I often hear in various interpretations the phrase: "The given examples show not the code incorrect from the viewpoint of porting to x64 systems, but the code incorrect in itself". I would like to discuss and theorize a bit on this point in the blog. Please, take this note with a bit of humor.
First, let's begin with saying that any code written in C++ is incorrect by itself. Only that code will be correct which consists of the empty function main, yet I'm not sure about it. It is impossible to write an ideal correct program in C/C++. For you should consider that the program should work on a 12-, 16-, 32-, 64-, ...-bit system. The program, if possible, shouldn't allocate memory dynamically because somewhere it is missing. Also, it shouldn't use functions like scanf for you may need to place the program into a controller where there is no input device. The program mustn't use type conversions. Any type conversion is a potential error on some platform. And perhaps it is better to write the program with the help of trigraphs - you never know... :)
Well, I mean that there are no ideally correct programs in C/C++. You can seek to create such a program but you will never create it. In reality, when writing programs an admissible level of correctness and supposition about the execution environment is chosen and the program is written within the framework of this model.
So, any code is incorrect by itself from the viewpoint of an ideal programmer with golden hands living in vacuum. But we can suppose that a particular code be correct in some particular conditions. When the conditions (the environment) change the code may become incorrect. In what way it becomes incorrect depends on the external changes. Search of errors occurring when the execution environment changes can be arranged in a group and successfully diagnosed, while the approach "everything in the program is incorrect" is irrational.
Let's consider an example. We have a program to port into a controller which won't have a console. The program has some number of cout, cin, printf, scanf. We should find and "deactivate" these functions. Suppose that input be performed through the ports connected to some handle on the device's case. There is no sense in saying that the code is bad, the programmer who wrote it is bad only because he hadn't foreseen that there can be no console and one cannot disable all these sections by one pressure. It won't help us. And there is no sense in trying to perform an ideal refactoring to create an ideal program. We should only find and fix the necessary fragments. One can invent a static analyzer of "input-output issues in controllers"-diagnosis kind. And it will be helpful! But, honestly, all this is due to imperfect code of course :-)
The example above is exaggerated but I just want to show that when one is writing code one cannot foresee everything. One doesn't know that in five years this code will be placed into a controller, ported on a 64-bit system or adapted to a submarine. It is rather difficult to foresee some things.
Programmers have and maintain that code which they have. It can contain a lot of magic numbers, THOUSANDS of expressions where signed and unsigned types are used together, where many warnings may be disabled because one has to use LARGE old third-party libraries. And no one will bother to perform total refactoring of such projects to make them more beautiful, portable etc. And if one insists on this - this person should be fired. :) In reality, you should solve real tasks. You should add new functionality, organize maintenance on existing systems. If necessary, you should port the code on 64-bits. But when you port the code on a 64-bit system, it is this task that will be solved and not the task of how to make the code maximum portable. And here we face the practical task of detecting particular magic numbers (but not all of them), unsafe expressions with signed and unsigned types (but not all of them).
My position may seem wrong to many people as if I'm urging to write bad code and then use various crutches (which I sell myself) to fix it in some places. I am simply a practitioner. And also I call many things by their names. :)
Mostly, program code is BAD. And it works more or less well because it is lucky. Unfortunately, programmers are persistent in not admitting it. Any "code-shaking" (changing of the compiler, execution environment etc) reveals a layer of particular types of errors. I understand that there are no "64-bit" errors. There are just errors in code. They are always present in code. But some errors will occur on a 64-bit system. I tell developers about these errors and hope it will help them. And it is these errors that I call "64-bit errors".
One of the problems that 64-bit software developers have to face is overflows of 32-bit variables in very long loops. PVS-Studio code analyzer is very good at catching issues of this type (see the Viva64 diagnostic set). A lot of questions concerning variable overflows are asked at StackOverflow.com. But since my answers may be treated as pure advertisement, rather than ...
In C language, you may use functions without defining them. Pay attention that I speak about C language, not C++. Of course, this ability is very dangerous. Let us have a look at an interesting example of a 64-bit error related to it. Below is the correct code that allocates and uses three arrays, 1 GB each: