CODEWITHSUNDEEP
cfunctionpointerslinked-listfunction-pointers
Ace
Ace

Reputation: 462

Passing a double pointer to a function to get starting address of linked list (C)

I am passing a double pointer to a function. The function reads data from a file into a linked list. I set CircuitData as the pointer to the first node. After I called the function and want to use CircuitData it is empty. Why does it not return the address of the first node(&newPtr)?

ListNodeType **CircuitData;
int numEl = 0;
int numNodes = 0;
CircuitData = NULL;

ReadFile(CircuitData, &numEl, &numNodes);
printf("%p", CircuitData);

Part of the ReadFile function that I'm calling:

void ReadFile(ListNodeType **CircuitData, int *numEl, int *numNodes){
    *numEl = 0;
    *numNodes = 0;

    ListNodeType *newPtr, *tempPtr;
    newPtr = malloc(sizeof(ListNodeType));
    CircuitData = &newPtr;
    newPtr->nextPtr = NULL;

Upvotes: 0

Views: 170

Answers (4)

Matt
Matt

Reputation: 6050

Define CircuitData as (ListNodeType *), and in function ReadFile, pass &CircuitData

Upvotes: 0

Paul R
Paul R

Reputation: 212959

The calling code needs to be like this:

ListNodeType *CircuitData = NULL;

ReadFile(&CircuitData, &numEl, &numNodes);

Upvotes: 0

Filipe Gonçalves
Filipe Gonçalves

Reputation: 21213

Remember that values are passed by copy in C. Thus, this assignment inside ReadFile:

CircuitData = &newPtr;

Will not be propagated to the caller. I think that you wanted this instead:

*CircuitData = newPtr;

For this to work, CircuitData must point to a valid memory location, don't initialize it to NULL. You might want to do this instead:

ListNodeType *CircuitData;

And then call the function like this:

ReadFile(ListNodeType &CircuitData, ...)

Upvotes: 1

Johan Henriksson
Johan Henriksson

Reputation: 687

First, the declared type of CircuitData should be a pointer:

ListNodeType* CircuitData;

To set the address stored in CircuitData (inside the ReadFile function) you would write

*CircuitData = newPtr;

This sets the data at the address CircuitData to the address stored in newPtr, which is what you want to do.

You also need to properly pass the address of the CircuitData pointer to the ReadFile function by calling 

ReadFile(&CircuitData, ...)

Upvotes: 2

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