Difference Between C and C++ Executables?

Question

I know there are differences in the source code between C and C++ programs - this is not what I'm asking about.

I also know this will vary from CPU to CPU and OS to OS, depending on compiler.

I'm teaching myself C++ and I've seen numerous references to libraries that can be used by both languages. This has started me thinking - are there significant differences between the binary executables of the two languages?

For libraries to be easily used by both, I would think they'd have to be similar on an executable level.

Are there many situations where a person could examine a executable file and tell whether it was created by C or C++ source code? Or would the binaries be pretty similar?

Answer 1

In most cases, yes, it's pretty easy. Here are just a few clues that I've seen often enough to remember them easily:

1. C++ program will typically end up with at least a few visible symbols that have been mangled.
2. C++ program will typically have at least a few calls to virtual functions, which are typically quite distinctive from code you'll typically see in C.
3. Many C++ compilers implement a calling convention for C++ that gives special consideration to passing the this pointer into C++ member functions. Again, since the this pointer simply doesn't exist in C, you'll rarely see a direct analog (though in some cases, they will use the same convention to pass some other pointer, so you need to be careful about this one).

Answer 2

A executable is a executable is a executable, no matter what language it's written in. If it's built for the target architecture, it'll run on the architecture.

The (arguably) most important difference between C and C++-compiled code, and the one relevant to libraries that can be linked both against C and C++ executables, is that of name mangling. Basically: when a library is compiled, it exports a set of symbols (function names, exported variables, etc.) that executables linked against the library can use. How these symbols are named is a fairly compiler/linker-specific, and if the subsequent executable is linked using a linker using an incompatible convention, then symbols won't resolve correctly. In addition, C and C++ have slightly different conventions. The Wikipedia article linked above has more of the details; suffice to say, when declaring exported symbols in a header file, you'll usually see a construction like:

#ifdef __cplusplus
extern "C" {
#endif

/* exported declarations here */

#ifdef __cplusplus
}
#endif

__cplusplus is a preprocessor macro only defined when compiling C++ code. The idea here is that, when using the header in C++, the compiler is instructed to use the C way of naming exported symbols (inside the "extern "C" { /* foo */ }" block, so the library can be linked both in C and C++ correctly.