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Structures as parameter variables

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• 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)

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  • Example:

    main program file (defines a)

    help program file (declares a)

    #include <stdio.h> /* --------------------------- 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 }

    #include <stdio.h> /* -------------------------- 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); }

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set2/struct3a.c + struct3b.c

    How to run the program:

    To compile: gcc -c struct3a.c gcc -c struct3b.c gcc struct3a.o struct3b.o To run:          ./a.out

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

    A struct typed parameter is passed by-value

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  • Program that verifies (illustrates) this fact:

    main program file (defines a)

    help program file (declares a)

    #include <stdio.h> /* --------------------------- 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); }

    #include <stdio.h> /* -------------------------- 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 !!!

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set2/struct4a.c + struct4b.c

    How to run the program:

    To compile: gcc -c struct4a.c gcc -c struct4b.c gcc struct4a.o struct4b.o To run:          ./a.out