Embedded dlls or linkable modules question

The ELF binaries produced by the GNU toolkit (gcc, as, ld and utilities) are suitable for dynamic loading.

You need a dynamic loader in your system. For an example, have a look at the Linux ld.so loader sources.

Please understand that the dynamic loader is not a simple thing. It is probably too large for an embedded system.

The first problem is relocation of absolute addresses. If your system supports position independend code (-fPIC) you are done.

Then put a jump table at the beginning of your binary, maybe plus a signature, and use function pointers in order to access the additional modules from the main block.

Mit freundlichen Grüßen

Frank-Christian Krügel

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Or you could use the resident library concept from the old AmigaDOS (which is based on an even older OS). Entry points to the library functions are a jump table at the beginning of the library. The OS maintains a directory of base addresses for libraries by name. Loading a library entails only relocating absolute addresses which is quite simple. If the libraries are reentrant, one copy of a given library suffices for multiple programs/processes/threads.

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I think that what you want used to be called "relocatable binary overlays". There are two issues:

Brute force and ignorance relocation can be done by compiling twice at different addresses and producing a relocation list, which is then attached to the overlay's binary. The number and types of relocation required will depend on the compiler.

Linking is just a matter of patching jump addresses in a table or linked list.

We've implemented these schemes a number of times.

Stephen