The use of .exp and what is the difference between .lib and .dll

cdlllinkerstatic-libraries

During compilation and linking, what is use of .exp? What is the difference between .lib and .dll? I know that .lib will be used, while linking and .dll will be used when running the program. But what exactly is the difference between .lib and .dll?

Does .lib file not contain the code for the functions coming from .dll files?
What is the need for using two separate files?

Please clarify.

Best Answer

In the case of an import library for a DLL, the .lib file does not contain any actual code at all. It basically contains just a list of the functions in the associated DLL -- enough for the linker to embed a reference to that DLL into something linked with the library, but not much else.

A .exp file is an export file -- basically just about the same as a .lib file. It's used (at least primarily) when you have a circular dependency. For example, assume you have a DLL that acts as a plug-in for an executable. The executable supplies some exported functions for use by plug-in DLLs, but also needs to be able to call some functions in the plug-ins as well (e.g. to load and initialize a plug-in).

The DLL won't link until the executable is built to provide a .lib file -- but the executable won't link until the DLL is built to provide a .lib file. To break the dependency, you run the linker against the executable, which fails (because it can't find a .lib file for the DLL), but will produce a .exp file. You then link the DLL against the .exp file for the executable. You can then re-run link to produce the executable, using the .lib file for the DLL.

Related Solutions

C++ – the difference between #include and #include “filename”

In practice, the difference is in the location where the preprocessor searches for the included file.

For #include <filename> the preprocessor searches in an implementation dependent manner, normally in search directories pre-designated by the compiler/IDE. This method is normally used to include standard library header files.

For #include "filename" the preprocessor searches first in the same directory as the file containing the directive, and then follows the search path used for the #include <filename> form. This method is normally used to include programmer-defined header files.

A more complete description is available in the GCC documentation on search paths.

The difference between ++i and i++

++i will increment the value of i, and then return the incremented value.
```
 i = 1;
 j = ++i;
 (i is 2, j is 2)
```
i++ will increment the value of i, but return the original value that i held before being incremented.
```
 i = 1;
 j = i++;
 (i is 2, j is 1)
```

For a for loop, either works. ++i seems more common, perhaps because that is what is used in K&R.

In any case, follow the guideline "prefer ++i over i++" and you won't go wrong.

There's a couple of comments regarding the efficiency of ++i and i++. In any non-student-project compiler, there will be no performance difference. You can verify this by looking at the generated code, which will be identical.

The efficiency question is interesting... here's my attempt at an answer: Is there a performance difference between i++ and ++i in C?

As @OnFreund notes, it's different for a C++ object, since operator++() is a function and the compiler can't know to optimize away the creation of a temporary object to hold the intermediate value.

Best Answer

Related Solutions

C++ – the difference between #include and #include “filename”

The difference between ++i and i++

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