CODEWITHSUNDEEP
clinked-listvalgrinddoubly-linked-list
JamesJohn990
JamesJohn990

Reputation: 11

where is error of "Invalid read of size 4" on the line below the definition of int level

  1. the definition of int level has Invalid read of size 4, and valgrind shows Memcheck, a memory error detector.
void sl_set(leapList *head, int *key, int *value) {
    leapList *prev[MAX_SKIPLIST_HEIGHT];
    leapList *curr = head;
    int level = head->height - 1;

    // Find the position where the key is expected
    while (curr != NULL && level >= 0) {
        prev[level] = curr;
        if (curr->next[level] == NULL) {
            -- level;
        } else {
            //int cmp = strcmp(curr->next[level]->key, key);
            //if (cmp == 0) {
            if ((curr->next[level]->key) == key) {
                // Found a match, replace the old value
                free(curr->next[level]->value);
                curr->next[level]->value = value;
                return;
            } else if ((curr->next[level]->key) > key) { // Drop down a level
                -- level;
            } else { // Keep going at this level
                curr = curr->next[level];
            }
        }
    }

    // Didn't find it, we need to insert a new entry
    leapList *new_entry = malloc(sizeof(leapList));
    new_entry->height = grand(head->height);
    new_entry->key = key;
    new_entry->value = value;
    int i;
    // Null out pointers above height
    for (i = MAX_SKIPLIST_HEIGHT - 1; i > new_entry->height; --i) {
        new_entry->next[i] = NULL;
    }
    // Tie in other pointers
    for (i = new_entry->height - 1; i >= 0; -- i) {
        new_entry->next[i] = prev[i]->next[i];
        sOw code as codeprev[i]->next[i] = new_entry;
    }
}

Upvotes: 0

Views: 450

Answers (1)

chqrlie
chqrlie

Reputation: 144959

For int level = head->height - 1; to produce an invalid read of size 4, there can be only one explanation: the pointer head is invalid and you have undefined behavior.

  • either head is a null pointer, but this should produce a segmentation fault,
  • or head is uninitialized, but again a segmentation fault would be highly likely,
  • or head points to a previously freed memory block or an object that has gone out of scope: valgrind detects this condition as the pointer should not be dereferenced but if the memory it points to is readable, the CPU might not generate an error, just meaningless data is read.

Look at the calling code and investigate where head is coming from.

Upvotes: 3

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