How do I use free() properly to free memory when using malloc?

Question

I have been using pointers and malloc and I do not know how to use free() properly. I have a program that allows the user to add a record of data when they select a specific option. My program allows this function to happen only once and then I get a segfault. I do not think I am using free() properly and was hoping someone could point out my problem. I have the global variables:

int records = 5;
int access = 0;                  //initialize access counter to 0
int count = 0;

struct childrensBook *first;    //initialize first pointer
struct childrensBook *last;     //initialize last pointer
struct childrensBook *temp;     //initialize temp pointer

I then have the start of my main function where I include predefined records:

int main(void)
{
    last = (struct childrensBook *) malloc(records * sizeof(struct childrensBook));     //allocate memory to "last" pointer
    first = last;       

    memcpy(last->title, "We're Going on a Bear Hunt", 27);                              //Beginning of all predefined Records
    memcpy(last->author, "Michael Rosen", 14);
    last->price = 7.99;
    last++;                                                                             //incrememnts pointer
    count = count + 1;                                                                  //begins counting of records

    memcpy(last->title, "Goodnight Moon", 15);
    memcpy(last->author, "Margaret Wise Brown", 20);
    last->price = 8.99;
    last++;                                                                             //incrememnts pointer
    count = count + 1;                                                                  //adds 1 to record count

    memcpy(last->title, "One Fish\n Two Fish\nRed Fish\n Blue Fish", 38);
    memcpy(last->author, "Dr. Seuss", 10);
    last->price = 8.99;
    last++;                                                                             //incrememnts pointer
    count = count + 1;                                                                  //adds 1 to record count

    memcpy(last->title, "Snowmen At Night", 17);
    memcpy(last->author, "Caralyn Buehner", 16);
    last->price = 16.99;
    last++;                                                                             //incrememnts pointer
    count = count + 1;                                                                  //adds 1 to record count

    memcpy(last->title, "Harold and the Purple Crayon", 29);
    memcpy(last->author, "Crockett Johnson", 17);
    last->price = 6.99

Below this is just my menu loop which I have not included. My problem lies in my add function where I am using free() which is here:

   addRecord()                                                                          //Add Function
{                                                       
    last=first;                                                                         //get pointer in correct position
    memcpy(&temp, &last, records *sizeof(struct childrensBook));                        //use memcpy to copy info from last to temp

    fprintf(stderr, "\nYou have added the record:\n==========================\nBook: %s\nWritten by: %s\nPrice: $%.2f\n", temp->title, temp->author, 
        temp->price);

    temp++; 
    free(last);       //problem?? I have tried using free(first), free(last) and free(temp) and none work....
    count = count + 1;

}//end addRecord

I also have done it this way and it still does not work:

            addRecord()                                                                                     //Add Function
    {                                                       
    last=first;                                                                         //get pointer in correct position
    temp = (struct childrensBook *) malloc(records * sizeof(struct childrensBook)); 
    memcpy(&temp, &last, records *sizeof(struct childrensBook));                        //use memcpy to copy info from last to temp

    fprintf(stderr, "\nYou have added the record:\n==========================\nBook: %s\nWritten by: %s\nPrice: $%.2f\n", temp->title, temp->author, 
        temp->price);

    temp++; 
    free(last);
    count = count + 1;

}//end addRecord

Please don't cast `malloc()`. It's not a good idea, for several reasons: http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc. — cyphar, Oct 03 '15 at 00:42
Seems like you are calling free several times (I think 2) and when you are trying to free freed object you have the crash, not to mention that you did not allocated the temp. — Coldsteel48, Oct 03 '15 at 00:45
`memcpy(&temp, &last, records *sizeof(struct childrensBook));` : `temp` isn't allocate. also `&` ?? — BLUEPIXY, Oct 03 '15 at 00:45

score 0 · Answer 1 · answered Oct 03 '15 at 01:39

last points to first which gets freed at the end of the first call to addRecord(). On the second call you will crash since last no longer points to allocated memory. But, more problematic is that you are not allocating any new memory for the new record. See realloc().

How do I use free() properly to free memory when using malloc?

1 Answers1