C++: Compiler and Linker functionality

Question

I want to understand exactly which part of a program compiler looks at and which the linker looks at. So I wrote the following code:

#include <iostream>
using namespace std;
#include <string>

class Test {
private:
    int i;

public:
    Test(int val) {i=val ;}
    void DefinedCorrectFunction(int val);
    void DefinedIncorrectFunction(int val);
    void NonDefinedFunction(int val);

    template <class paramType>
    void  FunctionTemplate (paramType val) { i = val }
};

void Test::DefinedCorrectFunction(int val)
{
    i = val;
}

void Test::DefinedIncorrectFunction(int val)
{
    i = val
}

void main()
{
    Test testObject(1);
    //testObject.NonDefinedFunction(2);
    //testObject.FunctionTemplate<int>(2);

}

I have three functions:

• DefinedCorrectFunction - This is a normal function declared and defined correctly.

• DefinedIncorrectFunction - This function is declared correctly but the implementation is wrong (missing ;)

• NonDefinedFunction - Only declaration. No definition.

• FunctionTemplate - A function template.

  Now if I compile this code I get a compiler error for the missing ';'in DefinedIncorrectFunction.
  Suppose I fix this and then comment out testObject.NonDefinedFunction(2). Now I get a linker error. Now comment out testObject.FunctionTemplate(2). Now I get a compiler error for the missing ';'.

For function templates I understand that they are not touched by the compiler unless they are invoked in the code. So the missing ';' is not complained by the compiler until I called testObject.FunctionTemplate(2).

For the testObject.NonDefinedFunction(2), the compiler did not complain but the linker did. For my understanding, all compiler cared was to know that is a NonDefinedFunction function declared. It didn't care for the implementation. Then linker complained because it could not find the implementation. So far so good.

Where I get confused is when compiler complained about DefinedIncorrectFunction. It didn't look for implementation of NonDefinedFunction but it went through the DefinedIncorrectFunction.

So I'm little unclear as to what the compiler does exactly and what the linker does. My understanding is linker links components with their calls. So for when NonDefinedFunction is called it looked for the compiled implementation of NonDefinedFunction and complained. But compiler didn't care about the implementation of NonDefinedFunction but it did for DefinedIncorrectFunction.

I'd really appreciate if someone can explain this or provide some reference.

Thank you.

Answer 1

The function of the compiler is to compile the code that you have written and convert it into object files. So if you have missed a ; or used an undefined variable, the compiler will complain because these are syntax errors.

If the compilation proceeds without any hitch, the object files are produced. The object files have a complex structure but basically contain five things

1. Headers - The information about the file
2. Object Code - Code in machine language (This code cannot run by itself in most cases)
3. Relocation Information - What portions of code will need to have addresses changed when the actual execution occurs
4. Symbol Table - Symbols referenced by the code. They may be defined in this code, imported from other modules or defined by linker
5. Debugging Info - Used by debuggers

The compiler compiles the code and fills the symbol table with every symbol it encounters. Symbols refers to both variables and functions. The answer to This question explains the symbol table.

This contains a collection of executable code and data that the linker can process into a working application or shared library. The object file has a data structure called a symbol table in it that maps the different items in the object file to names that the linker can understand.

The point to note

If you call a function from your code, the compiler doesn't put the final address of the routine in the object file. Instead, it puts a placeholder value into the code and adds a note that tells the linker to look up the reference in the various symbol tables from all the object files it's processing and stick the final location there.

The generated object files are processed by the linker that will fill out the blanks in symbol tables, link one module to the other and finally give the executable code which can be loaded by the loader.

So in your specific case -

1. DefinedIncorrectFunction() - The compiler gets the definition of the function and begins compiling it to make the object code and insert appropriate reference into Symbol Table. Compilation fails due to syntax error, so Compiler aborts with an error.
2. NonDefinedFunction() - The compiler gets the declaration but no definition so it adds an entry to symbol table and flags the linker to add appropriate values (Since linker will process a bunch of object files, it is possible this definitionis present in some other object file). In your case you do not specify any other file, so the linker aborts with an undefined reference to NonDefinedFunction error because it can't find the reference to the concerned symbol table entry.

To understand it further lets say your code is structured as following

File- try.h

#include<string>
#include<iostream>


class Test {
private:
    int i;

public:
    Test(int val) {i=val ;}
    void DefinedCorrectFunction(int val);
    void DefinedIncorrectFunction(int val);
    void NonDefinedFunction(int val);

    template <class paramType>
    void  FunctionTemplate (paramType val) { i = val; }
};

File try.cpp

#include "try.h"


void Test::DefinedCorrectFunction(int val)
{
    i = val;
}

void Test::DefinedIncorrectFunction(int val)
{
    i = val;
}

int main()
{

    Test testObject(1);
    testObject.NonDefinedFunction(2);
    //testObject.FunctionTemplate<int>(2);
    return 0;
}

Let us first only copile and assemble the code but not link it

$g++ -c try.cpp -o try.o
$

This step proceeds without any problem. So you have the object code in try.o. Let's try and link it up

$g++ try.o
try.o: In function `main':
try.cpp:(.text+0x52): undefined reference to `Test::NonDefinedFunction(int)'
collect2: ld returned 1 exit status

You forgot to define Test::NonDefinedFunction. Let's define it in a separate file.

File- try1.cpp

#include "try.h"

void Test::NonDefinedFunction(int val)
{
    i = val;
}

Let us compile it into object code

$ g++ -c try1.cpp -o try1.o
$

Again it is successful. Let us try to link only this file

$ g++ try1.o
/usr/lib/gcc/x86_64-redhat-linux/4.4.5/../../../../lib64/crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: ld returned 1 exit status

No main so won';t link!!

Now you have two separate object codes that have all the components you need. Just pass BOTH of them to linker and let it do the rest

$ g++ try.o try1.o
$

No error!! This is because the linker finds definitions of all the functions (even though it is scattered in different object files) and fills the blanks in object codes with appropriate values

Answer 2

I believe this is your question:

Where I get confused is when compiler complained about DefinedIncorrectFunction. It didn't look for implementation of NonDefinedFunction but it went through the DefinedIncorrectFunction.

The compiler tried to parse DefinedIncorrectFunction (because you provided a definition in this source file) and there was a syntax error (missing semicolon). On the other hand, the compiler never saw a definition for NonDefinedFunction because there simply was no code in this module. You might have provided a definition of NonDefinedFunction in another source file, but the compiler doesn't know that. The compiler only looks at one source file (and its included header files) at a time.

Answer 3

Say you want to eat some soup, so you go to a restaurant.

You search the menu for soup. If you don't find it in the menu, you leave the restaurant. (kind of like a compiler complaining it couldn't find the function) If you find it, what do you do?

You call the waiter to go get you some soup. However, just because it's in the menu, doesn't mean that they also have it in the kitchen. Could be an outdated menu, it could be that someone forgot to tell the chef that he's supposed to make soup. So again, you leave. (like an error from the linker that it couldn't find the symbol)