如何将指向struct的指针设置为NULL

Question

typedef struct _DocumentRow
{
    char * code /** The code */;
    char * designation /** The designation */;
    double quantity /** The quantity */;
    char * unity /** The unity */;
    double basePrice /** The base price */;
    double sellingPrice /** The selling price */;
    double discount /** The discount */;
    double rateOfVAT /** The rate of VAT */;
    struct _DocumentRow * next /** The pointer to the next row */;
} DocumentRow;

void DocumentRowList_init(DocumentRow ** list) {
    DocumentRow *L;
    list = ( DocumentRow ** ) malloc( sizeof( DocumentRow* ) );
    if ( list == NULL ) {
        fatalError( "memory is not enough" );
    }
    L = NULL;
    list = &L;
}

After using the function DocumentRowList_init , when I test if ( *list == NULL ) , it evaluates to false, why ? I have already set list = &L and L = NULL .

Answer 1

Looks like you want to change (initialize) something pointed to by list , here is how it usually be done:

void DocumentRowList_init(DocumentRow ** list) {
    *list = ( DocumentRow * ) malloc( sizeof( DocumentRow ) );
    if ( *list == NULL ) {
        fatalError( "memory is not enough" );
    }
}

Answer 2

You would have undefined behaviour here. L is a local variable, so when you return it's address via the pointer pointer, the variable no longer exists when DocumentRowList_init is returning.

So even though you assign NULL to it, it will point to invalid memory.

But list is local to DocumentRowList_init , so it will not return the value anyway, as you only assign it a value and then return.

If you want to return a structure of DocumentRow you'd have to use this

  *list = malloc( sizeof *L);

to allocate a structure and return the pointer to it.

Answer 3

void DocumentRowList_init(DocumentRow ** list) {
    DocumentRow *L;
    list = ( DocumentRow ** ) malloc( sizeof( DocumentRow* ) );
    if ( list == NULL ) {
        fatalError( "memory is not enough" );
    }
    L = NULL;
    list = &L;
}

Please mind that list=&L assigns list the address of a variable located on the stack. So once you exist the function the variable is out of scope and your list is left pointing to a some location (on the stack).