Thread: Array overflow

  1. #1
    Registered User
    Join Date
    Sep 2005

    Array overflow

    Hi peoples...

    I was wondering about an occurence i had regarding some code i was writing.

    I am writing a basic library catalogue system for a uni assignment and had the following question (see code below):

    after i create a array of structures, i read them in from a file. then when i add a new structure to the array from user input, it seems to tack it onto the end of the array fine, even though i havent created the necessary memory for it... i was wondering why this is happening?

    #define MAXLENGTH 30
    // data structure:
    struct book
       char title[MAXLENGTH];
       char authFirstName[MAXLENGTH];
       char authSurname[MAXLENGTH];
       int pubYear;
       float replacementCost;
    // open the file.
    FILE *openFile(char *fileName, char mode)
       FILE *openedFile;
       openedFile = fopen(fileName, &mode);
       if(openedFile == NULL)
          printf("File '%s' not opened correctly.\n", fileName);
          printf("File '%s' opened correctly.\n", fileName);
       return openedFile;
    // read the contents of the file
    void readFile(FILE *openedFile, struct book *bookDB, int iNumOfBooks)
       int i;
       for(i = 0; i < iNumOfBooks; i++)
          fgetc(openedFile); // <-- picks up the last \n char to start the fgets
          fgets(bookDB[i].title, MAXLENGTH, openedFile);
          fscanf(openedFile, "%s", bookDB[i].authFirstName);
          fscanf(openedFile, "%s", bookDB[i].authSurname);
          fscanf(openedFile, "%d", &(bookDB[i].pubYear));
          fscanf(openedFile, "%f", &(bookDB[i].replacementCost));
    // add a new book:
    int newBook(struct book *bookDB, int iNumberOfBooks)
       printf("Adding a book to the collection...\n");
       printf("Title:: ");
       fgets(bookDB[iNumberOfBooks].title, MAXLENGTH, stdin);
       printf("Auther firstname:: ");
       scanf("%s", bookDB[iNumberOfBooks].authFirstName);
       strcat(bookDB[iNumberOfBooks].authFirstName, ",");
       printf("Auther Surname:: ");
       scanf("%s", bookDB[iNumberOfBooks].authSurname);
       printf("Year of Publication:: ");
       scanf("%d", &(bookDB[iNumberOfBooks].pubYear) );
       printf("Replacement Cost:: ");
       scanf("%f", &(bookDB[iNumberOfBooks].replacementCost) );
       return iNumberOfBooks;
    // call to open the file, read the file and add a new book:
    printf("Please enter a filename to open:: ");
    scanf("%s", fileName);
    openedFile = openFile(fileName, 'r'); // opens a file
    fscanf(openedFile, "%d", &iNumberOfBooks); // gets the number of books from file.
    bookDB = (struct book*) malloc(sizeof(struct book)*iNumberOfBooks);
    readFile(openedFile, bookDB, iNumberOfBooks);
    iNumberOfBooks = newBook(bookDB, iNumberOfBooks);

    Sorry about the long windedness of the code, but i wasnt sure what you would/wouldnt be needed.

    any comments would be very welcome.

    Joel aka SoulRedemption.

  2. #2
    ATH0 quzah's Avatar
    Join Date
    Oct 2001
    If your program doesn't crash when you stick something in some space you haven't allocated, it's just luck on your part. Don't rely on it, just fix it.

    Hope is the first step on the road to disappointment.

  3. #3
    Registered User
    Join Date
    Sep 2005
    Heh, thanks. Thats just what i thought. Now, about fixing it, im assuming that would be to re allocate enough memory using malloc? or using a linked list or something?

    Im not sure - i dont have that much exp with this...


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