Unable to find the cause of segmentation fault of huffman coding to do entropy encoding and decoding-CodePudding

I have been looking at the code to do entropy encoding and decoding with of huffman coding in c, and I cannot seem to find the reason why I keep getting segmentation fault.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

const int MAX_TREE_HT = 100;

struct MinHeapNode {
    char data;
    unsigned freq;
    struct MinHeapNode* left, * right;
};

struct MinHeap {
    unsigned size;
    unsigned capacity;
    struct MinHeapNode** array;
};

struct MinHeapNode* newNode(char data, unsigned freq) {
    struct MinHeapNode* temp = (struct MinHeapNode*)malloc(sizeof(struct MinHeapNode));
    temp->left = temp->right = NULL;
    temp->data = data;
    temp->freq = freq;
    return temp;
}

struct MinHeap* createMinHeap(unsigned capacity) {
    struct MinHeap* minHeap = (struct MinHeap*)malloc(sizeof(struct MinHeap));
    minHeap->size = 0;
    minHeap->capacity = capacity;
    minHeap->array = (struct MinHeapNode**)malloc(minHeap->capacity * sizeof(struct MinHeapNode*));
    return minHeap;
}

void swapMinHeapNode(struct MinHeapNode** a, struct MinHeapNode** b) {
    struct MinHeapNode* t = *a;
    *a = *b;
    *b = t;
}

void minHeapify(struct MinHeap* minHeap, int idx) {
    int smallest = idx;
    int left = 2 * idx   1;
    int right = 2 * idx   2;

    if (left < minHeap->size && minHeap->array[left]->freq < minHeap->array[smallest]->freq)
        smallest = left;

    if (right < minHeap->size && minHeap->array[right]->freq < minHeap->array[smallest]->freq)
        smallest = right;

    if (smallest != idx) {
        swapMinHeapNode(&minHeap->array[smallest], &minHeap->array[idx]);
        minHeapify(minHeap, smallest);
    }
}

int isSizeOne(struct MinHeap* minHeap) {
    return (minHeap->size == 1);
}

struct MinHeapNode* extractMin(struct MinHeap* minHeap) {
    struct MinHeapNode* temp = minHeap->array[0];
    minHeap->array[0] = minHeap->array[minHeap->size - 1];
    --minHeap->size;
    minHeapify(minHeap, 0);
    return temp;
}

void insertMinHeap(struct MinHeap* minHeap, struct MinHeapNode* minHeapNode) {
      minHeap->size;
    int i = minHeap->size - 1;
    minHeap->array[i] = minHeapNode;
    while (i && minHeap->array[i]->freq < minHeap->array[(i - 1) / 2]->freq) {
        swapMinHeapNode(&minHeap->array[i], &minHeap->array[(i - 1) / 2]);
        i = (i - 1) / 2;
    }
}

void buildMinHeap(struct MinHeap* minHeap) {
    int n = minHeap->size - 1;
    int i;
    for (i = (n - 1) / 2; i >= 0; --i)
        minHeapify(minHeap, i);
}

void printArr(int arr[], int n) {
    int i;
    for (i = 0; i < n;   i)
        printf("%d", arr[i]);
    printf("\n");
}

int isLeaf(struct MinHeapNode* root) {
    return !(root->left) && !(root->right);
}

struct MinHeap* createAndBuildMinHeap(char data[], int freq[], int size) {
    struct MinHeap* minHeap = createMinHeap(size);
    int i;
    for (i = 0; i < size;   i)
        minHeap->array[i] = newNode(data[i], freq[i]);
    minHeap->size = size;
    buildMinHeap(minHeap);
    return minHeap;
}

struct MinHeapNode* buildHuffmanTree(char data[], int freq[], int size) {
    struct MinHeapNode *left, *right, *top;
    struct MinHeap* minHeap = createAndBuildMinHeap(data, freq, size);
    while (!isSizeOne(minHeap)) {
        left = extractMin(minHeap);
        right = extractMin(minHeap);
        top = newNode('$', left->freq   right->freq);
        top->left = left;
        top->right = right;
        insertMinHeap(minHeap, top);
    }
    return extractMin(minHeap);
}

void printCodes(struct MinHeapNode* root, int arr[], int top) {
    if (root->left) {
        arr[top] = 0;
        printCodes(root->left, arr, top   1);
    }
    if (root->right) {
        arr[top] = 1;
        printCodes(root->right, arr, top   1);
    }
    if (isLeaf(root)) {
        printf("%c: ", root->data);
        printArr(arr, top);
    }
}

void HuffmanCodes(char data[], int freq[], int size) {
    struct MinHeapNode* root = buildHuffmanTree(data, freq, size);
    int arr[MAX_TREE_HT], top = 0;
    printCodes(root, arr, top);
}

void encode(char* str, char* encoded_str, struct MinHeapNode* root) {
    if (root == NULL)
        return;
    if (isLeaf(root)) {
        while (*str != root->data)
            str  ;
        while (*str == root->data) {
            *encoded_str = '1';
            encoded_str  ;
            str  ;
        }
        return encode(str, encoded_str, root);
    }
    if (*str == root->data) {
        *encoded_str = '0';
        encoded_str  ;

        encode(str   1, encoded_str, root->left);
    } else {
        encode(str, encoded_str, root->right);
    }
}

void decode(char* encoded_str, struct MinHeapNode* root, char* decoded_str) {
    struct MinHeapNode* curr = root;
    while (*encoded_str) {
        if (*encoded_str == '0')
            curr = curr->left;
        else
            curr = curr->right;
        if (isLeaf(curr)) {
            *decoded_str = curr->data;
            decoded_str  ;
            curr = root;
        }
        encoded_str  ;
    }
}

int main() {
    char str[] = "this is a test";
    int freq[256] = { 0 };
    int size = 0, i;
    for (i = 0; str[i]; i  ) {
        freq[str[i]]  ;
        size  ;
    }
    int unique = 0;
    char data[size];
    for (i = 0; i < 256; i  ) {
        if (freq[i]) {
            data[unique] = (char)i;
            unique  ;
        }
    }
    HuffmanCodes(data, freq, unique);
    char encoded_str[100], decoded_str[100];
    struct MinHeapNode* root = buildHuffmanTree(data, freq, unique);
    encode(str, encoded_str, root);
    printf("Encoded string: %s\n", encoded_str);
    decode(encoded_str, root, decoded_str);
    printf("Decoded string: %s\n", decoded_str);
    return 0;
}

With the several try and error with commenting out section by section, it was found that it's caused somewhere inside the encoding function. This is not the code I wrote myself and is written with some references online. And the output should be the following.

 : 00
a: 010
e: 011
h: 100
i: 1010
s: 1011
t: 110
Encoded string: 0101110101011101110111111001101
Decoded string: this is a test

Instead I get the following.

a: 0
e: 10
h: 110
i: 1110
s: 11110
 : 111110
t: 111111
zsh: segmentation fault

Thanks in advance for your help.

CodePudding user response：

void encode(char* str, char* encoded_str, struct MinHeapNode* root) {
if (root == NULL)
    return;
if (isLeaf(root)) {
    while (*str != root->data)
    {
        printf("str[]=%c", *str); // Here you can see that you have an overflow *******
        str  ;
    }
    while (*str == root->data) {
        *encoded_str = '1';
        encoded_str  ;
        str  ;
    }
    return encode(str, encoded_str, root);
}
if (*str == root->data) {
    *encoded_str = '0';
    encoded_str  ;

    encode(str   1, encoded_str, root->left);
} else {
    encode(str, encoded_str, root->right);
}

}

Just add a condition in your while loop to make sure you don't overflow your string:

while (*str  && *str != root->data) // HERE and in all other loops
{
    str  ;
}

BUT ! first you need to fix the fact that : char encoded_str[100], decoded_str[100]; are empty and not null terminated so you can go through them in a loop :

encoded_str[end_index] = '\0';
while (*str ....) // it will stop at the '/0' in case you dont find *str == root->data.

the problem in your encode function is that you never go out of the recursion. hope this will help you :).

CodePudding user response：

Also your freq[str[i]] ; needs to be freq[str[i] & 0xff] ;. char is signed, so you will be trying to access negative index elements of freq for bytes greater than 127.