Struct and memory allocation with loop

Question

I have been trying to free() the memory at the end however my instructor stated that I have created 3 malloc'd pointers (with the current settings).

NOTE: I would like as detailed explanation possible regarding the malloc/what's actually going on in memory.

I would appreciate guidance on what I can do to make sure there is no memory leak.

I have written the following.

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>

    #define LUNCH_ITEMS 5
    #define REMAINING 2
    #define CHAR_SIZE 256

    int main(void)
    {
        struct Food
        {
            char *name; //name attribute of food
            int weight, calories;
        } lunch[LUNCH_ITEMS] = {{"apple", 4, 100}, {"salad", 2, 80},};
        int loopCount;
        //INPUT
        char *namePtr = NULL;
        for (loopCount = REMAINING; loopCount < LUNCH_ITEMS; ++loopCount)
        {
            char tempBuffer[CHAR_SIZE];
            printf("Enter name of item,the weight of item, and the calories in that item: \n");
            // store name string in a tempBuffer. weight and calories directly into lunch structure array address
            scanf("%255s %d %d", tempBuffer, &lunch[loopCount].weight, &lunch[loopCount].calories);
            // get the exact size of (memory of) the input string including add one for null char
            size_t exactMemory = strlen(tempBuffer) + 1;
            //dynamically allocate the exact amount of memory determined in the previous step
            namePtr = (char *)malloc(exactMemory * sizeof(char));
            // check if no memory is allocated for foodPtr
    if (namePtr == NULL)
    {
        fprintf(stderr, "Not enough memory available***\n Terminating Program");
        exit(1);
    }
    //store the pointer to it in the name member of the structure in
    //the current lunch array element.
    (lunch + loopCount)->name = namePtr;
    // Copy the food name (stored in tempbuffer) into the dynamically-allocated
    // memory using the memcpy function

            memcpy(namePtr, tempBuffer, exactMemory);
    //(lunch + loopCount)->name = namePtr;
        }
    //DISPLAY
        printf("Item                        Weight       Cals\n---------------------------------------------\n");
        for (loopCount = 0; loopCount < LUNCH_ITEMS; loopCount++)
        {
            int weight = lunch[loopCount].weight;
            int cals = lunch[loopCount].calories;
            printf("%-12.20s%22d%11d\n", (lunch + loopCount)->name, weight, cals);
            if (loopCount > REMAINING)
            {
                //(lunch+loopCount)->name = NULL;
                namePtr = NULL;
                free(namePtr);
                //free((lunch + loopCount)->name);
            }
        }
        //De-allocate all malloc'd memory
        return EXIT_SUCCESS;
    }

My output -

Item Weight Cals
-----------------
apple  4   100
salad  2    80
hello  22   33
maybe  44   45
right 100   200

Answer 1

I think your instructor's comment about 3 malloc'ed strings is a pretty strong clue. I notice you have an array of 5 items, with 2 items pre-populated. 5 - 2 is 3.

Also, please be aware that indexes in C start from 0. The 2 items you pre-initialize your array with will have index 0 and index 1. Index 2 will be the first entered data. Comparing that index using > 2 will actually cause you to skip over the index of the first piece of user supplied data.

Answer 2

Look at the initialization loop:

for (loopCount = REMAINING; loopCount < LUNCH_ITEMS; ++loopCount)
{
    // The code inside the loop will be executed for
    // loopCount being equal to 
    //     REMAINING
    //     REMAINING + 1
    //     ....
    //     LUNCH_ITEMS - 1
    //
    // So in your case, you execute this code for
    // loopCount equal to 2, 3 and 4
}

In other words, the code inside the loop is executed 3 times, i,e, you call malloc 3 times.

In the same way, look at the loop where you release the memory.

for (loopCount = 0; loopCount < LUNCH_ITEMS; loopCount++)
{
    // you execute this code for
    // loopCount equal to 0, 1, 2, 3 and 4

    // ....

    if (loopCount > REMAINING)
    {
        // Since REMAINING is 2, you execute this code for
        // loopCount equal 3 and 4
    }
}

So the code in the body of the if is only executed 2 times. You really want to do it 3 times (i.e. for loopCount equal 2, 3 and 4). So you need to change the code to:

    if (loopCount >= REMAINING)  // Notice the = sign
    {
        // Since REMAINING is 2, you execute this code for
        // loopCount equal 2, 3 and 4
    }

Now regarding malloc and free. When you release memory, i.e. using free, you must pass exactly the value returned to you by malloc

In the initialization you saved the pointer like this:

namePtr = (char *)malloc(exactMemory * sizeof(char));
// ...
(lunch + loopCount)->name = namePtr;  // Save pointer

So it is (lunch + loopCount)->name that shall be used for free. Like:

    if (loopCount >= REMAINING)  // Notice the = sign
    {
        free((lunch + loopCount)->name);

        // Optional part but always after calling free
        (lunch + loopCount)->name = NULL;
    }

BTW: Notice that

(lunch + loopCount)->name

is the same as

lunch[loopCount].name

which many consider easier to read.