Dynamic library not loading in R binary package build

Question

I am trying to build a package with compiled C code in R using 'RStudio' and 'devtools' in a Windows environment.

Only one of the function uses the C code in src folder. The source package works fine. I can use all the functions. I am able to compile the C code using devtools::document() and the corresponding .dll and .o file also appears in the src folder. Then I can load the code using dev_tools::load_all or Ctrl+Shift+L and run all the functions.

However when I am building and reloading the package using Ctrl+Shift+B, I am not able to use the particular function. The function is missing from the package even thought the documentation is retained. I also get the error telling that the corresponding .dll is not loaded.

Error in library.dynam.unload(name, system.file(package = name)) : 
  DLL ‘mypackage.dll’ was not loaded

I get the same results when I am using devtools::build with binary=TRUE.

However I can find the .dll file in the library Documents\R\win-library\3.0\mypackage\libs\i386\mypackage.dll. Why is this dynamic library from compiled code not being loaded?

PS: 1) devtools::has_devel() is giving TRUE 2) I am forced to use .C instead of .Call.

This is the result of the R CMD INSTALL

* installing to library 'C:/Users/lenovo/Documents/R/win-library/3.0'
* installing *source* package 'mypackage' ...
** libs
make: Nothing to be done for `all'.
installing to C:/Users/lenovo/Documents/R/win-library/3.0/mypackage/libs/i386
** R
** preparing package for lazy loading
** help
*** installing help indices
** building package indices
** testing if installed package can be loaded
* DONE (mypackage)

Answer 1

I was able to solve this on Windows 10 with R 3.5 by adding the following function to a file in the R/ folder.

.onUnload <- function (libpath) { library.dynam.unload("mypackage", libpath)}

Here is the reference where I found it.