R’s C API: Redirecting and Capturing Print Output
The printf2Rprintf package provides a demostration of how to write code using R’s C API. The main focus is on control the information stream specified for output. Work on this example was largely driven by a StackOverflow question related to capturing print output.
Usage
To install the package, you must first have a compiler on your system that is compatible with R. For help on obtaining a compiler consult either macOS or Windows guides.
With a compiler in hand, one can then install the package from GitHub by:
# install.packages("remotes")
remotes::install_github("coatless-rd-rcpp/r-c-api-remap-printf")
library("printf2Rprintf")Implementation Details
The main portion of the code requires defining a macro that sets printf to direct into Rprintf and include the #define STRICT_R_HEADERS to avoid errors.
// Load in the R header
#include <R.h>
// Note: This loads R_ext/Print.h that we need
// Define strict headers
#define STRICT_R_HEADERS
// Map printf to Rprintf
#define printf Rprintf
// Function prototype with void parameter list to avoid warnings
extern SEXP hello_world_c(void);
// Sample C function that prints: Hello World!
SEXP hello_world_c(void) {
printf("Hello World!\n");
return(R_NilValue);
}For details on R’s management of printing, please see Section 6.5 Printing of Writing R Extensions
From here, the capture can be passed off to either capture.output(), which directly assigns output to a variable, or sink(), which redirects output to a file whose contents must then be read back in using readLines(). The latter makes it possible to have a clear enclose over multiple lines of code to capture output while the prior is focused on securing output present from an inputted expression.
# Capture output for a function directly
captured_data = capture.output(hello_world())
# Using sink around multiple function calls to redirect output
# to a single file
sink("sink-examp.txt")
hello_world()
sink()
input_data = readLines("sink-examp.txt")
all.equal(input_data, captured_data)