Lesson 6. Errors in 64-bit code
Even if you correct all compilation errors and warnings, it does not mean that a 64-bit application will work well. So it is the description and diagnosis of 64-bit errors that we will deal with in the most lessons of our course. And one more thing - do not rely on the switch /Wp64 which is described by many people (often unreasonably) in forum discussions as a wonderful tool able to find 64-bit errors.
/Wp64 switch
The switch /Wp64 allows programmers to find some issues that may occur when compiling code for 64-bit systems. The check is implemented in this way: the types marked with the key word __w64 in 32-bit code are interpreted as 64-bit types while being checked.
For example, here is a code:
typedef int MyInt32;
#ifdef _WIN64
typedef __int64 MySSizet;
#else
typedef int MySSizet;
#endif
void foo() {
MyInt32 value32 = 10;
MySSizet size = 20;
value32 = size;
}The expression "value32 = size;" will lead to value cutting on a 64-bit system and therefore to a possible error. We want to diagnose this issue. But when we try to compile the 32-bit application, everything is correct and there is no warning.
To get ready to move the application to 64-bit systems we need to add the switch /Wp64 and the key word __w64 when defining the type MySSizet in the 32-bit version. After that the code looks so:
typedef int MyInt32;
#ifdef _WIN64
typedef __int64 MySSizet;
#else
typedef int __w64 MySSizet; // Add __w64 keyword
#endif
void foo() {
MyInt32 value32 = 10;
MySSizet size = 20;
value32 = size; // C4244 64-bit int assigned to 32-bit int
}Now we get the warning C4244 that will help us in porting the code to a 64-bit platform.
Note that the switch /Wp64 is ignored in the 64-bit compilation mode because all the types already have the necessary size and the compiler performs the necessary checking. So, as you can see, we will get the warning C4244 when compiling the 64-bit version even if the switch /Wp64 is disabled.
So, the switch /Wp64 helped developers get somehow ready to use the 64-bit compiler while working with 32-bit applications. All warnings revealed with the help of /Wp64 will turn into compilation errors or remain warnings when building the 64-bit code. And that is all aid you may except from the switch /Wp64 in detecting errors.
By the way, the switch /Wp64 is considered deprecated in Visual Studio 2008 because it is high time we started to compile 64-bit applications instead of going on to get ready for it.
64-bit errors
When we speak of 64-bit errors, we mean those cases when a code fragment that works well in the 32-bit version of an application causes errors after being recompiled in the 64-bit mode. 64-bit errors occur most frequently in the following kinds of code fragments:
- code based on wrong assumptions about type sizes (for example, an assumption that the pointer size is always 4 bytes);
- code processing large arrays whose size is more than 2 Gbytes on 64-bit systems;
- code responsible for data writing and reading;
- code containing bit operations;
- code with complex address arithmetic;
- obsolete code;
- and so on.
In fact, all errors occurring in the code when it is recompiled for 64-bit systems arise from inaccurate compliance with C/C++ standard ideology. But we do not find it very reasonable to follow this recommendation: "write correct programs and there will be no 64-bit errors". One cannot argue against it but it has little relevance to real projects. There is much C/C++ code in the world that has been written for many decades. The purpose of our lessons is to arrange all the 64-bit errors into a set of patterns that will help you detect defects and instruct you how to eliminate them.
Examples of 64-bit errors
We will speak a lot about 64-bit errors in future but here are two examples for you to understand what these errors are.
The first is an example of using the magic constant 4 that serves as the size of a pointer what is incorrect in 64-bit code. Note that this code worked quite well in the 32-bit version and was not diagnosed as dangerous by the compiler.
size_t pointersCount = 100;
int **arrayOfPointers = (int **)malloc(pointersCount * 4);The second is an example of an error in the data reading mechanism. This code is correct in the 32-bit version and the compiler does not react to it. But this code fails to correctly read the data saved by the 32-bit version of the program.
size_t PixelCount;
fread(&PixelCount, sizeof(PixelCount), 1, inFile);A comment for sophisticated programmers
I would like to comment right away upon the 64-bit error patterns and error examples that will be discussed in many following lessons. People often argue that actually these are not errors related to 64 bits but the errors arising from an incorrectly written and badly portable code. And they also say that many errors can be found when porting code not only to the 64-bit architecture but simply to any architecture where the base types have other sizes.
Yes, that is right! We keep this in mind. But our goal is not to study the issue of code portability as such. In these lessons we are going to solve a particular local task - to help developers in mastering 64-bit platforms that become more and more popular.
When speaking of 64-bit error patterns we will consider examples of code that is correct on 32-bit systems but may cause faults when being ported to the 64-bit architecture.
The course authors: Andrey Karpov (karpov@viva64.com), Evgeniy Ryzhkov (evg@viva64.com).
The rightholder of the course "Lessons on development of 64-bit C/C++ applications" is OOO "Program Verification Systems". The company develops software in the sphere of source program code analysis. The company's site: http://www.viva64.com.