---

Declaring struct variables

---

• Declaring structure variables

  • 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 define the struct (data type) before you define or declare any variable of that specific struct !!! (Otherwise you will get a data type undefined error !!!)

    ---

    ---

  • Example:

    main program file (defines a)

    help program file (declares a)

    /* --------------------------- Define the structure FIRST ---------------------------- */ struct BankAccount { int accNum; double balance; }; /* ------------------------------------ THEN 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; }; /* ---------------------------- THEN you can declare variable ---------------------------- */ 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)                                                
    • /home/cs255001/demo/C/set2/struct2a.c
    • /home/cs255001/demo/C/set2/struct2b.c

    How to run the program:

    To compile: gcc -c struct2a.c gcc -c struct2b.c gcc struct2a.o struct2b.o To run:          ./a.out

    ---

  • Further experiment:

    Comment out the struct BankAccount definition and recompile. You will get this compile error: cs255-1@aruba (5415)> gcc -c struct2a.c struct2a.c: In function `main': struct2a.c:21:5: error: invalid use of undefined type `struct BankAccount' a.accNum = 123; ^

  ---

  ---

  ---

  ---

• A better organization: use header file
  • As you can see in this program, we must repeat the definition of the struct BankAccount definition in all C program files:

    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); }

    Repeating a definition (e.g., by cut and paste) can result in errors caused by typos or "paste accidents"...

    ---

  • Better organization:

    It is common practice to put the struct and typedef definition(s) inside a header file: /* ------------------- my-types.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 <stdio.h> #include "my-types.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 <stdio.h> #include "my-types.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); }