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We have a very modular application with a lot of shared objects (.so). Some people argue that on low-end platforms with limited memory/flash, it is better to statically link everything into one big executable as shared objects have overhead.
What is your opinion on this ?
Best Regards,
Paul
Unless memory is extremely tight, the size of one copy of these files is not the primary determining factor. Given that this is an embedded system, you probably have a good idea of what applications will be using your libraries and when. If your application opens and closes the multiple libraries it references dutifully, and you never have all the libraries open simultaneously, then the shared library will be a significant savings in RAM.
The other factor you need to consider is the performance penalty. Opening a shared library takes a small (usually trivial) amount of time; if you have a very slow processor or hard-to-hit real-time requirements, the static library will not incur the loading penalty of the shared library. Profile to find whether this is significant or not.
To sum up, shared libraries can be significantly better than static libraries in some special cases. In most cases, they do little to no harm. In simple situations, you get no benefit from shared libraries.
Of course, the shared library will be a significant savings in Flash if you have multiple applications (or versions of your application) which use the same library. If you use a static library, one copy (which is about the same size as the shared library[1]) will be compiled into each. This is useful when you’re on a PC workstation. But you knew that. You’re working with a library which is only used by one application.
[1] The memory difference of the individual library files is small. Shared libraries add an index and symbol table so that
dlopen(3)can load the library. Whether or not this is significant will depend on your use case; compile for each and then compare the sizes to determine which is smaller in Flash. You’ll have to run and profile to determine which consumes more RAM; they should be similar except for the initial loading of the shared library.