---

Structures: user-defined types

---

• Defining structures
  • Structure:

    A structure in C is a group of related variables

  • Comparable thing in Java:

    A structure in C is like a class in Java containing only public variables and without any methods defined inside the class

    ---

    ---

  • Defining a structure

    Defining a structure = tell (convey) to the C compiler the make up (components) of the structure (= group of variables)

  • Syntax to define a structure:

    struct StructName { dataType1 member1Name ; dataType2 member2Name ; ... };

    Note:

    A structure definition must appear outside every function (I.e., a structure is a global data structure - it cannot appear inside a function scope)

  ---

  ---

  ---

  ---

  ---

  ---

• Defining structure variables
  • Defining variables that has a specific structure:

    Defining structure variables = reserve (= allocate) compute memory to store all component (variables) in the structure

    ---

  • After defining a structure, you can define any number of variables of that specific structure.

    (The structure definition acts like a blue print to construct variable of that structure).

  • Fact:

    Defining structure variables in C is like creating an object in Java However: Unlike Java, defining a structure variable in C is a one step process

    ---

    ---

    Syntax to define structure variables:

    struct StructName varName1, varName2, ... ;

    ---

  • Example:

    /* ------------------------ Structure definition ------------------------ */ struct BankAccount { int accNum; double balance; }; int main(int argc, char *argv[]) { struct BankAccount a, b; // Defines 2 structure variables .... }

    ---

    ---

  • Result of a structure variable definition:

    The following figure shows the effect of a structure variable definition: Explanation: A structure variable definition will allocate (= reserve memory space) all components (variables) that was specified in the structure definition clause. The variable name will be associated with the starting location of the allocated memory cells For example, in the above figure: The identifier a (symbolic constant) is equated to the value 5000 (= starting address of the components in the structure a) The identifier b (symbolic constant) is equated to the value 5012 (= starting address of the components in the structure b )

  ---

  ---

  ---

  ---

  ---

  ---

• Using (accessing) structure variables
  • Fact:

    Using structure variables in C is very similar to using object variables in Java

    ---

  • Syntax to use (= access) a specific component of a structure variable:

    structVarName.componentName

    Example:

    #include <stdio.h> /* ------------------------ Structure definition ------------------------ */ struct BankAccount { int accNum; double balance; }; int main(int argc, char *argv[]) { struct BankAccount a, b; a.accNum = 123; a.balance = 1000.0; b.accNum = 444; b.balance = 3000.0; printf("a.accNum = %d a.balance = %f\n", a.accNum, a.balance); printf("b.accNum = %d b.balance = %f\n", b.accNum, b.balance); }

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   gcc struct1.c To run:          ./a.out

  ---

  ---

  ---

  ---

• Declaring structure variables
  • Recall:

    Declaration = inform the data type of a global variable that has been defined elsewhere in the program to the C compiler (The C compiler needs the type information to decode the stored information)

    ---

  • Declaring (global) structure variables:

    A structure typed (global) variable is declared just like a built-in type variable, using the keyword extern

    ---

  • Syntax to declare a structure variable:

    extern struct StructName varName ;

    Important caveat:

    You must have defined the structure (data type) before you can define/declare a variable of that specific structure !!!

    ---

    ---

  • Example:

    main program file (defines a)

    help program file (declares a)

    /* --------------------------- Define the structure FIRST ---------------------------- */ struct BankAccount { int accNum; double balance; }; /* ------------------------------------ Now you can define a struct variable ------------------------------------ */ struct BankAccount a; int main(int argc, char *argv[]) { a.accNum = 123; a.balance = 1000.0; print(); // Invoke print() defined // inside a different file }

    /* -------------------------- Define the structure FIRST -------------------------- */ struct BankAccount { int accNum; double balance; }; /* ---------------------- Now you can declare a ---------------------- */ extern struct BankAccount a; // Declare structure variable a // that is defined ELSEWHERE void print() { printf("a.accNum = %d a.balance = %f\n", a.accNum, a.balance); }

    ---

  • Example Program: (Demo above code)                                                
    • The struct definition Prog file: click here
    • The struct declaration Prog file: click here

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile: gcc -c struct2a.c gcc -c struct2b.c gcc struct2a.o struct2b.o To run:          ./a.out

  ---

  ---

• A better organization
  • As you can see in this program, we must repeat the definition of the struct BankAccount definition:

    main program file (defines a)

    help program file (declares a)

    /* --------------------------- Define the structure FIRST ---------------------------- */ struct BankAccount { int accNum; double balance; }; /* ------------------------------------ Now you can define a struct variable ------------------------------------ */ struct BankAccount a; int main(int argc, char *argv[]) { a.accNum = 123; a.balance = 1000.0; print(); // Invoke print() defined // inside a different file }

    /* -------------------------- REPEAT the definition !!! -------------------------- */ struct BankAccount { int accNum; double balance; }; /* ---------------------- Now you can declare a ---------------------- */ extern struct BankAccount a; // Declare structure variable a // that is defined ELSEWHERE void print() { printf("a.accNum = %d a.balance = %f\n", a.accNum, a.balance); }

    This can result in errors caused by typos...

    ---

  • Better organization:

    It is common practice to put the all struct definition inside a header file: ------------------- bankaccount.h ------------------- struct BankAccount { int accNum; double balance; }; The header file(s) are then inserted at the start of each C program file main program file (defines a) help program file (declares a) #include <bankaccount.h> struct BankAccount a; int main(int argc, char *argv[]) { a.accNum = 123; a.balance = 1000.0; print(); // Invoke print() defined // inside a different file } #include <bankaccount.h> extern struct BankAccount a; // Declare structure variable a // that is defined ELSEWHERE void print() { printf("a.accNum = %d a.balance = %f\n", a.accNum, a.balance); }

  ---

  ---

  ---

  ---

  ---

  ---

• Struct typed variables as function parameters
  • Syntax to specify a struct typed parameter variable:

    returnType functionName ( .... , struct StructName param , .... ) { // (Function body) }

    Always remember that:

    Before you can define/declare any struct typed variables: You must first define the structure (Otherwise, the C compiler does not know how to access the various components in the structure)

    ---

  • Example:

    main program file (defines a)

    help program file (declares a)

    /* --------------------------- Define the structure FIRST ---------------------------- */ struct BankAccount { int accNum; double balance; }; /* ------------------------------------ Now you can define variables of that structure... ------------------------------------ */ int main(int argc, char *argv[]) { struct BankAccount a; a.accNum = 123; a.balance = 1000.0; print( a ); // Pass a struct // variable as parameter }

    /* -------------------------- Define the structure FIRST -------------------------- */ struct BankAccount { int accNum; double balance; }; /* ------------------------------- Now you can define a paramter variable of that structure..... ------------------------------- */ void print( struct BankAccount x ) { printf("x.accNum = %d x.balance = %f\n", x.accNum, x.balance); }

    ---

  • Example Program: (Demo above code)                                                
    • The main Prog file that passes the structure parameter: click here
    • The function with a struct parameter: click here

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile: gcc -c struct3a.c gcc -c struct3b.c gcc struct3a.o struct3b.o To run:          ./a.out

  ---

  ---

• Important fact: struct parameter variables are passed-by-value (i.e., copy)
  • Fact:

    A struct typed parameter is passed by-value

    ---

    ---

  • Program that verifies (illustrates) this fact:

    main program file (defines a)

    help program file (declares a)

    /* --------------------------- Define the structure FIRST ---------------------------- */ struct BankAccount { int accNum; double balance; }; /* ------------------------------------ Now you can define variables of that structure... ------------------------------------ */ int main(int argc, char *argv[]) { struct BankAccount a; extern void update( struct BankAccount x ); // Declare update() a.accNum = 123; a.balance = 1000.0; printf("Before calling update: a.accNum = %d a.balance = %f\n", a.accNum, a.balance); update (a); // Demonstrate that struct is // passed-by-value printf("AFTER calling update: a.accNum = %d a.balance = %f\n", a.accNum, a.balance); }

    /* -------------------------- Define the structure FIRST -------------------------- */ struct BankAccount { int accNum; double balance; }; /* ------------------------------- Now you can define a paramter variable of that structure..... ------------------------------- */ void update( struct BankAccount x ) { printf("Before update: x.accNum = %d x.balance = %f\n", x.accNum, x.balance); x.accNum = 1001; // Update parameter x.balance = 999999; // variable printf("AFTER update: x.accNum = %d x.balance = %f\n", x.accNum, x.balance); }

    Result:

    Before calling update: a.accNum = 123 a.balance = 1000.000000 Before update: x.accNum = 123 x.balance = 1000.000000 AFTER update: x.accNum = 1001 x.balance = 999999.000000 AFTER calling update: a.accNum = 123 a.balance = 1000.000000 UNCHANGED !!!

    ---

  • Example Program: (Demo above code)                                                
    • The main Prog file that passes the structure parameter: click here
    • The function that updates the struct parameter: click here

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile: gcc -c struct4a.c gcc -c struct4b.c gcc struct4a.o struct4b.o To run:          ./a.out

  ---

  ---

  ---

  ---

• Passing an array by value
  • Programming "Trick": (how to pass an array by value in C)

    Recall that array parameters are passed-by-reference in C (See: click here Fact: When a structure contains an array variable as a component. then the array is passed-by-value (because the entire structure is passed by value)

    ---

  • Example:

    Main function passes array inside struct

    Functions updates 2 elemenst in array

    /* ------------------------ Structure definition ------------------------ */ struct Array { int a[10]; // Array in struct }; extern void update( struct Array x ); int main(int argc, char *argv[]) { struct Array a; int i; for ( i = 0; i < 10; i++ ) a.a[i] = i; printf("Before calling update: "); for ( i = 0; i < 10; i++ ) printf("%d ", a.a[i]); putchar('\n'); update (a); // Demonstrate that array inside // a struct is passed-by-value printf("AFTER calling update: "); for ( i = 0; i < 10; i++ ) printf("%d ", a.a[i]); putchar('\n'); }

    /* ------------------------ Structure definition ------------------------ */ struct Array { int a[10]; }; void update( struct Array x ) { int i; printf("Before updating array in x: "); for ( i = 0; i < 10; i++ ) printf("%d ", x.a[i]); putchar('\n'); x.a[0] = 4444; x.a[9] = 4444; printf("AFTER updating array in x: "); for ( i = 0; i < 10; i++ ) printf("%d ", x.a[i]); putchar('\n'); }

    Result:

    Before calling update: 0 1 2 3 4 5 6 7 8 9 Before updating array in x: 0 1 2 3 4 5 6 7 8 9 AFTER updating array in x: 4444 1 2 3 4 5 6 7 8 4444 AFTER calling update: 0 1 2 3 4 5 6 7 8 9 (UNCHANGED)

    ---

  • Example Program: (Demo above code)                                                
    • The main Prog file that passes an array inside a structure as parameter: click here
    • The function that updates 2 elements in the array inside the struct parameter: click here

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile: gcc -c struct5a.c gcc -c struct5b.c gcc struct5a.o struct5b.o To run:          ./a.out

  ---

  ---

  ---

  ---

• Functions that return a struct
  • Example:

    struct BankAccount { int accNum; double balance; }; struct BankAccount f( ) { struct BankAccount p; // Define a BankAccount variable p.accNum = 4; p.balance = 4444.0; return p; // Return the BankAccount variable }

    ---

    ---

  • How to use a function that returns a struct type:

    struct BankAccount a; a = f ( );

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   gcc struct-return1.c To run:          a.out

  ---

  ---

  ---

  ---

• How big is the struct typed variable ???
  • Sizeof:

    The sizeof operator can be used with any user-defined data type to find out how many bytes of memory space the variable will need.

    Example:

    /* ------------------------ Structure definition ------------------------ */ struct BankAccount { int accNum; double balance; }; int main(int argc, char *argv[]) { printf("sizeof(struct BankAccount) = %d\n", sizeof(struct BankAccount) ); }

    Output:

    sizeof(struct BankAccount) = 16

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile:   gcc sizeof1.c To run:          ./a.out

    ---

    ---

  • $64,000 question:

    Wait a minute... didn't we learned that: sizeof(int) = 4 sizeof(double) = 8 How is it that sizeof(struct BankAccount) = 16 ??? struct BankAccount { int accNum; // 4 bytes double balance; // 8 bytes }; ---------------- 12 bytes !!! Should a struct BanAccount variable use 12 bytes ????

    Answer:

    There are memory address alignment requirements in the computer hardware (the manifacturer set these requirements to make the memory operations efficient) Commonly used requirments: A double typed variable must be located at a memory address that is divisible by 8 An int typed variable (and a float typed variable) must be located at a memory address that is divisible by 4 A short typed variable must be located at a memory address that is divisible by 2 A char typed variable can be located at any memory address that In order to store a struct BankAccount, the C compiler must use the following storage format:

    ---

    ---

  • Experiment that show the memory address alignment requirement:

    struct s1 { char a; int d; }; struct s2 { char a; char b; int d; }; struct s3 { char a; char b; char c; int d; }; int main(int argc, char *argv[]) { printf("sizeof(struct s1) = %d\n", sizeof(struct s1) ); printf("sizeof(struct s2) = %d\n", sizeof(struct s2) ); printf("sizeof(struct s3) = %d\n", sizeof(struct s3) ); }

    Output:

    sizeof(struct s1) = 8 sizeof(struct s2) = 8 sizeof(struct s3) = 8

    Explanation:

    The variables of these 3 structures are stored in the following manner: That's why all 3 structures will use 8 bytes to store their variables.

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile:   gcc sizeof2.c To run:          ./a.out

  ---

  ---

  ---

  ---

  ---

  ---

• Note: reference (pointer) variables to struct typed variables
  • Teaching Note:

    I will delay the discussion of reference (= pointer) variables to struct variables This topic will be discusses after I discussed the other kinds of user defined types in C (You can defined reference variables that point to variables of any of the user-defined data types.)