- We must
declare
all functions
before they are
used.
When a large number of functions are used, we will have a very long list of function declarations
Schematically:
Program file that make use of library functions Library program with useful functions (e.g.: push, pop, etc (stack)) #include <stdio.h> int f(float x); // Declare f !!! float g(int x); // Declare g !!! float h(float x); // Declare h !!! .... int main( int argc, char *argv[]) { short a = 2; int b; b = f( a ); // Works correctly now printf("a = %d, b = %d\n", a, b); }// File: /home/cs255001/demo/C/Multi-file-prog/r2.c #include <stdio.h> int f( float x ) { return(x*x); } float g( int x ) { return(x*x*x); } float h( float x ) { return(x*x*x*x); } ... (possibly more functions)...
- We can use the file inclusion feature of the C-preprocessor (see: click here) to help us organize the function declarations !!!
-
Common practice in
C programming:
- Write
a header file
prog.h for
each (library)
C source program file prog.c
The header file prog.h will contain:
- Declaration of
all functions that are
defined in the C program file
- Some other stuff that will be discussed later (specifically: user-defined types and global variables )
- Declaration of
all functions that are
defined in the C program file
- Include the
header file
prog.h in
every C program file
- This is done to declare the functions so the C compiler will not use implicit function prototyping
Because there is no harm to declare a function, every header (.h) file that you write is usually included in every program (.c) file
- Write
a header file
prog.h for
each (library)
C source program file prog.c
- Suppose we wrote the following
C (libary) program file
r2.c with the following
3 functions:
// File: /home/cs255001/demo/C/Multi-file-prog/r2.c #include <stdio.h> int f( float x ) { return(x*x); } float g( int x ) { return(x*x*x); } float h( float x ) { return(x*x*x*x); }
- We write/compose a
header file
r2.h
with the
function declarations for
all the functions
in r2.c:
// File: /home/cs255001/demo/C/Multi-file-prog/r2.h // This #ifndef trick is used to prevent recursive include..... #ifndef h_prog2_h #define h_prog2_h int f( float x ); float g( int x ); float h( float x ); #endif(You can create this file easily using cut and paste from the function definitions in file r2.c !!!)
- We can now declare
all the
functions defined in r2.c
by
including the header file r2.h
by using
#include "r2.h"
Example:
- This program file containing
the main( ) program that
call
all the
functions in
r2.c:
#include <stdio.h> #include "r2.h" // Declares all functions in r2.c int main( int argc, char *argv[]) { short a = 2; int b; b = f( a ); printf("a = %d, b = %d\n", a, b); b = g( a ); printf("a = %d, b = %d\n", a, b); b = h( a ); printf("a = %d, b = %d\n", a, b); }The #include "r2.h" clause will declare all functions defined in r2.c
- This program file containing
the main( ) program that
call
all the
functions in
r2.c:
- Example Program:
(Demo above code)
- Prog file: home/cs255000/demo/C/Multi-file-prog/r1.c + r2.c + r2.h
How to run the program:
cd /home/cs255000/demo/C/Multi-file-prog gcc -c r1.c gcc -c r2.c gcc r1.o r2.o a.out Output: cs255-1@aruba (4995)> a.out a = 2, b = 4 a = 2, b = 8 a = 2, b = 16 No errors !!!
- Fact:
- There is
no harm for
a C program file to contain
BOTH:
- A function declaration and
- The function definition for that same function
Example: (taken from this lecture webpage click here) this C program contains both a declration and a definition of the function int f(short x):
#include <stdio.h> int f(short x); // function declaration !!! int main(int argc, char *argv[]) { short x = 2; // *** short !!! int y = 0; y = f(x); // Assumes: int f(int x) printf("x = %d, y = %d\n", x, y); } /* --------------------------------------------------- Function f( ) is defined AFTER main's use of f( ) --------------------------------------------------- */ int f(short x) // Function definition !!! { return(x*x); }
- There is
no harm for
a C program file to contain
BOTH:
- So,
it's a very common practice
in C programming to:
- Include the header file inside the program source file where the functions are defined.
Example:
// File: /home/cs255001/demo/C/Multi-file-prog/r2.c #include <stdio.h> #include "r2.h" // Include header files for functions in r2.c !! int f( float x ) { return(x*x); } float g( int x ) { return(x*x*x); } float h( float x ) { return(x*x*x*x); }The program c2.c will compile correctly !!!
- Furhermore,
we gain
the advantage that
the C compiler can
detect
any error in
the function declaration:
- When you compile the C program source file r2.c, the C compiler will check (= verify !!!) if the function declarations in r2.h match the actual function definitions !!!
- Example:
- If you change the
function definition for
f( ) to:
int f ( double x ) // Suppose we want more accuracyand you compile the modified r2.c program, the C compiler will detect that the function declaration in r2.h is inconsistent with your updated function f( ) !!!
cs255-1@aruba (5004)> gcc -c r2.c r2.c:6:5: error: conflicting types for 'f\u' int f( double x ) ^ In file included from r2.c:4:0: r2.h:7:7: note: previous declaration of 'f' was here int f( float x ); ^I will demo this in class....
- If you change the
function definition for
f( ) to: