Function Free() does not delete the linked list?

This is a discussion on Function Free() does not delete the linked list? within the C Programming forums, part of the General Programming Boards category; I created a program which was a link list in the only one function. Then I tried to delete the ...

  1. #1
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    Question Function Free() does not delete the linked list?

    I created a program which was a link list in the only one function. Then I tried to delete the linked list with function free(), but the function couldn't do it, or I don't know how function free runs exactly.

    *First of all, I created an linked list, then print them.
    *Secondly I used to function free for deleting all of things.
    *Then, I encountered that the list was not removed exactly.Somethings were still there.

    If I deactive "startPtr=NULL;" with " // ", I can see that actually the linked list haven't been deleted from the memory.
    Note: The code of "startPtr=NULL;" whom I spoke is between "//Deleting Nodes" and "//Check out the list if deleted or not."

    So, Does function free perform the deletion? If you say yes, what is the situation the following?

    1.When active "startPtr=NULL;", the "//Check out the list if deleted or not." codes are deactive:
    FIGURE(1)
    Name:  active.png
Views: 257
Size:  6.7 KB



    2.But we see that when deactive "startPtr=NULL;", the "//Check out the list if deleted or not" codes aren't deactive. They runs, and they can show some items - such as grades,last names - from pseudo deleted list:
    FIGURE(2)
    Name:  deactive.png
Views: 336
Size:  10.2 KB

    To summurize my questions;
    a. Does function free() delete its argument from the memory?
    *If you say yes, why didn't free() delete some items as you can see Figure(2)?
    *If you say no, what do function free() do?

    b. Do startPtr=NULL just hide items apperantly?

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    
    
    int main()
    {
        struct gradeNode{
            char lastName[20];
            double grade;
            struct gradeNode *nextPtr;
        };
    
    
        typedef struct gradeNode GradeNode;
        typedef GradeNode *GradeNodePtr;
    
    
        GradeNodePtr startPtr=NULL;
        GradeNodePtr newPtr=NULL;
        GradeNodePtr previousPtr=NULL;
        GradeNodePtr currentPtr=NULL;
        GradeNodePtr tempPtr=NULL;
    
    
        //Node 1
        newPtr = malloc(sizeof(GradeNode));
        strcpy(newPtr->lastName,"Jones");
        newPtr->grade = 91.5;
        newPtr->nextPtr = NULL;
    
    
        startPtr = newPtr;
    
    
    
        //Node 2
        newPtr = malloc(sizeof(GradeNode));
        strcpy(newPtr->lastName,"Smith");
        newPtr->grade = 0.00;
        startPtr->nextPtr = newPtr;
    
    
    
    
        //Node 3
        newPtr = malloc(sizeof(GradeNode));
        strcpy(newPtr->lastName,"Adams");
        newPtr->grade = 85.0;
    
    
        previousPtr=NULL;
        currentPtr=startPtr;
    
    
        newPtr->nextPtr = currentPtr;
        startPtr = newPtr;
    
    
    
    
        //Node 4
        newPtr = malloc(sizeof(GradeNode));
        strcpy(newPtr->lastName,"Thompson");
        newPtr->grade = 73.5;
    
    
        previousPtr = (startPtr->nextPtr)->nextPtr;
        currentPtr=NULL;
    
    
        previousPtr->nextPtr = newPtr;
        newPtr->nextPtr = currentPtr;
    
    
    
    
    
    
        //Node 5
        newPtr = malloc(sizeof(GradeNode));
        strcpy(newPtr->lastName,"Pritchard");
        newPtr->grade = 66.5;
    
    
        previousPtr = startPtr->nextPtr;
        currentPtr = (startPtr->nextPtr)->nextPtr;
    
    
        previousPtr->nextPtr = newPtr;
        newPtr->nextPtr = currentPtr;
    
    
    
    
        //Printing the nodes.
        currentPtr = startPtr;
        while(currentPtr!=NULL){
            printf("Lastname = %s\nGrade = %6.2f\n\n",currentPtr->lastName,currentPtr->grade);
            currentPtr = currentPtr->nextPtr;
        }
    
    
        //Deleting the nodes.
        currentPtr = startPtr;
        while(currentPtr != NULL){
            tempPtr=currentPtr;
            currentPtr=currentPtr->nextPtr;
            free(tempPtr);
        }
        
    
        //startPtr=NULL;  //   LOOK AT HERE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!LOOK AT HERE?????????????????????????????????????LOOK AT HERE!!!!!!!!!!
    
    
    
        //Check out the list if deleted or not.
        printf("\nThe list was deleted?\n");
        currentPtr = startPtr;
        while(currentPtr!=NULL){
            printf("Lastname = %s\nGrade = %6.2f\n\n",currentPtr->lastName,currentPtr->grade);
            currentPtr = currentPtr->nextPtr;
        }
        return 0;
    }

  2. #2
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by hefese
    a. Does function free() delete its argument from the memory?
    *If you say yes, why didn't free() delete some items as you can see Figure(2)?
    *If you say no, what do function free() do?
    Conceptually, free() deallocates the memory associated with its pointer argument, thus the memory is available to be allocated by say, a subsequent call of malloc(). However, what exactly happens is implementation defined, e.g., you may still be able to access that memory after the use of free() before calling malloc(), and it may well contain the exact same contents, or it could be zeroed. Or, maybe you will not be able to access the memory at all.

    Consequently, trying to determine if free() has "deleted" those items by examining the memory after calling free() tells you little, and in fact results in undefined behaviour.
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    SAMARAS std10093's Avatar
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    I checked to see if the list is linked properly and found that it is.I also checked that the deletion of the nodes is as should be too!But then with the startPtr=NULL to comments the printing of the list behaves as the nodes were not free'ed!The first three nodes have garbage in the field name.

    now reading laserlights post i see that she was expecting so..

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    Quote Originally Posted by laserlight View Post
    Conceptually, free() deallocates the memory associated with its pointer argument, thus the memory is available to be allocated by say, a subsequent call of malloc(). However, what exactly happens is implementation defined, e.g., you may still be able to access that memory after the use of free() before calling malloc(), and it may well contain the exact same contents, or it could be zeroed. Or, maybe you will not be able to access the memory at all.

    Consequently, trying to determine if free() has "deleted" those items by examining the memory after calling free() tells you little, and in fact results in undefined behaviour.
    I thought that function free() deletes structure's members and their values what 'argument in function free' points to completely, until I read you say. I understand that this was not like that. Thank you for these information.



    Quote Originally Posted by std10093 View Post
    I checked to see if the list is linked properly and found that it is.I also checked that the deletion of the nodes is as should be too!But then with the startPtr=NULL to comments the printing of the list behaves as the nodes were not free'ed!The first three nodes have garbage in the field name.

    now reading laserlights post i see that she was expecting so..
    Thank you for your interest.
    Last edited by hefese; 08-11-2012 at 08:29 AM.

  5. #5
    SAMARAS std10093's Avatar
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    Quote Originally Posted by hefese View Post
    I thought that function free() deletes structure's members what its argument points to completely, until I read you say. I understand that this was not like that. Thank you for these information.
    Thank you for your interest.
    Your welcome.However have in mind that if for example the field name was not declared statically but dynamically(the array had the length of the name that it would store),then first you should free the array name that was dynamically allocated(with malloc() perhaps) and then free the node.If you free the node you have no access to it's fields then.The fact that startPtr points at the start of your list here is not sure to happen everytime.In fact most times it points to garbage.So if you free your node,then you have no access to the array,and thus you can not deallocate it(with free).

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    Quote Originally Posted by std10093 View Post
    Your welcome.However have in mind that if for example the field name was not declared statically but dynamically(the array had the length of the name that it would store),then first you should free the array name that was dynamically allocated(with malloc() perhaps) and then free the node.If you free the node you have no access to it's fields then.The fact that startPtr points at the start of your list here is not sure to happen everytime.In fact most times it points to garbage.So if you free your node,then you have no access to the array,and thus you can not deallocate it(with free).
    OK. Thank you again.

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