According to valgrind, an invalid read of size 4 was made exactly at dictionary.c, function addTrie, line 256. "If ptr is NULL, no operation is performed" Apparently, this invalid read is the same one that causes the segmentation fault. Better check that you're not trying to read from a NULL pointer or something!

A1: From the manual pages for free, "If ptr is NULL, no operation is performed".

If you have two pointers that point to the same location in memory and you called free on one of them, you'll certainly free the memory that's pointed to by both of them. I see no problem at all with initializing one of them to NULL.

A2: Using a temporary pointer. For example, here's a simple algorithm for inserting a word into a trie whose root is an array of 26 chars (since no word starts with an apostrophe) and each node of which contains an array of 27 nodes named children and a bool member that indicates the end of a word

create a node pointer named temp and set it to NULL

for each char in word
{
    create an int named index and set it to the index of the current char
    if the current char is the first char
        set temp to root[index]
    else
        set temp to temp -> children[index]

    if temp doesn't have memory allocated for it
    {
        allocate memory for it
        set all the elements of the current node's children member to NULL
    }

    the current char is the last char? set the bool member to true; else, set it to false
}

This might not be the best algorithm to do it though!

A3: I think declaring the root of the trie as an array indicates that you're on the right track. Implementing a trie and the functions associated with it isn't the easiest thing to do comparing to something like taking the hash table approach, for example.

It's more efficient in loading and checking (O(k) where k is the length of the longest word) than a hash table (O(n) where in is the length of the longest chain). Yet it's less efficient when it comes to memory usage.

According to valgrind, an invalid read of size 4 was made exactly at dictionary.c, function addTrie, line 256. "If ptr is NULL, no operation is performed" Apparently, this invalid read is the same one that causes the segmentation fault. Better check that you're not trying to read from a NULL pointer or something!

A1: From the manual pages for free, "If ptr is NULL, no operation is performed".

If you have two pointers that point to the same location in memory and you called free on one of them, you'll certainly free the memory that's pointed to by both of them. I see no problem at all with initializing one of them to NULL.

A2: Using a temporary pointer. For example, here's a simple algorithm for inserting a word into a trie whose root is an array of 26 chars (since no word starts with an apostrophe) and each node of which contains an array of 27 nodes named children and a bool member that indicates the end of a word

create a node pointer named temp and set it to NULL

for each char in word
{
    create an int named index and set it to the index of the current char
    if the current char is the first char
        set temp to root[index]
    else
        set temp to temp -> children[index]

    if temp doesn't have memory allocated for it
    {
        allocate memory for it
        set all the elements of the current node's children member to NULL
    }

    the current char is the last char? set the bool member to true; else, set it to false
}

This might not be the best algorithm to do it though!

A3: I think declaring the root of the trie as an array indicates that you're on the right track. Implementing a trie and the functions associated with it isn't the easiest thing to do comparing to something like taking the hash table approach, for example.

It's more efficient in loading and checking (O(k) where k is the length of the longest word) than a hash table (O(n) where n is the length of the longest chain). Yet it's less efficient when it comes to memory usage.