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The importance of function declaration in multi-file C programs

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• Review: C compiler can make an incorrect implicit function prototype assumptions
  • Consider the following correct C program:

    #include <stdio.h> int f( float x ); // Declare f !! int main( int argc, char *argv[]) { short a = 2; int b; b = f( a ); printf("a = %d, b = %d\n", a, b); } int f( float x ) { return(x*x); }

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  • When we delete the function declaration:

    #include <stdio.h> 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(float x) { return(x*x); }

    the C compiler will make a incorrect function prototype assumption:

    int f ( int x )

    when the actual function definition is:

    int f(float x) { return(x*x); }

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  • We also saw that the C compiler was able to detect the conflicting function prototype when it processes the function definition (and discovers the error):

    cs255-1@aruba (4845)> gcc impl-declare3.c impl-declare3.c: In function 'main': impl-declare3.c:9:8: warning: implicit declaration of function 'f' [-Wimplicit-function-declaration] y = f(x); // C compiler will assume: int f(int x) ^ impl-declare3.c: At top level: impl-declare3.c:24:5: error: conflicting types for 'f' int f( float x ) ^ impl-declare3.c:25:1: note: an argument type that has a default promotion can't match an empty parameter name list declaration { ^ impl-declare3.c:9:8: note: previous implicit declaration of 'f' was here y = f(x); // C compiler will assume: int f(int x) ^

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/Multi-file-prog/q-orig2.c

    How to run the program:

    To compile:   gcc q-orig2.c          To run: cannot be run

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• The inability for the C compiler to verify its implicit function prototype assumptions in multi-files C program
  • Recall:

    The C compiler processes (= translates) each C program file independently from each other.

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  • Consider the above example (where the C compiler was able to verify its incorrect function prototype assumption:

    #include <stdio.h> 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(float x) { return(x*x); }

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  • Suppose we split this C program into 2 program files:

    File 1 (q1.c)	File 2 (q2.c)
    #include <stdio.h> int main( int argc, char *argv[]) { short a = 2; int b; b = f( a ); // **** assumes: int f( int x ) printf("a = %d, b = %d\n", a, b); }	#include <stdio.h> int f( float x ) { return(x*x); }

    Observe that:

    File q1.c does not contain the function definition for f( ) Therefore: The C compiler can not discover the conflicting function prototype only by processing the file q1.c

    When we compile these program files:

    cd /home/cs255001/demo/C/Multi-file-prog gcc -c q1.c q1.c: In function 'main': q1.c:9:8: warning: implicit declaration of function 'f' [-Wimplicit-function-declaration] b = f( a ); // **** assumes: int f( int x ) ^ gcc -c q2.c gcc q1.o q2.o ***** A warning is not fatal !!! ***** Program compiles successfully !!!

    However, the program will not run correctly:

    cs255-1@aruba (4978)> a.out a = 2, b = 0 (correct output is: b = 4)

    Because:

    The C compiler did not convert the input to a float (IEEE 754) representation (it converted to an int representation !!!)

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• Fixing the incorrect function prototype assumption with a function declaration
  • This problem is easy to fix: we just need to declare the function f( ) (just like before):

    File 1 (q1.c)	File 2 (q2.c)
    #include <stdio.h> int f(float x); // Declare f !!! 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); }	#include <stdio.h> int f( float x ) { return(x*x); }

    I'll do the demo in class

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• Moral of this experiment
  • Moral:

    It is very important to declare all functions that are used in your C program

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  • Next:

    We will learn to organize the function declarations using header files !!!

    We will discuss this next