio.columbia.edu (Shiping Zhang)
Newsgroups: comp.lang.c
Subject: Re: SIMPLE malloc & pointer question
Message-ID: <1990Aug7.142453.6895@cubmol.bio.columbia.edu>
Date: 7 Aug 90 14:24:53 GMT
References: <7206@helios.TAMU.EDU>
Reply-To: ping@cubmol.bio.columbia.edu (Shiping Zhang)
Organization: Dept. of Biology, Columbia Univ., New York, NY
Lines: 44

In article <7206@helios.TAMU.EDU> jdm5548@diamond.tamu.edu (James Darrell McCauley) writes:
>I have the following code:
>---cut here
>#include<stdio.h>
>#include<malloc.h>
>main()
>{
>  int *a,*b;
>
>  b=(int *)malloc( (unsigned) 4*sizeof(int));
>  b[2]=5;
>  printf("main(): b[2]=%d\n",b[2]);
>  inita(a,b);
>  printf("main(): a[2]=%d\n",a[2]);
>}
>
>inita (a,b)
>int a[],b[];
>{
>  a=(int *)malloc( (unsigned) 4*sizeof(int));
>  a[2]=3;
>  printf("inita(): a[2]=%d\n",a[2]);
>  printf("inita(): b[2]=%d\n",b[2]);
>}
>---cut here
>which produces the following output:
>
>main(): b[2]=5
>inita(): a[2]=3
>inita(): b[2]=5
>Segmentation fault (core dumped)
>

Remember that in C, function arguments are passed by value and
their original values are not changed no matter what operation
has been applied to them in the called functions. So in inita(),
a is assigned a value (the address of some memery location), but
the value is not saved when return from that function and a points
to nowhere or anywhere.
The simplest way to solve this problem in this case is just to move
the first statement of inita() to somewhere in the main function
before inita is called.

-ping