Issues with free and dynamic memory

This is a discussion on Issues with free and dynamic memory within the C Programming forums, part of the General Programming Boards category; This is my first time working with C (worked with Fortran and VB for years), and I have run into ...

  1. #1
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    Question 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 canít figure out. Iím 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 ďaddress errorĒ, and I have to completely clear the calculators memory.

    I have been working on this for 4 days and Iím 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.

  2. #2
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    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().

  3. #3
    and the hat of wrongness Salem's Avatar
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    > I get a fatal ďaddress errorĒ, 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.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  4. #4
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    Thank you gentlemen, It works now.

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