OK, I have the problem, I do not know exactly the correct terms in order to find what I am looking for on google. So I hope someone here can help me out.
When developing real time programs on embedded devices you might have to iterate a few hundred or thousand times until you get the desired result. When using e.g. ARM devices you wear out the internal flash quite quickly. So typically you develop your programs to reside in the RAM of the device and all is ok. This is done using GCC's functionality to split the code in various sections.
Unfortunately, the RAM of most devices is much smaller than the flash. So at one point in time, your program gets too big to fit in RAM with all variables etc. (You choose the size of the device such that one assumes it will fit the whole code in flash later.)
Classical shared objects do not work as there is nothing like a dynamical linker in my environment. There is no OS or such.
My idea was the following: For the controller it is no problem to execute code from both RAM and flash. When compiling with the correct attributes for the functions this is also no big problem for the compiler to put part of the program in RAM and part in flash. When I have some functionality running successfully I create a library and put this in the flash. The main development is done in the 'volatile' part of the development in RAM. So the flash gets preserved.
The problem here is: I need to make sure, that the library always gets linked to the exact same location as long as I do not reflash. So a single function must always be on the same address in flash for each compile cycle. When something in the flash is missing it must be placed in RAM or a lining error must be thrown.
I thought about putting together a real library and linking against that. Here I am a bit lost. I need to tell GCC/LD to link against a prelinked file (and create such a prelinked file).
It should be possible to put all the library objects together and link this together in the flash. Then the addresses could be extracted and the main program (for use in RAM) can link against it. But: How to do these steps?
In the internet there is the term prelink as well as a matching program for linux. This is intended to speed up the loading times. I do not know if this program might help me out as a side effect. I doubt it but I do not understand the internals of its work.
Do you have a good idea how to reach the goal?
You are solving a non-problem. Embedded flash usually has a MINIMUM write cycle of 10,000. So even if you flash it 20 times a day, it will last a year and half. An St-Nucleo is $13. So that's less than 3 pennies a day :-). The TYPICAL write cycle is even longer, at about 100,000. It will be a long time before you wear them out.
Now if you are using them for dynamic storage, that might be a concern, depending on the usage patterns.
But to answer your questions, you can build your code into a library .a file easily enough. However, GCC does not guarantee that it links the object code in any order, as it depends on optimization level. Furthermore, only functions that are referenced in a library file is pulled in, so if your function calls change, it may pull in more or less library functions.