Change default value of libsc preference

README.md

@@ -92,7 +92,12 @@ julia> using Preferences, UUIDs
 julia> set_preferences!(
            UUID("7d669430-f675-4ae7-b43e-fab78ec5a902"), # UUID of P4est.jl
            "libp4est" => "/path/to/your/libp4est.so", force = true)
-
+```
+On Windows you also need to set the path to the local build of the shared library
+of [`libsc`](https://github.com/cburstedde/libsc/tree/master), which is a subpackage
+of `p4est`. On other systems, this is not necessary as the library is already linked
+by `libp4est.so`, but it can be used to employ also a custom build of `libsc`.
+```julia
 julia> set_preferences!(
            UUID("7d669430-f675-4ae7-b43e-fab78ec5a902"), # UUID of P4est.jl
            "libsc" => "/path/to/your/libsc.so", force = true)

docs/make.jl

@@ -99,6 +99,7 @@ makedocs(
   pages = [
     "Home" => "index.md",
     "Introduction" => "introduction.md",
+    "Troubleshooting and FAQ" => "troubleshooting.md",
     "API reference" => "reference.md",
     "Authors" => "authors.md",
     "Contributing" => "contributing.md",

docs/src/troubleshooting.md

@@ -0,0 +1,5 @@
+# Troubleshooting and FAQ
+
+## [`using P4est` gives `UndefVarError: libp4est not defined`](@id undef-var-error)
+
+If you get the error message `UndefVarError: libp4est not defined` when `using P4est` the error is likely to be caused by the fact that P4est.jl uses a system provided MPI library, but no system provided `p4est` version. Therefore, check in your active project that the `LocalPreferences.toml` has a section `[MPIPreferencs]` as well as a section `[P4est]` with valid entry for `libp4est`. If you don't have any `LocalPreferences.toml` in your active project also check your default environment under `~/.julia/environments/vx.y/`, where `vx.y` is the julia version you're using.

src/P4est.jl

@@ -8,7 +8,7 @@ using Reexport: @reexport
 # load all preferences here and access them from the `module LibP4est`.
 using Preferences: @load_preference
 const _PREFERENCE_LIBP4EST = @load_preference("libp4est", "P4est_jll")
-const _PREFERENCE_LIBSC = @load_preference("libsc", "P4est_jll")
+const _PREFERENCE_LIBSC = @load_preference("libsc", _PREFERENCE_LIBP4EST)
 
 
 # Include p4est bindings