Malloc Problem in C

Question

Solution: Allocated memory is freed up after a program exits. Have to read+write from disk back into a linked list, and then rewrite to update the database! Thank you everyone =)

Hello, I've basically been working on this database program for the past few nights but I just continuously reach dead ends. The assignment is due today so if you could help me out, it would be very much appreciated. =T

The database is implemented with a Linked List and consists of a few files: sdbm.c, sdbm.h, new.c, get.c, insert.c, put.c, and remove.c. sdbm.c holds the methods for the database based on the sdbm.h interface, and the other files contain main methods that use the methods from sdbm.

The first problem comes with the insert program, which seems to work fine when I try to add in a key and value pair ... that is, until we try to call the insert program again. The memory allocated seems to have disappeared! I've been researching, trying to figure out why even though I have malloced, why does it disappear after the insert program exits. Here is some code:

• The node structure + global variable:

struct dbase_Node {
  char *keyValue;
  char *element;
  struct dbase_Node *next;
};

typedef struct dbase_Node Node;

Node *head;

========

• The insert method

static bool sdbm_insert_back(Node **headRef, const char *key, const char *value)
{
  Node *new = (Node *)malloc(sizeof(Node));
  if (new == NULL)
    return false;
  else {
    new->keyValue = malloc(strlen(key));
    new->element = malloc(strlen(value));
    strcpy(new->keyValue, key);
    strcpy(new->element, value);

    new->next = *headRef;
    *headRef = new;
    return true;
  }
}

• The sync method

bool sdbm_sync()
{
  if (!isOpen()) { return false; }

  if (fopen(databaseName, "w" ) == NULL) {
    error = SDBM_FOPEN_FAILED;
    return false;
  }

  Node *current = head;

  while (current != NULL) {
    fprintf(database, "Key: %s\n", current->keyValue);
    fprintf(database, "Value: %s\n", current->element);
    current = current->next;
  }
  return true;
}

I run the following:

./new [database] <-- works fine ./insert [database] [key] [value] <--seems to work fine

And then after I try to insert more, the already added nodes have disappeared ...

Answer 1

new->keyValue = malloc(strlen(key));
new->element = malloc(strlen(value));
strcpy(new->keyValue, key);
strcpy(new->element, value);

This causes an off-by-one buffer overflow. You need to allocate space for the terminating \0, so use strlen(key) + 1 and strlen(value) + 1.

Or even better, use strdup():

new->keyValue = strdup(key);
new->element = strdup(value);

---

The problem of your data being lost after the program exists is very simple but not simple to fix: You are not storing it anywhere but in memory. And memory is automatically free'd when the program exits. So you'll have to store it e.g. in a file.

Answer 2

This line in your question:

./new [database] <-- works fine ./insert [database] [key] [value] <--seems to work fine

implies you are running the program once to create a database, then again to create the key/value pairs. Are you anywhere persisting your data to disk?

Answer 3

Do I read that right? For each of those operations you're starting a new process? If so there's your problem: Once a process terminates all the memory it allocated is freed.

What you need is persistent storage. In the usual operating systems process memory is not persistent (there are some OS in which allocated memory is persistent, but still bound to a certain process, so even in such a OS this kind of program won't work).

Answer 4

I feel the issue is in the else part, u need to check if it's the first node in the List if so allocate memory, store into head. Next time u call it if the list is not null then link head->next to ur newly created one.

Answer 5

Suggestion: in sdbm_sync close the file after writing the data.

Also in sdbm_sync where is database defined? Is the fopen line missing somthing? ???

bool sdbm_sync()
{
  if (!isOpen()) { return false; }
  /* ******** MISSING DATABASE ? ? ? ? ?
  FILE *database;
  if ((database = fopen(databaseName, "w" )) == NULL) {
  ******** */
  if (fopen(databaseName, "w" ) == NULL) {
    error = SDBM_FOPEN_FAILED;
    return false;
  }

  Node *current = head;
  while (current != NULL) {
    fprintf(database, "Key: %s\n", current->keyValue);
    fprintf(database, "Value: %s\n", current->element);
    current = current->next;
  }
  if (!fclose(database)) /* unexpected error closing database */;
  return true;
}