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From: duvalj@bionette.cgrb.orst.edu (Joe Duval)
Newsgroups: comp.os.msdos.programmer
Subject: malloc() and free() is order important
Message-ID: <20681@orstcs.CS.ORST.EDU>
Date: 1 Oct 90 21:26:25 GMT
Sender: usenet@orstcs.CS.ORST.EDU
Reply-To: duvalj@bionette.cgrb.orst.edu (Joe Duval)
Distribution: na
Organization: Oregon State University - CMBL
Lines: 75

Hi,
  I am using Turbo C on a 386-20 with 1 Meg of RAM.
  I have a program that needs to read in data from a couple of <30K files 
and display it on the screen in a scrollable window.  (I have the window
stuff down).  I am having a problem when I free up the space held by the
two data file and the windows.  And I have noticed that my program will
completely hang if I try to free up one of the variables before the others.
I have yet to get my program to work correctly.

My question is does the order of mallocing and freeing data matter?  Should
whatever I malloc first be free'd last or the opposite?  My program is showing
inconsistency that seems to relate to the malloc'ing and free'ing I am doing.

Here is how my program "works":
/* the input will be straight ASCII text produced by another Turbo C program.
   I want to be able to read in the data for the two files and display it on
   the screen.
*/

# include <appropriate files>

#define LINEDELTA 100
#define STRLEN     80

typedef char element[STRLEN] /* will be a line of a data file, lines can be up
								to 80 characters long */

int fillarray (char *filename, element *array)
{
	/* fill the variable array with text from the file in filename
	   realloc memory using LINEDELTA to increase the size of the array
	   if the size of the array is not big enough
    */
}
  
main ()
{	element	*data1, *data2;
  
	data1 = (element *) malloc (LINEDELTA*sizeof(element *));
	data2 = (element *) malloc (LINEDELTA*sizeof(element *));

  /* One of the files is usually about 20 times the size of the other.  If I
	 read in the smaller one first, the data in the smaller one gets trashed
	 when I read in the data to the second one.  What can I do to avoid that.
	 things are fine if I read in the data for the larger one first (I may 
	 not know which of the two is the larger at run time)
   */
	lines1 = fillarray ("test1", data1);
	lines2 = fillarray ("test2", data2);

   /* set up two windows.  Uses a structure to keep track of lots of things
	  about a window  ala Al Stevens "Memory Resident Utilities, Screen I/O
	  and Programming Techniques"
	  Windows structures are kept in a linked list

	  Display the data 
   */


/* program gets through to here with no gliches usually :-) */
/* BUT it only will go so far through these lines,  inconsistently crashing
   on one of these lines */

   free (data1);
   free (data2);
   delete_window (window1); /* free's up the space occupied by this window */
   delete_window (window2); /* and deletes it from the linked list */
}

Thanks for reading and I'll be waiting for responses

--
Joe Duval		duvalj@bionette.cgrb.orst.edu
Losing your temper generally represents the incipient stage of rectal-
cranial inversion.