How do I modify a pointer that has been passed into a function in C?

You need to pass in a pointer to a pointer if you want to do this.

void barPush(BarList ** list,Bar * bar)
{
    if (list == NULL) return; // need to pass in the pointer to your pointer to your list.

    // if there is no move to add, then we are done
    if (bar == NULL) return;

    // allocate space for the new node
    BarList * newNode = malloc(sizeof(BarList));

    // assign the right values
    newNode->val = bar;
    newNode->nextBar = *list;

    // and set the contents of the pointer to the pointer to the head of the list 
    // (ie: the pointer the the head of the list) to the new node.
    *list = newNode; 
}

Then use it like this:

BarList * l;

l = EMPTY_LIST;
barPush(&l,&b1); // b1 and b2 are just Bar's
barPush(&l,&b2);

Jonathan Leffler suggested returning the new head of the list in the comments:

BarList *barPush(BarList *list,Bar *bar)
{
    // if there is no move to add, then we are done - return unmodified list.
    if (bar == NULL) return list;  

    // allocate space for the new node
    BarList * newNode = malloc(sizeof(BarList));

    // assign the right values
    newNode->val = bar;
    newNode->nextBar = list;

    // return the new head of the list.
    return newNode; 
}

Usage becomes:

BarList * l;

l = EMPTY_LIST;
l = barPush(l,&b1); // b1 and b2 are just Bar's
l = barPush(l,&b2);

Generic answer: Pass a pointer to the thing you want to change.

In this case, it would be a pointer to the pointer you want to change.

Remember, in C, EVERYTHING is passed by value.

You pass in a pointer to a pointer, like this

int myFunction(int** param1, int** param2) {

// now I can change the ACTUAL pointer - kind of like passing a pointer by reference 

}