Thread: C - freeing memory to avoid leak

  1. #1
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    C - freeing memory to avoid leak

    I have a program below that manages a library full of Books and when I test for memory leaks using valgrind and then enter 'CTRL+Z' to quit, it says there are hundreds of bytes lost. When the program exits naturally as its supposed to rather than me forcing an exit however, there are 0 bytes lost.

    I deallocated my code using 'free' at the end of main.c and I was wondering if there's something wrong with the way I'm doing it? I was told every time I use 'malloc' I also have to use the 'free keyword' and aside from the 2 times I use malloc in main.c, I also use malloc in addBook() in library.c when initializing the Book object. I'm not sure if this is where the memory leak is occurring because I didn't use free in this function?

    I included my header file as well so you can see the kind of data types/arrays I'm creating.

    main.c

    Code:
        int main(){
    
    
            LibraryType* library  = malloc(sizeof(LibraryType)); 
            BookCollectionType* book = malloc(sizeof(BookCollectionType));
            
            addBook(book, "The Hunger Games", 2012, 405.5); 
            addBook(book, "Twilight", 2007, 302); 
            addBook(book, "The Martian", 2015, 203.2); 
    
    
            }
    
    
        //.....end of program.....//
    
    
            for(int i = 0; i < MAX_BOOKS; ++i){
                free(p->books[i]);
                free(library->bookCollection[i]);
            }
            free(book);
            free(library);
    
    
        }


    library.c
    Code:
        int addBook(BookCollectionType* bookCollection, char* t, int c, float p){
    
    
            bookCollection->books[bookCollection->numBooks] = malloc(sizeof(BookType));
            bookCollection->books[bookCollection->numBooks]->title = t;
            bookCollection->books[bookCollection->numBooks]->numCopies = c;
            bookCollection->books[bookCollection->numBooks]->pages = pages;
            bookCollection->numBooks++;
            return 1;
    
    
        }
    defs.h
    Code:
        typedef struct{
            char* title;
            int year;
            int numCopies;
            float pages;
        }BookType;
    
    
    
    
        typedef struct {
            BookType* books[MAX_PAGES]; 
            int numBooks;
        }BookCollectionType;
    
    
    
    
        typedef struct{
            char* libraryName;
            BookCollectionType* bookCollection[MAX_BOOKS];
        }LibraryType;
    Last edited by eagerissac; 10-21-2020 at 05:37 AM.

  2. #2
    C++ Witch laserlight's Avatar
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    I would focus on these lines first:
    Code:
    LibraryType* library = malloc(sizeof(LibraryType));
    BookCollectionType* book = malloc(sizeof(BookCollectionType));
    
    addBook(book, "The Hunger Games", 2012, 405.5);
    addBook(book, "Twilight", 2007, 302);
    addBook(book, "The Martian", 2015, 203.2);
    Basically, why do you have a library variable, and then also have a book variable?

    I would expect something like this instead:
    Code:
    LibraryType* library = malloc(sizeof(LibraryType));
    library->bookCollection[0] = malloc(sizeof(BookCollectionType));
    library->numCollections = 1;
    addBook(library->bookCollection[0], "The Hunger Games", 2012, 405.5);
    addBook(library->bookCollection[0], "Twilight", 2007, 302);
    addBook(library->bookCollection[0], "The Martian", 2015, 203.2);
    So the idea is that you have a library that consists of one or more book collections. You create a book collection and add three books to that collection. Notice that I added a member named numCollections to LibraryType, so it should look like this:
    Code:
    typedef struct{
        char* libraryName;
        BookCollectionType* bookCollection[MAX_BOOKS];
        int numCollections;
    }LibraryType;
    This is akin to what you did with BookCollectionType.

    Having done this, you do something with the library, e.g., print the books, and then when done, you clean up:
    Code:
    for (int i = 0; i < library->numCollections; i++)
    {
        for (int j = 0; j < library->bookCollection[i]->numBooks; j++)
        {
            free(library->bookCollection[i]->books[j]);
        }
        free(library->bookCollection[i]);
    }
    free(library);
    Note that your named constants are poorly named:
    • MAX_BOOKS sounds like the maximum number of books, but you're using it to mean the maximum number of book collections, so it would be better named MAX_BOOK_COLLECTIONS
    • MAX_PAGES sounds like the maximum number of pages in a book, but you're using it to mean the maximum number of books in a collection, so it would be better named MAX_BOOKS
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  3. #3
    and the hat of int overfl Salem's Avatar
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    If it frees all the memory you allocated during a normal exit, that should be fine.

    Freeing all the memory in various exception cases is rather more involved.
    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.

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