Redirect posix file calls in C

Question

We have a "library" (a selection of code we would rather not change) that is written from the perspective that it has access to 2 files directly. It uses "open", "read" and "seek" posix calls directly on a file descriptor.

However, now we have a proprietary file system that cannot be accessed through standard IO calls. Seeing that we do not want to re-write the code, it would be great if we could redirect the IO calls to known functions that could then be used as an interface.

Is there any way of changing the calls used above so that the "read" and "seek" can be over-written with new function calls?

Thanks.

Answer 1

When you say you don't want to change the library code, do you mean you want to use existing binary code, or just source? If you have the source and can recompile, I would simply pass -Dread=my_read -Dopen=my_open etc. to the compiler when building the library, and then provide your own my_read etc. functions.

Answer 2

One thing you can try is library function interposition.

Answer 3

In addition to already mentioned function interposition and renaming function calls using a macro, another Linux-only option is to use Filesystem in Userspace. This way you can make your proprietary filesystem accessible to other applications which use the standard POSIX filesystem API. FUSE hello world example is surprisingly short.