[ILINK32 Error] Fatal: Unable to open file .obj
Let's start with Delphi packages.
What is suspicious is the way the linker reports the error - such as [ILINK32 Error] Fatal: Unable to open file MyComponent.obj. You search the entire hard drive for MyComponent.obj only to find the closest thing to be MyComponent.dcu, which is compiled in Delphi from MyComponent.pas. So how and where do you get MyComponent.obj?
The answer is, this .obj file is actually in a container with the extension of .LIB. If your package with MyComponent.pas is called MyPackage.bpl, then you need to look for MyPackage.lib. While an installed package should get automatically added to the default list of included packages when you create a new project file, it doesn't always happen (i.e. bug). All you have to do then is manually open up your current project file (.cbproj) in a text editor and add MyPackage.lib into the tag. Reload your project and you should be able to link successfully.
If you are not dealing with Delphi packages and you get that error message, that means you're trying to use a static library, or a run-time package (and some times even design-time package which are not dropped onto a form / data module). This could easily be fixed and is the expected way linkers work. There are 2 ways you can fix this:
1) Add the lib file to the project (I don't like this, as you rely on the user of the static library to remember to add the lib file every time they use it in a new project)
2) Add the following line to a header file which is guaranteed to be included when the static library is used:
#pragma comment(lib, "your library name.lib")
That's it.
I haven't tried C++ Builder 2009, but I hope CodeGear have found a better way to do this. Actually, I'd suggest that they simply make all Delphi compiler generated C++ Builder files include that #pragma comment(lib, ...) line so that we never have to muck around with the .cbproj files any more. In fact, this would also mean that there will be no annoying messages that prompts you to remove packages you don't need when you create and try to save a new project.
附注：有的时候.obj包含在另外一个.lib文件中（即.obj文件的文件名与.lib文件的文件名不一致）
如果添加了.lib文件后，依然报这种错误，可能是由于.lib文件所在的路径没有加入到工程中！
还有一种错误：Unable to open file 'xxx_0.OBJ'，项目中有xxx.cpp文件，但没有xxx_0.cpp文件，这可能是由于项目中存在两个xxx.cpp文件，分别存放在两个不同的目录，且其中一个文件在文件系统中可能是不存在的。
原文链接：http://zachsaw.blogspot.com/2008/10/ilink32-error-fatal-unable-to-open-file.html