C structs are like records in other programming languages. The associate a group of variables (possibly of different types) that are somehow related.
Example: a struct representing a book might relate string variables author, title and publisher; and an integer variable number of pages. In C, this struct type could be defined as follows:
#define AUTHLEN 41
#define TITLELEN 81
#define PUBLEN 81
typedef char authtype[AUTHLEN];
typedef char titletype[TITLEN];
typedef char pubtype[PUBLEN];
typedef struct {
authtype author;
titletype title;
pubtype publisher;
int numpages;} bookrec;
Here, author, title, publisher and
numpages are called the fields of the
struct. Type bookrec is now the type of such
structs. A variable thetext of type
bookrec is declared as follows:
bookrec thetext;
Like arrays, a variable of a struct type can be initialized when (and only when) it is declared:
bookrec thetext = {"Barrett and Wagner",
"C and UNIX",
"John Wiley and Sons",
446};
The order of the initial values must match the order of the fields in the type definition.
Each field of a struct variable can be referred to by the variable name
followed by . followed by the field name. Examples (given
the previous declaration of thetext):
/* print the number of pages */
printf("This book has %d pages.\n", thetext.numpages);
/* change the number of pages */
thetext.numpages = 448;
/* read in a new publisher */
scanf("%s", thetext.publisher);
/* read in a new number of pages */
scanf("%d", &thetext.numpages);
Note how addresses are handled here.
Example: a function to print a bookrec:
void printbook(bookrec book) {
printf("Author: %s\n", book.author);
printf("Title: %s\n", book.title);
printf("Publisher: %s\n", book.publisher);
printf("Number of pages: %d\n", book.numpages);
}
Functions can take structs as parameters and return structs as results.
Example: a program that handles structs representing points
in the real plane
#include <stdio.h>
typedef struct {
double x;
double y;
} point;
point newpoint(void);
void printpoint(point);
main() {
point p;
p = newpoint();
printpoint(p);
}
point newpoint(void) {
/* return an initialized point */
point tp;
tp.x = 0;
tp.y = 0;
return (tp);
}
void printpoint(point tp) {
/* print a point */
printf("The point value is: (%lf, %lf).\n", tp.x, tp.y);
}
Structs are passed by value by default, so a function that needs to change some field(s) of a struct must have a parameter of type address of struct.
Example: a function to set the x field of a point
void setx(point *paddr, double newval) {
/* set the x field to newval */
(*paddr).x = newval;
}
Note that we first get the value at the address of the struct, and then
refer to the field. C also provides the -> operator to
refer to fields of a struct parameter that has been passed by address:
void setx(point *paddr, double newval) {
/* set the x field to newval */
paddr->x = newval;
}
Example: see ~taw2/pub/struct.c
Structs can:
Example. The following makes librarytype the type of arrays of
bookrecs (a library of books):
#define AUTHLEN 41
#define TITLELEN 81
#define PUBLEN 81
#define LIB_MAX 100
typedef char authtype[AUTHLEN];
typedef char titletype[TITLEN];
typedef char pubtype[PUBLEN];
typedef struct {
authtype author;
titletype title;
pubtype publisher;
int numpages;} bookrec;
typedef bookrec librarytype[LIB_MAX];
/* some expressions using this type */
/* declare a variable of type librarytype */
librarytype library;
/* how many pages does the third book
in the library have? */
printf("Number of pages: %d\n", library[2].numpages);
/* add 5 pages to the 10th book in the library */
library[9].numpages += 5;
/* read in a new author for the second
book in the library */
scanf("%s", library[1].author);
Example: see ~taw2/pub/library.c
Structs can be declared without using a typedef by using a
tagname that appears between struct and {.
Example:
struct point {
double x;
double y;
};
Here, point is the tagname. To use this struct type, the
keyword struct must be used with it in declarations and prototypes.
Examples:
struct point point1, point2; void setx(struct point *paddr);
This syntax is will be useful in the future. A typedef can still
be used along with this way of defining struct types:
typedef struct point pointtype;
makes struct point and pointtype equivalent type
names.