Structures with Pointers to Structures

Recall from a previous lecture we created a structure of type PERSON as typedef struct person { char name[30]; int age; } PERSON; But now suppose that we want more than just a 30 (or is it 29??) character name. Suppose we want to be able to refer to the first name, last name and even middle initial separately. We could make another structure of type NAME and use it inside of PERSON. We did something similar to this in an earlier lecture with POINT and LINE. typedef struct name { char firstName[20]; char middleInitial; char lastName[20]; } NAME; and now define PERSON as typedef struct person { NAME name; int age; } PERSON;

Recall the syntax

Now, given the following declaration

PERSON bob;

to what do the following expressions refer and what is their data type?
Answers listed below.

bob bob.name bob.name.firstName bob.name.lastName[5]

The memory

Given the declaration of PERSON bob; above, how does the compiler allocate memory?

All memory needed for the variable bob is contiguous, just as you would expect. We would also expect that sizeof(PERSON) would be 45 if we just count up the number of bytes needed. The actual sizeof a PERSON (or any other type of structure) is not a predictable as you might think. There is actually wasted space inside most structures. Here's a short program that proves space is being wasted inside a PERSON.

#include <stdio.h> typedef struct name { char firstName[20]; char middleInitial; char lastName[20]; } NAME; typedef struct person { NAME name; int age; } PERSON; int main() { printf("The size of a PERSON is %d bytes\n", sizeof(PERSON)); return 0; } The size of a PERSON is 48 bytes

If we chose not to have so much space being used inside the structure, we can create a different picture of memory. Recall that we can create pointers to structures. Can we have a member of a structure be a pointer to another structure? Of course.

Consider this definition of PERSON

typedef struct person { NAME *pName; int age; }PERSON; What does the picture of memory for PERSON look like now?

Now the memory to hold the NAME is not part of PERSON. PERSON only has a pointer to a NAME. Where's the memory that holds the NAME? The memory for NAME must be malloc'd.

What's the syntax now?

Now that PERSON has a pointer to NAME, the syntax is different.
Given the declaration PERSON bob; What is the syntax for referring to a. bob's name b. bob's first name c. the 3rd character of bob's last name Answers below.

Answers to the first set of questions regarding PERSON bob
Answers to the questions regarding proper syntax.
If pName is a pointer to "struct NAME" then