R's C API: Redirecting and Capturing Print Output

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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.


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("devtools")

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
// Map printf to Rprintf
#define printf Rprintf

// Sample C function that prints: Hello World!
SEXP hello_world_c() {
  printf("Hello World!\n");

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

input_data = readLines("sink-examp.txt")

all.equal(input_data, captured_data)


GPL (>= 2)

coatless/printf2Rprintf documentation built on May 16, 2019, 6:58 p.m.