understanding double pointers in functions and linked lists

[yougene]

My book gives an example of a linked list algorithm. To test my understanding I memorized the logic of "insert" and rewrote it. It looked the same except for one thing. The book example has a double pointer parameter, whereas mine used only a single pointer. My insert didn't work until I modified it to also use two points of indirection. I think I figured out why but I want you guys to confirm or deny.

When I use a single point of indirection and pass the value in a pointer variable it is being passed call by value. When I use a double pointer I get to use the address operator (&) to pass the pointer which tells the compiler it is to be passed call by reference.

If this is the case, it sure wasn't mentioned in the book! Does a primitive type only get passed call by reference when it has the address operator input in the function argument?

#include <stdio.h>
#include <stdlib.h>


struct listNode {
	char data;
	struct listNode *nextPtr;
};

typedef struct listNode ListNode;
typedef ListNode *ListNodePtr;
/*  Does this mean any structure declared using ListNodePtr
    will automatically be a pointer without using the * cast?  */

//void insert( ListNodePtr *, char );
void insert( ListNode **sPtr, char value );
char delete( ListNodePtr *, char );
int isEmpty( ListNodePtr );
void printList( ListNodePtr );
void instructions( void );

int main()
{
	ListNode *startPtr = NULL;
	int choice;
	char item;

	instructions();
	printf( "? " );
	scanf( "%d", &choice );

	while( choice != 3 ) {

		switch( choice ) {
			case 1:
				printf( "Enter a character: " );
				scanf( "\n%c", &item );
				/*  startPtr is a pointer.  What does it mean to use the address operator on a pointer?  */
				insert( &startPtr, item );
				printList( startPtr );
				break;

			case 2:
				if ( !isEmpty( startPtr ) ) {
					printf( "Enter character to be deleted: " );
					scanf( "\n%c", &item );

					if ( delete( &startPtr, item ) ) {
						printf( "%c deleted.\n", item );
						printList( startPtr );
					}
					else
						printf( "%c not found.\n\n", item );
				}
				else
					printf( "List is empty.\n\n" );

				break;
			default:
				printf( "Invalid choice.\n\n" );
				instructions();
				break;
		}
		printf( "? " );
		scanf( "%d", &choice );
	}

	printf( "%c\n", startPtr->data );
	printf( "End of run.\n" );
	return 0;
}


void insert( ListNode **sPtr, char value )
{
	ListNode *newPtr, *previousPtr, *currentPtr;

	newPtr = malloc( sizeof( ListNode ) );

	if ( newPtr != NULL ) {
		newPtr->data = value;
		newPtr->nextPtr = NULL;

		currentPtr = *sPtr;
		previousPtr = NULL;

		while ( currentPtr != NULL && value > currentPtr->data ) {
			previousPtr = currentPtr;
			currentPtr = currentPtr->nextPtr;
		}
		if ( previousPtr == NULL ) {
			newPtr->nextPtr = *sPtr;
			*sPtr = newPtr;
		}
		else {
			previousPtr->nextPtr = newPtr;
			newPtr->nextPtr = currentPtr;
		}
	}

}


void instructions( void )
{
	printf( "Enter your choice:\n"
		   "   1 to insert an element into the lsit.\n"
		   "   2 to delete an element from the list.\n"
		   "   3 to end.\n" );
}


/*
void insert( ListNode *sPtr, char value )
{
	ListNodePtr newPtr, previousPtr, currentPtr;

	newPtr = malloc( sizeof( ListNode ) );

	if ( newPtr != NULL ) {
		//  data is not a pointer, why is the -> used to access it then?  
		newPtr->data = value;
		newPtr->nextPtr = NULL;

		previousPtr = NULL;
		currentPtr = sPtr;
//		Is currentPtr pointing to the value at sPtr or the address of sPtr?  

		while ( currentPtr != NULL && value > currentPtr->data ) {
			previousPtr = currentPtr;
			currentPtr = currentPtr->nextPtr;
		}

		if ( previousPtr == NULL ) {
			newPtr->nextPtr = sPtr;
			sPtr = newPtr;
		}
		else {
			previousPtr->nextPtr = newPtr;
			newPtr->nextPtr = currentPtr;
		}
	}
	else
		printf( "%c not inserted. No memory available.\n", value );
}
*/

char delete( ListNodePtr *sPtr, char value )
{
	ListNodePtr previousPtr, currentPtr, tempPtr;

	/*  Does this mean the value of a struct name is the memory address?  */
	/*  Could I use a non-pointer struct and change the mem address it points to?  */
	if ( value == ( *sPtr )->data ) {
		tempPtr = *sPtr;
		*sPtr = ( *sPtr )->nextPtr;
		free( tempPtr );
		return value;
	}
	else {
		previousPtr = *sPtr;
		currentPtr = ( *sPtr )->nextPtr;

		while ( currentPtr != NULL && currentPtr->data != value ) {
			previousPtr = currentPtr;
			currentPtr = currentPtr->nextPtr;
		}

		if ( currentPtr != NULL ) {
			tempPtr = currentPtr;
			previousPtr->nextPtr = currentPtr->nextPtr;
			free( tempPtr );
			return value;
		}
	}

	return '\0';
}


int isEmpty( ListNodePtr sPtr )
{
	return sPtr == NULL;
}


void printList( ListNodePtr currentPtr )
{
	if ( currentPtr == NULL )
		printf( "List is empty.\n\n" );
	else {
		printf( "The list is:\n" );

		while ( currentPtr != NULL ) {
			printf( "%c --> ", currentPtr->data );
			currentPtr = currentPtr->nextPtr;
		}

		printf( "NULL\n\n" );
	}
}

[tabstop]

Everything is always passed by value. You are passing the address of startPtr by value. Of course, that means that since you have an address, you can write to that address, so you can change the contents of startPtr.

[yougene]

Thanks that clears some questions up.

[yougene]

Is the name of a structure without any accessor operators a pointer to the structure?

[laserlight]

Is the name of a structure without any accessor operators a pointer to the structure?

No. If I guess your line of thought correctly, unlike an array, a structure does not decay to a pointer when passed as an argument to a function. You have to pass the address of the structure explicitly.

[yougene]

Ahh I see.
But if I declare a pointer to a struct and point it to an existing struct I don't have to use the (*) operator either.

struct blah *apointer;
struct blah structure;

apointer = &structure;

//no * required here
apointer.member;

Is this syntactical sugar or does this represent something about what structures are?

[tabstop]

Except apointer.member is syntactically invalid. You have to use apointer->member instead.

[yougene]

Don't you also use -> if member is a pointer and "apointer" isn't?
The use of -> is where my question originally arose from.

[laserlight]

Don't you also use -> if member is a pointer and "apointer" isn't?

No, you don't. a->b is syntactic sugar for (*a).b so it has nothing to do with whether b is a pointer.

[yougene]

Gotchya thanks