So, I have some code, kind of like the following, to add a struct to a list of structs:
void barPush(BarList * list,Bar * bar)
{
// if there is no move to add, then we are done
if (bar == NULL) return;//EMPTY_LIST;
// allocate space for the new node
BarList * newNode = malloc(sizeof(BarList));
// assign the right values
newNode->val = bar;
newNode->nextBar = list;
// and set list to be equal to the new head of the list
list = newNode; // This line works, but list only changes inside of this function
}
These structures are defined as follows:
typedef struct Bar
{
// this isn't too important
} Bar;
#define EMPTY_LIST NULL
typedef struct BarList
{
Bar * val;
struct BarList * nextBar;
} BarList;
and then in another file I do something like the following:
BarList * l;
l = EMPTY_LIST;
barPush(l,&b1); // b1 and b2 are just Bar's
barPush(l,&b2);
However, after this, l still points to EMPTY_LIST, not the modified version created inside of barPush. Do I have to pass list in as a pointer to a pointer if I want to modify it, or is there some other dark incantation required?
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);