tarsnap | Command-line client code for Tarsnap

ENONEM and ERANGE are macros that are defined in errno.h. As for errno, it is an integer variable set when some errors are occurred.

Thus, to access those variables/defines you need to add #include in your file.

For more documentation : man errno

Your description is unclear whether the variables can be reused in several iterations without any kind of initialization. But let's assume this is the case.

then what if you declare the variable per thread, and then let OpenMP do the parallelization in the for loop

Please re-familiarize yourself with the "Building some native code" case study. Specifically, you have informed Rust what the interface of the library is, but you haven't told the compiler where the code is. That's what the error you are getting says: "I can't find the implementation of crypto_scrypt"

You need to add the library to the linker path and instruct it to be linked against.

From the linked case study, your build script can inform the compiler where the library is and what to link against: