How to create an "empty" space in an executable at a definite address (gcc,linux)?

Question

What I essentially want to do is have another program write data into this "empty space" for the executable to "work" on

I thought of appending a signature to the application and then writing the data, searching for it later, but that doesn't quite sound right...

Now, other important thing ... I know it should be possible to create a code cave by using code like :

void function(void) {
__asm {
nop
nop
nop
nop
};
}

then, even this is practically the same (apart from the fact that it will be in the .data section, so not executable):

const char data[3];

The problem then is that the other application will not have a definite address to write to.

Answer 1

At least for PEs and ELFs, you can append data to the end of the executable without affecting the program at all.

A standard approach is to append your data to the executable, and then append a number indicating how many bytes have been appended. The executable then opens itself for reading, looks at the last N bytes indicating the data length, and then seeks backwards by that value, to the beginning of the appended data.

This article goes into pretty good detail on how to use the above method to make a self-extracting executable. That's a little different from what you want, but the principle of reading data contained in the executable remains the same.