Unexpect behaviour with pointer array

Question

im currently implementing a basic linked list structure on C.

Basically im parsing a txt file, and I buffer each line into a Node which contains a pointer to an array of integers. The problem is, that my data (the array of integers) doesn't save correctly / i don't know how to iterate through it correctly.

These are the structures that I use:

typedef struct Node { struct Node* next; struct Node* prev; int* data; int len; } Node; typedef struct LinkedList { Node* head; Node* tail; } LinkedList;

and I use this method to create a new node from a buffered string:

int nodeManager(FILE* filePointer, char* buffer, char* token, LinkedList* linkedList) { token = strtok(buffer, DELIMITER); int* data = (int*)malloc(sizeof(int)); int dataIndex = 0; if (data == NULL) { fprintf(stderr, DATA_ALLOCATION_ERROR); freeLinkedList(linkedList); fclose(filePointer); return EXIT_FAILURE; } const char* insertPosition = token; while ((token = strtok(NULL, DELIMITER))) { data = realloc(data, dataIndex + 1); if (data == NULL) { fprintf(stderr, DATA_ALLOCATION_ERROR); freeLinkedList(linkedList); fclose(filePointer); return EXIT_FAILURE; } char *res; int num = (int) strtol(token, &res, 10); data[dataIndex] = num; dataIndex++; } Node* newNode = (Node*)malloc(sizeof(Node)); if (newNode == NULL) { freeLinkedList(linkedList); free(data); fclose(filePointer); return EXIT_FAILURE; } newNode -> prev = NULL; newNode -> next = NULL; newNode -> len = dataIndex; newNode -> data = data; if(strcmp(insertPosition, INSERT_TO_START) == 0) { addToStartLinkedList(linkedList, newNode); } else { addToEndLinkedList(linkedList, newNode); } return EXIT_SUCCESS; // TODO - Change }

A line of input looks like this:

start,1,2,3,4,5,6,7,10,22,44,55,66,66,77

for some reason, node -> data doesn't get all the values that I assign in this method and I can't really tell why.

I tried to print the values like this:

 for (int i = 0; i < newNode -> len; i++) { printf("%d,", (newNode -> data)[i]); }

However, as I said, something quite doesn't work with my assigment, or I just don't know how to access the values correctly.

Would love to get some insight - thanks.

4386427 · Accepted Answer · 2020-08-19 13:24:22Z

This part is wrong:

data = realloc(data, dataIndex + 1);

You have forgotten to multiply with sizeof(int) or better sizeof *p

BTW: Be careful about doing realloc directly into data. On failure realloc may return NULL and then you have a memory leak. You should use a temporary pointer like:

int * temp = realloc(data, SOME_SIZE); if (temp == NULL) { // oh dear, add error handling here - maybe a return or whatever fits } else { // All good data = temp; }

OT: Passing the file pointer to the function in order to be able to close the file is (IMO) a rather strange design. Instead, the caller should check the return value and close the file (if desired).

@WillyCboi: To realloc “safely,” you can use MyType *NewPointer = realloc(OldPointer, NewSize); if (!NewPointer) { Do error handling here, such as writing an error message to stderr and terminating the program. } OldPointer = NewPointer;. Although, if your error handling consists only of reporting an error and terminating the program, it does not matter if you use data = realloc(data, dataIndex + 1); in a typical system, as the operating system will reclaim all memory when your program terminates. (It is actually wasteful to “clean up” things that are purely internal to the program.)

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