Assembly Language for Beginners

1.4 Returning Values

1.3.4 Empty Functions in Practice.

Despite the fact empty functions seem useless, they are quite frequent in low-level code.

First of all, they are quite popular in debugging functions, like this one:

Listing 1.6: C/C++ Code

void dbg_print (const char *fmt, ...)
{
#ifdef _DEBUG
// open log file
// write to log file
// close log file
#endif
};

void some_function()
{

dbg_print ("we did something\n");

};

In a non-debug build (as in a “release”),_DEBUGis not defined, so thedbg_print()function, despite still
being called during execution, will be empty.

Similarly, a popular method of software protection is to make one build for legal customers, and another
demo build. A demo build can lack some important functions, as with this example:

Listing 1.7: C/C++ Code

void save_file ()
{
#ifndef DEMO
// actual saving code
#endif
};

Thesave_file()function can be called when the user clicksFile->Saveon the menu. The demo version
may be delivered with this menu item disabled, but even if a software cracker will enable it, only an empty
function with no useful code will be called.

IDA marks such functions with names likenullsub_00,nullsub_01, etc.

1.4 Returning Values

Another simple function is the one that simply returns a constant value:

Listing 1.8: C/C++ Code

int f()
{
return 123;
};

Let’s compile it.

1.4.1 x86

Here’s what both the GCC and MSVC compilers produce (with optimization) on the x86 platform:

Listing 1.9: Optimizing GCC/MSVC (assembly output)

f:
mov eax, 123
ret