### Introduction

This is another entry in my series of R + C based posts (you can see a full list here). This article focuses on a somewhat esoteric skill: constructing a global R object at the C level in a persistent way. By “persistent”, I mean that this object will only be created once (at package load time), and will be reusable throughout the life of the R session. You’ll be able to call it from other C files, and can even return the object to the R side. The other “trick” that will be used is a way to run arbitrary C code on R package load, using .onLoad() + .Call(). This is actually much more generic than what we will use it for in this article, so it is worth paying attention to in case you have other uses for it. Along the way, I’ll also use C header files to share C functions/objects between files, and discuss a bit about how I set up my R packages that use C code. Most of these ideas are not my own. They are adaptations of ideas used by Lionel Henry in vctrs and rlang.

Why are persistent R objects callable from C useful? I can think of two reasons.

• The first is performance. You might have a simple R object (for instance, an integer vector holding 1) that generally takes a small amount of time to create, but is generated and destroyed thousands of time across your C code base. To save a little bit of time, you might want to make this a persistent, unchangeable, global variable.
• The other is just for readability. Rather than having to deal with PROTECT()ing and UNPROTECT()ing common variables like int_one in the partial example below:
SEXP int_one = PROTECT(Rf_ScalarInteger(1));

// Create an R list of length 1, put int_one in it
SEXP result = PROTECT(Rf_allocVector(VECSXP, 1));
SET_VECTOR_ELT(result, 0, int_one);

UNPROTECT(2); // unprotect int_one and result
return result;

You can instead declare int_one as a global variable with a more permanent meaningful name, like shared_int_one, and use it without worrying about protection:

SEXP result = PROTECT(Rf_allocVector(VECSXP, 1));

// can use shared_int_one without creating a new one
SET_VECTOR_ELT(result, 0, shared_int_one);

UNPROTECT(1); // only have to care about result protection
return result;

When you have a large C based R package, these kinds of things really pay off in terms of increasing readability and cohesiveness of your package, especially if the global variable takes a few lines of C code to create each time. Additionally, if naming conventions for these kinds of variables are used consistently, you’ll immediately be able to recognize what shared_empty_dbl is without having to look it up in the code base. This makes reading over C code a more pleasant experience.

The rest of this post will focus on creating a package that constructs some of these global variables. Specifically, we will look at creating a shared empty integer and a shared character vector, and then we will see how to return them back to the R side. One thing to keep in mind is that these kinds of things take a lot of setup on the C side for the first object, but adding subsequent objects is much simpler.

If you haven’t read Now You C Me, and you aren’t too familiar with working on an R package with C code in it, you might want to go check out that post before continuing. It will teach you the basics of working with an R package containing C code.

The final product is an R package called cshared. It contains one R function, get_shared_objects(). I’ll discuss the bits and pieces of the package throughout the post, but that will be the ultimate reference for the end result.

### Setup

First, some setup. We’ll leverage {usethis} and {devtools} to get our new package up and running. I’m assuming you are working in RStudio for this. The Now You C Me post describes these steps in much greater detail.

# Create a new R package, cshared
usethis::create_package("~/path/to/location/for/the/package/cshared")

# Use roxygen2
usethis::use_roxygen_md()

# As prompted by use_roxygen_md()
devtools::document()

# Set up cshared-package.R, which also gives usethis a place to add extra
# roxygen namespace tags, which is used by use_c() later on.
usethis::use_package_doc()

# Create a src/shared.c file, and add the all important registration info
# to cshared-package.R
usethis::use_c("shared")

# Initialize the C DLL, otherwise document() will complain

# As prompted by use_c()
devtools::document()

At this point you should be in an R package, and if you’ve opened shared.c you should see this staring at you:

#define R_NO_REMAP
#include <R.h>
#include <Rinternals.h>

I actually like to move these defines / includes into a package API header file that I can #include in all of my .c files, so personally I’m going to create a cshared.h file next, and move this over there. There’s not a shortcut for this, so in RStudio do File -> New File -> C++ File then save it as cshared.h in the src/ folder. Copy those three lines to that file, and remove them from shared.c, replacing them with the following single include statement, which will have the same effect:

#include "cshared.h"

To prevent cshared.h from accidentally being included twice in the same file, we should also add some header include guards:

#ifndef CSHARED_H
#define CSHARED_H

#define R_NO_REMAP
#include <R.h>
#include <Rinternals.h>

#endif

### C -> R

Okay, now we have the basic structure set up, so let’s wire up a C function to be callable from the R side. For now, it will create a list containing an empty integer vector and a character vector holding "tidyverse", and return it to the R side. Later it will return the same list but holding the shared versions of these objects. Add the following function to shared.c:

#include "cshared.h"

SEXP cshared_get_shared_objects() {
// An empty integer vector
SEXP empty_int = PROTECT(Rf_allocVector(INTSXP, 0));

// Character vector of size 1, containing "hello world"
SEXP tidyverse = PROTECT(Rf_allocVector(STRSXP, 1));
SET_STRING_ELT(tidyverse, 0, Rf_mkChar("tidyverse"));

// Initialize the output list, then insert our objects into it
SEXP out = PROTECT(Rf_allocVector(VECSXP, 2));
SET_VECTOR_ELT(out, 0, empty_int);
SET_VECTOR_ELT(out, 1, tidyverse);

// Must unprotect 3 PROTECT() calls before exiting!
UNPROTECT(3);
return out;
}

To call this from R, we need an init.c file that registers the C routine to the R side. We’ve done something like this in the other blog post, so create init.c and fill it with:

#include <R.h>
#include <Rinternals.h>
#include <stdlib.h> // for NULL

/* .Call calls */
extern SEXP cshared_get_shared_objects();

static const R_CallMethodDef CallEntries[] = {
{"cshared_get_shared_objects", (DL_FUNC) &cshared_get_shared_objects, 0},
{NULL, NULL, 0}
};

void R_init_cshared(DllInfo *dll) {
R_registerRoutines(dll, NULL, CallEntries, NULL, NULL);
R_useDynamicSymbols(dll, FALSE);
}

Over on the R side, we now need an R function that calls this cshared_get_shared_objects routine. Call usethis::use_r("shared") and fill the resulting R file with:

#' Get the shared objects
#'
#' @examples
#'
#' get_shared_objects()
#'
#' @export
get_shared_objects <- function() {
.Call(cshared_get_shared_objects)
}

Lastly, run devtools::load_all() and devtools::document() to recompile the package and ensure that the shiny new get_shared_objects() is exported.

You should now be able to call:

get_shared_objects()
#> [[1]]
#> integer(0)
#>
#> [[2]]
#> [1] "tidyverse"

### Let’s Share

The next step is to replace our empty_int and tidyverse C variables with shared global variables that were created at package load time. This will clean up our code a bit, and make cshared_get_shared_object() a bit easier to read. But accomplishing this requires some thought! What we want is a way to initialize some C SEXP objects when the R package is loaded. Generally, when we want to perform any action when a package is loaded we use the .onLoad() hook (see ?.onLoad for more). To make it actually initialize our C variables, we will use .Call() from inside .onLoad() to call a C function that does the initialization.

The general outline we are going to follow is:

• Create a C global variable, initialized to NULL.
• Create a C initialization function where we modify that global variable and set it to its actual value.
• Register this initialization function as a routine callable from R like we did with cshared_get_shared_objects().
• Call it from .onLoad().

We will start with empty_int, and then add tidyverse. I find that it is useful to store these global variables in a utils.c file, with a companion utils.h file that holds the definitions, allowing you to share them with other .c files. So, to start, create utils.h and place the following in it:

#ifndef CSHARED_UTILS_H
#define CSHARED_UTILS_H

#include "cshared.h"

SEXP cshared_shared_empty_int;

#endif

All this holds is the “definition” of the global object cshared_shared_empty_int. By “definition” I just mean that we don’t actually initialize the thing here, we just say “hey, there is this thing called ‘cshared_shared_empty_int’, it is going to be a SEXP, and somewhere else it is going to be initialized, but if you #include "utils.h" you can use this thing”.

Now create utils.c, where we will actually initialize the object:

#include "cshared.h"
#include "utils.h"

SEXP cshared_shared_empty_int = NULL;

SEXP cshared_init_utils() {
cshared_shared_empty_int = Rf_allocVector(INTSXP, 0);
R_PreserveObject(cshared_shared_empty_int);
MARK_NOT_MUTABLE(cshared_shared_empty_int);

Rprintf("Initialized!");

return R_NilValue;
}

Here, SEXP cshared_shared_empty_int = NULL; declares it as a global variable, but just sets it to NULL. We can’t set it directly to an empty integer vector because that isn’t a “compile time value”, it is a “run time value”, meaning it can’t be known before the program starts.

cshared_init_utils() is the initialization function that we are eventually going to call from R in .onLoad(). It does the following:

• Updates cshared_shared_empty_int to actually hold an empty integer vector.
• Calls R_PreserveObject() on it to ensure it isn’t garbage collected.
• Calls MARK_NOT_MUTABLE() on it to ensure it can’t be overwritten accidentally throughout the life of the R session.

I’ve also added a print statement to prove that every time the package is loaded, this code is run.

Now we have to register it to the R side, so modify init.c to export cshared_init_utils(). That looks like:

#include <R.h>
#include <Rinternals.h>
#include <stdlib.h> // for NULL

/* .Call calls */
extern SEXP cshared_get_shared_objects();
extern void cshared_init_utils();

static const R_CallMethodDef CallEntries[] = {
{"cshared_get_shared_objects", (DL_FUNC) &cshared_get_shared_objects, 0},
{"cshared_init_utils", (DL_FUNC) &cshared_init_utils, 0},
{NULL, NULL, 0}
};

void R_init_cshared(DllInfo *dll) {
R_registerRoutines(dll, NULL, CallEntries, NULL, NULL);
R_useDynamicSymbols(dll, FALSE);
}

If we devtools::load_all() now, we should have access to the cshared_init_utils routine object. This is what we need to .Call() from .onLoad(). I generally put my .onLoad() in zzz.R, as it is an auxiliary function. It should be pretty simple:

.onLoad <- function(libname, pkgname) {
.Call(cshared_init_utils)
}

If we devtools::load_all() again, this will trigger .onLoad(), and you should see…

devtools::load_all()
#> Initialized!

Great! So now we know that code is being run. At this point, go back and remove the Rprintf() line from cshared_init_utils().

Head back to shared.c. At the top, just under #include "cshared.h", add #include "utils.h" which will give you access to cshared_shared_empty_int. Now update cshared_get_shared_objects() to use it. The function is becoming a bit easier to read!

#include "cshared.h"
#include "utils.h" // To access cshared_shared_empty_int

SEXP cshared_get_shared_objects() {
// Character vector of size 1, containing "hello world"
SEXP tidyverse = PROTECT(Rf_allocVector(STRSXP, 1));
SET_STRING_ELT(tidyverse, 0, Rf_mkChar("tidyverse"));

SEXP out = PROTECT(Rf_allocVector(VECSXP, 2));
SET_VECTOR_ELT(out, 0, cshared_shared_empty_int); // <- using it here!
SET_VECTOR_ELT(out, 1, tidyverse);

UNPROTECT(2);
return out;
}

Again, run devtools::load_all() and call get_shared_objects(). It should work as before, but this time it is returning a list holding the shared integer vector along with the tidyverse string!

### The tidyverse string

The final step is to make the tidyverse string global and shared. Now that we have the infrastructure set up, this is much more straightforward. Update utils.h with a strings_tidyverse variable:

#ifndef CSHARED_UTILS_H
#define CSHARED_UTILS_H

#include "cshared.h"

SEXP cshared_shared_empty_int;

SEXP strings_tidyverse;

#endif

Update utils.c with:

#include "cshared.h"
#include "utils.h"

SEXP cshared_shared_empty_int = NULL;

// This is new
SEXP strings_tidyverse = NULL;

SEXP cshared_init_utils() {
cshared_shared_empty_int = Rf_allocVector(INTSXP, 0);
R_PreserveObject(cshared_shared_empty_int);
MARK_NOT_MUTABLE(cshared_shared_empty_int);

// This is new
strings_tidyverse = Rf_allocVector(STRSXP, 1);
R_PreserveObject(strings_tidyverse);
SET_STRING_ELT(strings_tidyverse, 0, Rf_mkChar("tidyverse"));
MARK_NOT_MUTABLE(strings_tidyverse);

return R_NilValue;
}

This does much of the same as what we did with cshared_shared_empty_int. It creates a character vector of size 1 to overwrite the NULL global variable, preserves it, sets the first element value to "tidyverse", then marks it as immutable.

Finally we can go back to shared.c and use strings_tidyverse.

#include "cshared.h"
#include "utils.h" // To access cshared_shared_empty_int and strings_tidyverse

SEXP cshared_get_shared_objects() {
SEXP out = PROTECT(Rf_allocVector(VECSXP, 2));
SET_VECTOR_ELT(out, 0, cshared_shared_empty_int);
SET_VECTOR_ELT(out, 1, strings_tidyverse);

UNPROTECT(1);
return out;
}

One thing that I hope is clear is how much more focused cshared_get_shared_objects() is. It’s much easier to see what the purpose of the function is when you don’t have to worry about creating these common shared objects. Additionally, you only have to UNPROTECT() 1 value, out, which makes things slightly easier to keep track of. I also appreciate the fact that we can give our global objects evocative names like strings_tidyverse. If I had another string object I wanted to make into a global variable, I could call it strings_dplyr. When I come across other C code that uses this variable, I immediately know what its value is because of this consistent naming convention.

### Conclusion

These global variables are a neat trick for making code clearer, more internally consistent, and occasionally a bit faster. Additionally, being able to call arbitrary C code on R package load is a useful tool in more ways than just global variable initialization (which we didn’t get to explore in this post). In a later post, I hope to show how to use this trick to initialize a variable holding a call object that let’s you efficiently call an R function from C.