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The correct way to compile a multiple files C program: separate compilation

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• The correct (professional) way to compile a multi-file C program
  • Compile procedure:

    Compile each C program separately using the following command: gcc -c progName.c This command will: Check the C program source file progName.c for syntax errors and if no errors, translates the program into machine (= object) code The output is an object code file named progName.o Schematically: After compiling all the source C programs, you "link" (= combine) all the object code files using the following command: gcc -o execProgName progName1.o progName2.o .... This command will: Collect all the variable names and function names used in the entire program Replace the variable names and function names with the memory locations where these variables and functions are stored in memory This linkage step will result in an error when some variable or function is not defined The linkage step does not check for type errors (that's done in the compile step) Schematically:

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• Example of compiling a C program that consists of multiple program files
  • Previously, we fixed the following C program using a function declaration:

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

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  • Suppose we split this C program into 2 files by moving f( ) to another file:

    File 1 (p1.c)	File 2 (p2.c)
    #include <stdio.h> int f( short x ); // Declare f !! int main( int argc, char *argv[]) { short a = 2; int b; b = f( a ); // **** Use f( ) printf("a = %d, b = %d\n", a, b); }	#include <stdio.h> int f( short x ) { return(x*x); }

    Important note:

    The declaration of function f( ) provides the necessary information to the C compiler to enable it to perform the necessary conversions ! Therefore: The definition of the function f( ) need not be placed inside the same program file !!!

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

    gcc -c p1.c // Produces: p1.o gcc -c p2.c // Produces: p2.o gcc p1.o p2.o // Produces: a.out

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/Multi-file-prog/p1.c + p2.c

    How to run the program:

    To compile:   gcc -c p1.c                                gcc -c p2.c                       gcc p1.o p2.o To run: ./a.out

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• How to re-compile a multi-files C program
  • Important fact:

    You only need to re-compile a C program source file when: You have made changes (= edits) to the content in the C program source file !!!

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  • Example: suppose we make changes the p2.c file

    File 1 (p1.c)	File 2 (p2.c)
    #include <stdio.h> int f( short x ); // Declare f !! int main( int argc, char *argv[]) { short a = 2; int b; b = f( a ); // **** Use f( ) printf("a = %d, b = %d\n", a, b); }	#include <stdio.h> int f( short x ) { return(x*x*x); // Change in p2.c }

    We can re-compile the multi-file C program using the following commands:

    1. Re-compile the updated C program source files: gcc -c p2.c // Produces a new p2.o 2. Link again: gcc p1.o p2.o // Produce a new a.out

    Use /home/cs255001/demo/C/Multi-file-prog/p1.c + p2.c to do the demo

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• Demo of linkage error
  • The linking step detects undefined (missing) variables and functions

    To illustrate the linkage step, consider this multi-files C program where the function f( ) in p2.c is changed to g( ):

    File 1 (p1.c)	File 2 (p2.c)
    #include <stdio.h> int f( short x ); // Declare f !! int main( int argc, char *argv[]) { short a = 2; int b; b = f( a ); // **** Use f( ) printf("a = %d, b = %d\n", a, b); }	#include <stdio.h> int g( short x ) // Change in p2.c { return(x*x); }

  • Compile again... you will see that f( ) is not found error:

    cs255-1@aruba (5244)> gcc -c p1.c // No syntax errors in p1.c cs255-1@aruba (5245)> gcc -c p2.c // No syntax errors in p2.c cs255-1@aruba (5246)> gcc p1.o p2.o // Linkage error !! p1.o: In function `main': p1.c:(.text+0x1c): undefined reference to `f' collect2: error: ld returned 1 exit status

    Reason:

    main( ) called the function f( ) But function f( ) is not defined in the program files p1.c and p2.c

    Use /home/cs255001/demo/C/Multi-file-prog/p1.c + p2.c to do the demo