In C, you can create your own data types; in the next section you will see that FILE is an example of this. A "structure" is a collection of one or more variables which are grouped together under a single name; in other languages these are sometimes called "records". There are two keywords which you will use when creating structures:

• struct -- used to describe a new data type
• typedef -- used to associate a name to the struct or another type

This example, which you might find in computer graphics, creates a structure to hold a pair of 2D coordinates to define a line.

Example struct statement:

#include <stdio.h>
struct line
{
	int x1, y1;   //coordinates of beginning of the line
	int x2, y2;   //coordinates of end of the line
};

main()
{
	struct line line1;
}

In the above example, a variable called "line1" is defined to be of type line, as defined in the structure above.

Typedef associates a name with the structure; this should typically appear at the start of the program.

typedef struct line
{
	int x1, y1;
	int x2, y2;
} LINE;

int main()
{
	LINE line1;
}

To access the individual parts of a variable of a structure, use the "dot notation".

LINE line1;   // declare variable "line1" to be of type LINE

line1.x1 = 3;
line1.y1 = 5;
line1.x2 = 7;
line1.y2 = 10;

This notation can also be used to evaluate conditionals:

if (line1.x2 == 7)
{
	printf("x coordinate at the end of the line is seven\n");
}

Structures can also be passed and returned from functions; just be sure that the function prototype comes after the structure is declared.

typedef struct line
{
	int x1, y1;
	int x2, y2;
} LINE;

// function prototype definition
LINE find_perpendicular_line(LINE);

Structures should be used to make programming easier, and to make it easier for other programmers to read your code. Library writers often use structs to define and maintain a clean interface to their library. A user of said library should then be able to interact with it simply by understanding the interface struct (and the associated comments which should always go with it!).

Another example: a struct for defining complex numbers

typedef struct complex
{
	float imag;
	float real;
} CPLX;

Pointers to Structs

As with any datatype in C, it can frequently be useful to work with pointers to structs. This is very similar to what has already been discussed in the section on pointers, but it is useful to see how to reference members of a struct when we have a pointer to it. Using the above line example, we can use -> instead of . to access the structure"s members. Note that -> and . are not directly interchangeable. . should be used for struct variables, while -> operates on struct pointers.

Line line1;
// initialization of line1's members not shown here, see above
Line* line_pointer = &line1;
(*line_pointer).x1 = 10 // access and change x1
line_pointer->x1 = 10   // another way to do the same thing.