Recap of Structs

Remember that structs are just collections of items which may be of different types. Structs in C are analogous to

• Records in Pascal
• Classes (with just variables) in Java

In C, structs use named, not structural type equivalence, and untagged structs are each a different type.

Struct Declaration/Definition

struct [<optional tag>] {
    <type declaration>;
    <type declaration>;
    ...
} [<optional variable list>];

You can in fact initialize at compile time.

struct mystruct_tag {
    int myint;
    char mychar;
    char mystr[20];
};

struct mystruct_tag ms = {42, 'f', "goofy"}; //Compile time thing

This initializer is legal because a character array can be initialized to a string as a special case. However, if you try to assign an array to an array, you will get an error

Example of illegal code

struct mystruct_tag {
    int myint;
    char mychar;
    char mystr[20];
} ms;

ms.mystr = "foo";

Copying Structs

struct s {
    int i;
    char c[8];
} s1, s2;

s1.i = 42;
s1.c = 'a';
s2 = s1;

Note that while arrays decay to pointers, structs behave differently. When you do s2 = s1, C copies all the values byte by byte from s1 to s2.

It's generally bad programming form, but you can make strings copyable by doing

struct x {
    char s[10];
} a, b;

a = b;

Anonymous Struct

An anonymous struct is a struct that doesn't have a defined datatype.

struct {
    char mychar;
    int myint;
} mystruct;

In this case, mystruct is the only struct of this datatype in existence.

Where Do Members Get Stored In Memory?

Most C compilers will do alignment in practice. Don't make the assumption that you can sum up the components. Just use sizeof on the struct.

Pen and Paper Way of Addressing

struct {
    char mychar;
    int myint;
    char mystr[19];
} mystruct;

How would we set the 5th character of mystr to x? mystruct.mystr[4] = 'x';

• First find the address of mystruct &mystruct
• Add offset to mystruct to access mystr element &mystr + offset to mystr = &mystr + 8`
• Using the type of the member, find the offset to the desired element &mystr + 8 + 4 * sizeof(char)

Answer: &mystr + 12

Structure Operations

Remember, structures may be

• Copied
• Assigned
• Have their address taken with &
• Have their members accessed
• Be passed as arguments to functions
• Be returned from functions

However, structures CAN NOT be compared!