指向无法访问二维数组值的结构的指针

Question

I am currently in the process of writing a program that acts as a circuit. I have a gate structure that takes a 2D char array in order to hold variable names, yet when I try to access these variable names stored in the array outside of the while loop, the content is empty.

typedef struct Gate
{
    kind_t kind;
    int size;     // size of DECODER and MULTIPLEXER
    char **params; // length determined by kind and size (CHANGED FROM INT TO CHAR)
                  // includes inputs and outputs, indicated by variable numbers
} Gate;

typedef struct Node
{
    Gate *data;
    struct Node *next;
} Node;

// Linked list of gates & attributes
    while (fscanf(fp, "%16s", str) != EOF)
    {
        if (strcmp(str, "AND") == 0)
        {
            head = makeGate(fp, head, AND);
            length++;
        }
        else if (strcmp(str, "OR") == 0)
        {
            head = makeGate(fp, head, OR);
            length++;
        }
        else if (strcmp(str, "NAND") == 0)
        {
            head = makeGate(fp, head, NAND);
            length++;
        }
        else if (strcmp(str, "NOR") == 0)
        {
            head = makeGate(fp, head, NOR);
            length++;
        }
        else if (strcmp(str, "XOR") == 0)
        {
            head = makeGate(fp, head, XOR);
            length++;
        }
        else if (strcmp(str, "NOT") == 0)
        {
            //head = makeGate(fp, head, NOT);
            //length++;
        }
        else if (strcmp(str, "PASS") == 0)
        {
            //head = makeGate(fp, head, PASS);
            //length++;
        }
        else if (strcmp(str, "DECODER") == 0)
        {
            //
        }
        else if (strcmp(str, "MULTIPLEXER") == 0)
        {
            //
        }
        printf("%s\n", head->data->params[2]);
    }

    // plugs in values to circuit
    for (int i = 0; i < 3; i++)
    {
        printf("Stored string: %s\n", head->data->params[i]);
    }

`

Node *makeGate(FILE *fp, Node *head, kind_t inGate)
{
    char str[17];
    Node *new_node = (Node *)malloc(sizeof(Node)); // Node of linkedlist that contains gate structure
    new_node->data = (Gate *)malloc(sizeof(Gate)); // Gate structure that keeps information about a gate
    new_node->next = head;

    new_node->data->kind = inGate;
    new_node->data->size = 3;
    new_node->data->params = malloc(3 * sizeof(char*));
    for (int i = 0; i < 3; i++)
    {
        new_node->data->params[i] = malloc(17 * sizeof(char));
    }

    fscanf(fp, "%16s", str);
    new_node->data->params[0] = str;
    fscanf(fp, "%16s", str);
    new_node->data->params[1] = str;
    fscanf(fp, "%16s", str);
    new_node->data->params[2] = str;
    return new_node;
}

` The printf statement inside the while loop works perfectly fine and is there purely for testing, however the for loop that prints each value of the array is different and prints nothing.

I tried to fix this multiple times to no avail, I originally found this problem as I noticed that I had gotten memory leak, and when I freed where the memory leak should be, it throws that I am freeing a address that is not malloced.

My only thought is I am somehow losing/skipping a node, but I am out of ideas

Answer 1

The following does not copy data from str into the struct ( you'd need strcpy ):

new_node->data->params[0] = str;

What it does is copy the address of str into each element. They all point to the same buffer/string. And, str goes out of scope when the function returns.

You can [and should ] just scan into the struct directly.

So, change:

fscanf(fp, "%16s", str);
new_node->data->params[0] = str;

fscanf(fp, "%16s", str);
new_node->data->params[1] = str;

fscanf(fp, "%16s", str);
new_node->data->params[2] = str;

Into:

fscanf(fp,"%16s",new_node->data->params[0]);
fscanf(fp,"%16s",new_node->data->params[1]);
fscanf(fp,"%16s",new_node->data->params[2]);