Structured Data Types A structure can be used

Structured Data Types • A structure can be used to combine data of different types into a single (compound) data value. • Unlike an array name, a struct name is not a pointer. • Unlike an array, a struct is passed as a scalar parameter (pass-by-value) 1

Structured Data Types • Like all data types, structures must be declared and defined. • C has three different ways to define a structure – variable structures – tagged structures – type-defined structures 2

Structured Data Types A variable structure definition defines a struct variable. struct { unsigned char red; unsigned char green; unsigned char blue; } pixel; variable name Members DON’T FORGET THE SEMICOLON 3

Structured Data Types A tagged structure definition defines a type. We can use the tag to define variables, parameters, and return types. struct point_t double x; double y; }; { structure tag Member names DON’T FORGET THE SEMICOLON To Use: struct point_t point 1, point 2; note the use of struct must use the tag 4

Structured Data Types • Example 2 (tagged structure definition) struct pixel_t { unsigned char red; unsigned char green; unsigned char blue; }; Structure tag Members DON’T FORGET THE SEMICOLON To Use: struct pixel_t pixel; 5

Structured Data Types • A typed-defined structure is the most powerful way to declare a structure. We can use the tag to define variables, parameters, and return types. typedef struct pixel_type { unsigned char red; unsigned char green; unsigned char blue; } pixel_t; New type name • To declare a variable of the new type, use: pixel_t pixel; 6

Structured Data Types Example 2 (typed-defined structure) typedef struct point. Type { double x; double y; } point_t; New type name To declare variables of the type, use: point_t point 1; point_t point 2; These structure variable definitions create member variables x and y associated with the structure. 7

Structured data types • Member variables of a struct are accessed using the dot operator. pixel 1. red = 200; pixel 1. green = 200; pixel 1. blue = 0; point 1. x = 10. 0; point 1. y = 5. 5; // type is double • These variables may be used exactly like any other variables. • You can assign one struct to another of the same type, such as point 2 = point 1; (each member is copied) 8

Example typedef struct student_type { int id; char grade; } student_t; int main( ) { student_t student; student. id = 2201; student. grade = ‘A’; fprintf(stdout, “id: %d, grade: %cn”, student. id, student. grade); return 0; } 9

Initializing Structures • At declaration time, members of a struct can be initialized in a manner similar to initializing array elements. pixel_t pixel = { 255, 0, 100 }; • The sequence of values is used to initialize the successive variables in the struct. The order is essential. • It is an error to have more initializers than variables. • If there are fewer initializers than variables, the initializers provided are used to initialize the data members. The remainder are initialized to 0 for primitive types. 10

Structured Data Types typedef struct pixel_type { unsigned char red; unsigned char green; unsigned char blue; } pixel_t; 11

Use of sizeof() with structures • The sizeof() operator should always be used in dynamic allocation of storage for structured data types and in reading and writing structured data types. However, it is somewhat easy to do this incorrectly. 12

Pointers to structures: We can declare a pointer to a pixel_t in the following manner: pixel_t *pixptr; We must allocate memory to the pointer before using it. We can either assign to pixptr the address of a pixel_t struct or use malloc to allocate memory: pixptr = (pixel_t *)malloc( sizeof(pixel_t) );

Pointers to structures: Declaring a pointer and allocating memory at the same time: pixel_t *pixptr = (pixel_t *)malloc( sizeof(pixel_t) ); To set or reference components of the pixptr, we can use: (*pixptr). red = 250; // make *pixptr magenta (*pixptr). green = 0; (*pixptr). blue = 250;

Pointers to structures: An alteranative “short hand” notation has evolved for accessing elements of structures through a pointer: pixptr->red = 0; pixptr->green = 250; pixptr->blue = 250; This shorthand form is almost universally used.

Structures containing pointers: • A structure can contain a pointer to a data object that is allocated elsewhere. – Assignment operator for this type of struct will copy a pointer member without copying the object to which the pointer points = = > “shallow copy” – If you want the object copied rather than the pointer to the object, you must allocate space for the new object and then copy the values = = > “deep copy” • A structure can contain a pointer of the same type of structure (this is called a self-referential struct) and can be used to build linked lists, trees, etc.

Structures containing structures • It is common for structures to contain elements which are themselves structures or arrays of structures. In these cases, the structure definitions should appear in "inside-out" order. • This is done to comply with the usual rule of not referencing a name before it is defined. 17

Structures containing structures typedef struct date. Type { int month; int day; int year; } date_t; typedef struct name. Type { char *firstname; char mi[2]; char *lastname; } name_t; 18

Structures containing structures typedef struct address. Type { char *street; char *city; char state[3]; char *zip; } addr_t; typedef struct person. Type { name_t name; addr_t address; date_t dob; } person_t; 19

Arrays of structures • We can also create an array of structure types: pixel_t pixel. Map[400 * 300]; student_t roster[125]; • To access an individual element of the array: pixel. Map[20]. red = 250; roster[50]. gpa = 3. 75; 20

Arrays of structures containing arrays • We can also create an array of structures that contain arrays: time. Card_t employees[50]; • To access an individual element: employees[10]. hours. Worked[3] = 10; 21

Arrays within structures An element of a structure may be an array typedef struct { char name[25]; double pay. Rate; int hours. Worked[7]; } time. Card_t; time. Card_t my. Time; Elements of the array are accessed in the usual way: my. Time. hours. Worked[5] = 6; 22

Structures as parameters to functions • A struct, like an int, may be passed to a function. • The process works just like passing an int in that: – The complete structure is copied to the stack – The function is unable to modify the caller's copy of the variable 23

Structures as parameters to functions #include <stdio. h> typedef struct s_type { int a; double b; } sample_t; void funct(sample_t x) { fprintf(stdout, "x. a = %dn", x. a); fprintf(stdout, "x. b= %lfn", x. b); x. a = 1000; x. b = 55. 5; } int main() { sample_t y; y. a = 99; y. b = 11. 5; funct(y); fprintf(stdout, "y. a = %dn", y. a); fprintf(stdout, "y. b = %lfn", y. b); return 0; } 24

Structures as parameters to functions Sample Run: . /a. out x. a = 99 x. b= 11. 500000 y. a = 99 y. b = 11. 500000 25

Structures as parameters to functions • The disadvantages of passing structures by value are that copying large structures onto the stack – is very inefficient and – may even cause program failure due to stack overflow. typedef struct { int w[100000]; } sample_t; /* */ passing a struct of type sample_t above will cause 1 Gigabyte to be copied onto the stack. sample_t one. GB; for(i = 0; i < 1000000; i++) slow_call(one. GB); } { 26

Passing the address of a struct • A more efficient way is to pass the address of the struct. • Passing an address requires that only a single word be pushed on the stack, regardless of how large the structure is. • Furthermore, the called function can then modify the structure. 27

Passing the address of a struct #include <stdio. h> typedef struct { int a; double b; } sample_t; /* Use the * operator. funct modifies the struct */ void funct (sample_t *x) { // note the use of -> operator fprintf(stdout, "x->a = %dn", x->a); fprintf(stdout, "x->b = %lfn", x->b); x->a = 1000; x->b = 55. 5; } int main() { sample_t y; y. a = 99; y. b = 11. 5; // use the address operator, &, in the call funct(&y); fprintf(stdout, "y. a = %dn", y. a); fprintf(stdout, "y. b = %lfn", y. b); return 0; } 28

Passing the address of a struct Sample run: . /a. out x->a = 99 x->b = 11. 500000 y. a = 1000 y. b = 55. 500000 29

Passing the address of a struct • What if you do not want the recipient to be able to modify the structure? • In the prototype and function header, use the * operator. – Use the const modifier void funct(const sample_t *x) ; 30

Using the const modifier #include <stdio. h> typedef struct s_type { int a; double b; } sample_t; void funct(const sample_t *x) { fprintf(stdout, "x. a = %dn", x->a); fprintf(stdout, "x. b = %dn", x->a); x->a = 1000; x->b = 55. 5; } int main( ) { sample_t y; y. a = 99; y. b = 11. 5; /* to pass the address use the & operator */ funct(&y); fprintf(stdout, "y. a = %dn", y. a); fprintf(stdout, "y. b = %dn", y. b); return 0; } – The above code will generate a compile-time error. 31

Using the const modifier gcc struc 5. c: In function 'funct': struc 5. c: 12: error: assignment of read-only location struc 5. c: 13: error: assignment of read-only location 32

Structures as return values from functions • Scalar values (int, float, etc) are efficiently returned in CPU registers. • Historically, the structure assignments and the return of structures was not supported in C. • But, the return of pointers (addresses), including pointers to structures, has always been supported. 33

Structures as return values from functions typedef struct { int a; double b; } sample_t; sample_t *funct ( ) { sample_t s; s->a = 1000; s->b = 55. 5; return (&s); } int main() { sample_t *y; y = funct( ); fprintf(stdout, "y->a = %dn", y->a); return 0; }. /a. out return. Param. c: In function 'funct': return. Param. c: 8: warning: function returns address of local variable 34

Structures as return values from functions • The reason for the warning is that the function is returning a pointer to a variable that was allocated on the stack during execution of the function. • Such variables are subject to being wiped out by subsequent function calls. 35

Structures as return values from functions • It is possible for a function to return a structure. • This facility depends upon the structure assignment mechanisms which copies one complete structure to another. – This avoids the unsafe condition associated with returning a pointer, but – incurs the possibly extreme penalty of copying a very large structure 36

Structures as return values from functions #include <stdio. h> typedef struct int a; double b; } sample_t; s_type { sample_t funct ( ) { sample_t s; s. a = 1000; s. b = 55. 5; return s; } int main() { sample_t y; sample_t z; y = funct(); z = y; printf("%d %dn", y. a, z. a); return 0; }. /a. out 1000 37

Summary • Passing/returning instances of structures potentially incurs big overhead. • Passing/returning addresses incurs almost no overhead • Accidental modifications can be prevented with const – Therefore, it is recommended that, in general, you do not pass nor return an instance of a structure unless you have a very good reason for doing so. • This problem does not arise with arrays. – The only way to pass an array by value in the C language is to embed it in a structure – The only way to return an array is to embed it in a structure. 38