Can I allocate a char pointer with own memory?

Question

I'm in the second semester of my college so I just started coding. Recently we learned something about binary trees. So I just wanted to code my own ones. I decided to code a binary tree contact book.

First I'm saving my struct into a .txt file. (I want it in a txt file, not a binary, because then i can read it after the program.) After that I try to load it again in a node to rebuild my binary tree.

Here we go with a shortened version. I commented the important parts.

#define CLEN 100

struct binarytree {
    struct binarytree *left;
    struct binarytree *right;
    char *firstname;
    char *lastname;
    char *city;
    char *street;
    char *addr;
    char *tel;
} typedef btree;

//-----------------------------------------

btree *creatnullnode(void);
btree *loadtree(char *filename);

//-----------------------------------------

btree *creatnullnode(void) {
    btree *node = malloc(sizeof(btree));
    node->left  = NULL;
    node->right = NULL;

    //TODO: the memmory is not right allocated..
    node->firstname = (char*)malloc(CLEN * sizeof(char));
    node->lastname  = (char*)malloc(CLEN * sizeof(char));
    node->city      = (char*)malloc(CLEN * sizeof(char)); 
    node->street    = (char*)malloc(CLEN * sizeof(char));
    node->addr      = (char*)malloc(CLEN * sizeof(char));
    node->tel       = (char*)malloc(CLEN * sizeof(char));
    return node;
}

btree *loadtree(char *filename) {
    FILE *fp;
    btree *tree = NULL;
    btree *node = creatnullnode();
    char ch = "";
    int lines = 0;

    fp = fopen(filename,"r");
    if (!fp) {
        printf("Error. no file\n");
        return NULL;
    } else {
        while (!feof(fp)) {
            ch = fgetc(fp);
            if (ch == '\n')
                lines++;
        }
        fseek(fp, 0,(int)lines % 2);

        //TODO: right here the memory of every char can't be read anymore
        fscanf(fp, "%s\t\t%s\t\t\t%s\t%s\t\t%s\t\t%s\n",
               &node->firstname, &node->lastname, &node->addr, &node->city, 
               &node->street, &node->tel);

        tree = insertnode(tree, node);

        fseek(fp, 0, 0);
        //rekursiveload(&tree, fp);      //TODO: - ausprogrammieren -
    }

    fclose(fp);
    return tree;
}

While debugging I saw that the memory did not get correctly allocated. But i don't know how to fix it.

after allocationg the char[] is set to: node->firstname = 0x007db250 "ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍýýýýM¸Þµ¦æ" Debugger says: <Error reading the characters of the string.> after fscanf

Answer 1

There are several issues in your code:

• while (!feof(fp)) is always wrong for testing end of file: Why is “while (!feof(file))” always wrong? You should instead write this:

      while ((ch = fgetc(fp)) != EOF) {
          ...
  

• you should create a new node for each line that you read from the file. Currently you reuse the same memory for each node and overwrite the fields with the new data. insertnode, which is missing from the code fragment, most probably creates a loop in the list, causing undefined behavior when you try and free it.

• char ch = ""; is incorrect: "" is a string, not a char, and ch must be defined as an int to read bytes with fgetc() and store EOF too.

• fseek(fp, 0,(int)lines % 2); is meaningless. What are you trying to achieve? you can try and rewind the stream with rewind(fp) or fseek(fp, 0L, SEEK_SET), but you will only be able to read a single line .

• fscanf(fp, "%s\t\t%s\t\t\t%s\t%s\t\t%s\t\t%s\n", &node->firstname, ... has multiple issues: you cannot prevent incorrect input from causing too many characters to be stored into the destination arrays, and you should pass pointers to the destination arrays, not addresses of pointers. In other words, the code should be:

      char eol;
      if (fscanf(fp, "%99s%99s%99s%99s%99s%99s%c",
          node->firstname, node->lastname, node->addr, 
          node->city, node->street, node->tel, &eol) != 7 || eol != '\n') {
          /* invalid input... */
      }
  

  A much safer approach to read this input is to read a single line into a larger array of char and use sscanf() to parse this line into the node fields... but looking at your format string, it seems you are dealing with TAB separated values and

• Neither fscanf(), nor sscanf() nor even strtok() can properly parse TAB separated values from a text file. You need to write you own code for this. I suggest you use strcspn() to compute the field lengths and strndup() to allocate strings from a range in a char array.