Issues with free and dynamic memory

This is my first time working with C (worked with Fortran and VB for years), and I have run into an error that I can’t figure out. I’m working on a program for my ti-89 calculator that will read values from a file and then display. The code below is a simpler program that I wrote to help with debugging.

The problem:
Memory is allocated, and then freed, but when I try to re-allocate memory I get a fatal “address error”, and I have to completely clear the calculators memory.

I have been working on this for 4 days and I’m stumped; does anyone see an issue with the following code.

Code:

---

/*############################################################################*/
#include <stdlib.h>
#include <graph.h>
#include <kbd.h>
#include <compat.h>
#include <alloc.h>
#include <string.h>
/*############################################################################*/
unsigned short int key;
unsigned short int max=0;
unsigned short int i;
char **list = NULL;
/*############################################################################*/
void load_string(char *s);
void free_up();
/*############################################################################*/
void _main(void) {
 while (key != 264){
    load_string((char *)"dummy string");
    free_up();
    key = ngetchx();
 }
 free_up();
}
/*############################################################################*/
void load_string(char *st){
 char **tmp;
 char *tmp_st;
 if(st != NULL){
    max++;
    if (( tmp=(char **)realloc(list,max*sizeof(char *)) )==NULL){
      // realloc failed, end program
      max--;
      free_up();
      ClrScr();
      DrawStr(0, 0,"System out of memory(1),", A_NORMAL);
      DrawStr(0, 10,"press any key to exit!", A_NORMAL);
      ngetchx();
      exit(0);
    }else{
      // try to allocate space for st
      list=tmp;
      if ((tmp_st=malloc(strlen(st) + 1))==NULL){
          //no space to allocate
          list[max-1]=NULL;
          free_up();
          ClrScr();
          DrawStr(0, 0,"System out of memory(2),", A_NORMAL);
          DrawStr(0, 10,"press any key to exit!", A_NORMAL);
          ngetchx();
          exit(0);
      }else{
                list[max-1]=tmp_st;
                if (strcpy(tmp_st,st)==NULL){
                    //could not copy data, end program
                    free_up();
                    ClrScr();
            DrawStr(0, 0,"couldn't copy to memory,", A_NORMAL);
            DrawStr(0, 10,"press any key to exit!", A_NORMAL);
            ngetchx();
            exit(0);
                }
      }
    }
 }else{
    // st was null, end program
    free_up();
    ClrScr();
    DrawStr(0, 0,"Input was Null, press any", A_NORMAL);
    DrawStr(0, 10,"key to exit!", A_NORMAL);
    ngetchx();
    exit(0);
 }
}
/*############################################################################*/
void free_up(void){
 if (max > 0){
    for (i=0;i<max;i++){
      if (list[i] != NULL){
          free((void *)list[i]);
      }
    }
    free ((void *)list);
  }
  max=0;
}

---

Dan S.

Set list to be NULL at the end of the free_up() function. The reason for your problem is that free_up() frees up the memory, so list is a dangling reference. One of the first things that load_string() does to list is pass it to realloc().

> I get a fatal “address error”, and I have to completely clear the calculators memory.
You need to do
list = NULL;
at the end of your free_up() function.

Other problems
> #include <alloc.h>
malloc / realloc / free etc are declared in stdlib.h which you also include.
alloc.h is an obsolete and non-portable header.

There is no need to cast any of the results of malloc/calloc/free - see the FAQ

I also can't see why you would need this cast either.
load_string((char *)"dummy string");

> if (strcpy(tmp_st,st)==NULL)
strcpy never fails like this - it always returns the first parameter.

Thank you gentlemen, It works now.