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Compiling a C program consisting of multiple files - the importance of declaring a function

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• Compiling a C program that consists of multiple C program files
  • Consider to following C program:

    #include <stdio.h> int square( int x ) { int r; // Define a local variable r = x * x; // Statement return ( r ); // Return statement } int main( int argc, char *argv[] ) { int a, b; a = 4; b = square( a ); // Call function square printf( "Square of %d = %d\n", a, b); }

    Notice that:

    The variables are now integers The function square() also returns an integer

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  • Fact:

    A C program can consist of multiple (program) files The name of each program file must end in .c

    In general, to compile a C program that consists of the files file1.c, file2.c, file3.c, ...., use this command:

    gcc [-o outputName] file1.c file2.c file3.c .... Requirement: One of the files file1.c, file2.c, file3.c ... must contain a main() function

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• Separate compiling 1: see if can figure out why this works
  • Suppose we split the program into 2 files:

    File 1 (func1a.c)	File 2 (func1b.c)
    #include <stdio.h> int square( int x ) { int r; r = x * x; return ( r ); }	#include <stdio.h> int main( int argc, char *argv[] ) { int a, b; a = 4; b = square( a ); printf( "Square of %d = %d\n", a, b); }

    We can compile this C-program using the following command:

    gcc func1a.c func1b.c (output is a.out) or: gcc -o myProg func1a.c func1b.c (output is myProg)

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  • Example Program: (Demo above code)                                                
    • The func1a.c Prog file: click here
    • The func1b.c Prog file: click here

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile:   gcc func1a.c func1b.c To run:          ./a.out

    Output:

    Square of 4 = 16

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  • Reason why this work:

    The implicit assumption made by the C compiler (that all parameters and the return value of a function are int type) happens to be correct !!!

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• What can go horribly wrong with a multiple file C program
  • Consider the previous example, but now the data type of the variables are double:

    double square( double x ) { double r; // Define a local variable r = x * x; // Statement return ( r ); // Return statement } int main( int argc, char *argv[] ) { double a, b; a = 4; b = square( a ); // Call function square printf( "Square of %lf = %lf\n", a, b); }

    This program (stored in one file) will compile and run correctly:

    Square of 4.000000 = 16.000000

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  • 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 func2.c To run:          ./a.out

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  • Now, let's split this C program into 2 program files:

    File 1 (func2a.c)	File 2 (func2b.c)
    #include <stdio.h> double square ( double x ) { double r; // Local variable r = x * x; // Statement return ( r ); // Return }	#include <stdio.h> int main( int argc, char *argv[] ) { double a, b; a = 4.0; b = square( a ); // Call square printf( "Square of %lf = %lf\n", a, b); }

    Result:

    Square of 4.000000 = 0.000000

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  • Example Program: (Demo above code)                                                
    • Prog file func2a.c: click here
    • Prog file func2b.c: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   gcc func2a.c func2b.c To compile:   a.out Notice the wrong result...

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  • Reason why this program is wrong:

    The C compiler assumed the parameters and return value of square() were int Therefore: When you pass a double to the function square(): double a; b = square(a); // Assumed: square(int) // ---> convert double to int FIRST !!! the C compiler mistakenly converted the double representation into int before passing the value to square() !!!! This also happens to the return value: The return value is assumed to be int The int (2's complement) encoding is first converted to double before assigning it to the variable b !!!

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  • How to fix this: we must inform the C compiler that:

    the type information of the parameters of each function used the type information of the return value of each function used.

    by:

    Declaring the function !!!!

    (I will show you a working example in the next webpage)