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README.Rmd
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---
output: github_document
---
<!-- README.md is generated from README.Rmd. Please edit that file -->
```{r, include = FALSE}
knitr::opts_chunk$set(
collapse = TRUE,
comment = "#>",
fig.path = "man/figures/README-",
out.width = "100%"
)
library(cstructr)
```
# cstructr
<!-- badges: start -->
[](https://github.com/coolbutuseless/cstructr/actions)
<!-- badges: end -->
`{cstructr}` is a demonstration package illustrating how a C struct may be
wrapped and manipulated from within R as an external pointer.
This is a "full-service" implementation which makes the internal values
of the struct accessible from R via getters and setters.
For bespoke solutions, a lot of the code could be excised to just keep
some core functions.
#### Note: The code in this package was auto-generated by parsing the C struct definition and the C library function declaration.
#### C Struct Definition
```{c eval=FALSE}
typedef struct {
int N;
double strength;
} MyCStruct;
```
#### C Library Function Declaration
```{c eval=FALSE}
double multiply(MyCStruct *mycstruct);
```
## Installation
You can install from [GitHub](https://github.com/coolbutuseless/cstructr) with:
``` r
# install.package('remotes')
remotes::install_github('coolbutuseless/cstructr')
```
## Create and Manipulate
```{r example}
library(cstructr)
mycstruct <- MyCStruct(N = 1, strength = 13)
mycstruct
class(mycstruct)
typeof(mycstruct)
mycstruct$N
mycstruct$strength
mycstruct$N <- 3
mycstruct
rm(mycstruct)
gc()
```
Create multiple structs at once
```{r}
lots <- MyCStructs(n = 3)
class(lots)
lots
```
## Call a C library function
1. Write a "bridge function" wrapper (in C) that maps from R objects to C objects
2. Call the C library function from this wrapper
3. Return the result to R in an appropriate container
An example of a C function in a library is as follows. C libraries know
nothing about R or SEXP objects.
```{c eval=FALSE}
double multiply(MyCStruct *mycstruct) {
return mycstruct->N * mycstruct->strength;
}
```
The "bridge" function between R and the C library call is written in C as follows.
This function unpacks the externalptr to a real `MyCStruct` pointer, calls the
C library function, and then returns the results as an R real numeric value.
```{c eval=FALSE}
SEXP multiply_(SEXP mycstruct_ptr_) {
MyCStruct *mycstruct = external_ptr_to_MyCStruct_ptr(mycstruct_ptr_);
double res = multiply(mycstruct);
return ScalarReal(res);
}
```
The call from R is:
```{r eval = FALSE}
multiply <- function(mycstruct) {
.Call(multiply_, mycstruct)
}
```
All these functions combine such that we can create a pointer to a C struct
in R, and then call the C library function as follows
```{r}
library(cstructr)
mycstruct <- MyCStruct(N=2, strength = 4.5)
multiply(mycstruct)
```
## Auto-generation of bridge function
A lot of the code to wrap a C function and make it callable from R is standard boilerplate.
For simple library functions, you could auto-generate a lot of this R + C code from the
C function declaration.