header image

Linking C with R in Windows

R is great for many aspects of statistical analysis but it is often criticized for running iterative programs (e.g., for loops) slowly.  There are many new packages that are attempting to rectify this by allowing the user to 1) use multiple processing cores (see Todd Jobe’s Blog for an example), and 2) by calling compiled code within R (the purpose of this post).  Both solutions can take time to implement but are often worth the development time as they can really pay off in the long run.  I have found that solution #1 usually is best if there are a few iterations that each take several seconds to process and solution #2 is usually best if there are many thousands of iterations over a very trivial calculation.

There is actually a decent amount on the web about how to format  your syntax in R and C to make things work out well (see this page for example); however, almost all of the instructions and tutorials out there assume that you are able to compile your C code yourself.  If you’re like me an you use a PC that runs Windows you may not be very familiar with compiling your own code. This is where I can lend some instruction based upon my own experiences.

Tutorial Outline:
1) Getting Windows ready to go
2) Writing the C code
3) Compiling the C code
4) Loading and calling C code in R

1) Getting Windows ready to go:

First you are going to have to install some tools that will allow you to build R packages and compile C code from the command line (R installation and Administration – Appendix D).

Luckily all of the tools you will need have been bundled together for you as Rtools.exe which can be downloaded from any CRAN mirror (e.g., http://cran.cs.wwu.edu/bin/windows/Rtools)

Once you have downloaded and installed those tools, you will need to change the PATH of your environment variables. I think this is necessary so that the R tools you just installed can be called from the command line or DOS prompt. Here is a link that describes three different ways to perform this: R FAQ: 2.15. We need to redefine the path such it that includes (see R installation and Administration – Appendix D):


Note that there may be a way when installing R tools to have this path changed for you automatically, although I found that I had to perform this task manually.  Here I will describe in detail how to do this because information is scant on the web on this topic.  As noted above there are three ways to change the PATH of your environment variables, here I will describe the third method (quoted directly from R FAQ: 2.15):

“For all applications via Windows. How you set an environment variable is system specific: under Windows 2000/XP/2003 you can use `System’ in the control panel or the properties of `My Computer’ (under the `Advanced’ tab). Under Vista, go to `User Accounts’ in the Control Panel, and select your account and then `Change my environment variables’.”

Once you are in the change environment variable box, select “New”. Name the new variable “PATH” and then set the value to (at a minimum):


Note that above c:\R\R-2.15\bin\i386 should be tailored to your specific path directory of R that you downloaded the Rtools for. Then click “OK” twice and your environment variables should now be set and you should be ready to compile some C code via the command line with R.

After performing these steps be sure to restart your machine.

2) Writing the C code:

Before we compile our C code we need to generate some sample code or lift some from the web. Here are two example functions written in C that you can copy and paste into a text file you name “sequence_examples.c” (exactly what you name it does not matter).

Filename: "sequence_examples.c"
Return a vectors of sequentially summed values
start -- value to start the sum at
size -- the number of elements to return
sumVect -- the vector of summed output values

void sumSeq(int *start, int *size, int *sumVect){
    This function provides a simple sequential sum
    where F[n] = F[n-1] + n

    int i, j ;
    j = 0 ;
    for(i = *start; i < (*start + *size); i++){
        if(i == *start){
            sumVect[j] = i ;
            sumVect[j] = sumVect[j-1] + i ;
        j ++ ;

void fiboSeq(int *size, int *sumVect){
    This function returns the Fibonacci sequence
    where F[n] = F[n-1] + F[n-2]

    int i ;
    sumVect[0] = 0 ;
    sumVect[1] = 1 ;
    for(i = 2; i < *size; i++){
        sumVect[i] = sumVect[i-1] + sumVect[i-2] ;

3) Compiling the C code:

Once you have R ready to go and your bit of C code you’ll need to compile it into a .dll file that can be dynamically loaded into R.  To compile the .C file you must open the Windows Command Prompt.  This is a program that allows you to write DOS code straight into the computer’s innards (for lack of a better word).  Once the Command Prompt is open you’ll have to direct it towards the folder that contains the .C file you wish to compile. For example if my file was on my desktop, I would enter the following into the command prompt:

cd C:\Users\dmcglinn\Desktop

The cd in the above statement stands for change directory.  Now all you have to do is send the compile command to the Command Prompt with your file name. For example if I wanted to compile the file sequence_example.c I would send the following command to the Command Prompt

R CMD SHLIB sequence_examples.c

If no errors occur (error statement will appear in the Command Prompt, they are actually usually fairly informative) then two files should be produced: sequence_examples.o and sequence_examples.dll.  The file sequence_examples.dll is the only one you will need to link in with R.

4) Loading and calling C code in R

Now that our C code is compiled we are ready to bring it into the R environment like so:


.C("sumSeq", start = as.integer(10), size = as.integer(5),
   sumVect = as.integer(rep(0, 5)))

.C("fiboSeq", size = as.integer(5),
   sumVect = as.integer(rep(0, 5)))

These R commands should return lists that contain the input to the C function and the resulting output. Because we are only usually interested in the output it is a good idea to be more specific about what portion of the list you would like returned.

.C("sumSeq", start = as.integer(10), size = as.integer(5),
   sumVect = as.integer(rep(0, 5)))$sumVect

.C("fiboSeq", size = as.integer(5),
   sumVect = as.integer(rep(0, 5)))$sumVect

That’s all there is to it. To make this cleaner next I would place my call to the .C function in an R wrapper function that handles the passing of my R variables to the C functions for me.

Please leave feedback or suggestions below. Good luck!


  1. By: Luke on March 3, 2011 at 12:48 pm      

    Thank you soooo much for posting this. You rock. Thank you.

  2. By: Dan McGlinn on March 10, 2011 at 9:29 am      

    Hey Luke,
    I’m glad this was helpful to you!

  3. By: Brady on April 14, 2011 at 10:30 am      


    I hope all is well in NC. The prairies are greening up beautifully out here (though a little slower than normal, we need some rain).

    You might want to check out the ‘inline’ package. It does the compiling, linking, and loading of C/C++ code stored as strings in R. I’ve been experimenting/playing with it, along with the ‘Rcpp’ package. Both work quite nicely. I’m using linux, however, haven’t tried it in windows.

    All the best,


  4. By: Dan McGlinn on April 14, 2011 at 10:45 am      

    Hey Brady,

    Great to hear from you! I miss the prairies particularly at this time of year! Thanks for the tips on those packages. I had tried to get them to work briefly but had little success. Maybe I should take another shot at it.


  5. By: Tom Boucher on June 29, 2011 at 11:39 am      

    Hi All,
    I have the ‘inline’ package running on Windows 7, with some very simple C code being compiled, linked and loaded. So far, this is very nice. Thanks for your help.



  6. By: Carol on November 11, 2011 at 5:48 pm      

    Thank you soooo much for this. You saved my life!

  7. By: Freddy on May 14, 2012 at 6:36 pm      

    Muchas gracias, excelente documentación.

  8. By: Philipp on November 7, 2012 at 6:23 pm      


    some feedback:

    * Second PATH example is listing the last path twice.
    * C source has HTML entities instead of operators.
    * R source lacks the final closing parentheses.
    * Fibbonaci breaks on size smaller than 2.


  9. By: Dan McGlinn on January 17, 2013 at 12:48 am      

    Hey Philipp,

    Thank you for pointing out those mistakes. I think I fixed them all except for the note about the Fibbonaci function. I was able to use the existing code to work when the size was smaller than 2. You do have to make sure to also update the size of the the sumVect argument. For example,

    .C(“fiboSeq”, size = as.integer(1), sumVect = as.integer(rep(0, 1)))$sumVect

    will correctly return 0.

    Thanks again!

  10. By: lisa on April 18, 2013 at 9:52 pm      

    Thanks it’s gorgeous!

    I’ve tried 2 days linking R with C and I only succeed last night in my Mac.. Now I can fully use it in my office desktop (which is Windows)!

    However when I compiled my .c file in cmd there’s a warning and I didn’t get it.. But that does not influence the generation of .o and .dll files. So I just ignore it..

    But I’d be very much appreciated if you could help me with the warning :)

    Thanks anyway! It’s really awesome!

    ————- My warnings:
    cygwin warning:
    MS DOS style path detected: …
    Preferred POSIX equivalent is: /cygdrive/….
    CYGWIN environment variable option “nodosfile warning” turns off this warning

  11. By: Dan McGlinn on April 25, 2013 at 1:32 am      

    Hey Lisa, I’m glad the post was useful for you. I’m not sure about that warning. It sounds like its just a cygwin warning and not an error coming from the c complier so yea its safe to ignore. If you are still curious about the warning I would post a question at http://stackoverflow.com/

  12. By: Tonio on August 8, 2013 at 10:47 am      

    Hey Dan,

    i totally agree with Lisa, i really tried long with different guides until I found this one. You saved me quite some time!

    Concerning Lisa´s question, you are right, it is a cygwin warning, but just about a preference, that he loads/would like to load it under a different path.

  13. By: Saad on October 20, 2013 at 11:20 am      

    Thanks! Saved me a lot of time. Great work!

  14. By: TimOsborn on November 11, 2013 at 11:07 pm      

    Very helpful – thank you.

    Output is:

    [1] 5

    [1] 0 1 1 2 3

  15. By: oliveira on November 28, 2013 at 9:42 pm      


    I do not understand C. When I compile in C the following error appears:

    void hello(int *n)
    int i;
    for(i=0; i < *n; i++) {
    Rprintf("Hello, world!\n");

    [Error]R.h: No such file or directory

    What do I do? You must download this library somewhere?

  16. By: oliveira on November 28, 2013 at 9:44 pm      

    thank you!

  17. By: dmcglinn on November 30, 2013 at 11:17 pm      

    Hey Oliveira, Glad this was helpful to you. It sounds like you figured out your question. Let me know if you still have an error when you try to compile the C code.


  18. By: Roger DeAngelis on December 28, 2013 at 5:34 pm      

    The devil is in the details. I was close after about 4 hrs of trying. You probably saved me another 4hrs. I had forgot to add the i386 folder to the path.


  19. By: David on February 20, 2014 at 12:11 pm      

    note, if you are on a 64bit machine change the path to :


  20. By: dmcglinn on February 20, 2014 at 10:56 pm      

    Thanks David!

  21. By: Lynh on February 26, 2014 at 11:37 am      


    when I call C code in R with:

    > dyn.load(“sequence_examples.dll”).

    The error:
    Error in inDL(x, as.logical(local), as.logical(now), …) :
    unable to load shared object ‘C:/Users/Lynh/Documents/sequence_examples.dll’:
    LoadLibrary failure: %1 is not a valid Win32 application.

    Could you help me,
    Thank you very much

  22. By: dmcglinn on February 27, 2014 at 1:45 pm      

    Hey Lynh, I’m not sure right off the bat but it could be because you compiled the code as a 64-bit application but are running 32-bit version of R to try to load the .dll. See David’s comment up above where he points out how to specify the path for the 64-bit version of R rather than the 32-bit version that I have provided instructions for. Did you get any errors when you ran R CMD SHLIB sequence_examples.c ?

  23. By: Jayalal on March 10, 2014 at 2:16 am      

    Hello Daniel McGlinn,
    I am so thankful to you; such a nice and useful info.
    I was desperately trying to call C functions from my R environment. Today I got it, with the help of you.
    Now I am proceeding further.
    A PhD (in Computer Science) student from Chennai, India

  24. By: Jayalal on March 10, 2014 at 3:28 am      

    Hi all,
    I got used to it and it started working for my Windows PC.
    Now the next step: How can I get the returned values, it to my R environment.
    Please help me.

  25. By: Gilles on March 31, 2014 at 12:04 pm      

    tks for all these information.
    Unfortunately, I have some problem for compiling my C code (I was able to compile this code without problem before using a different PC in 32bits).

    My path is:

    My version of R is the most recent one in 64 bits: 3.0.3 Platform: x86_64-w64-mingw32/x64 (64-bit)

    When trying to compile my C code in DOS, it says
    cygwin warning:
    MS-DOS style path detected: C:/R/R-3.0.3/etc/x64/Makeconf
    Preferred POSIX equivalent is : /cygdrive/c/R/R-3.0.3/etc/x64/Makeconf
    CYGWIN environment variable option “nodefilewarning” turns off this warning.
    Consult the user’s guide about POSIW paths

    make: Nothing to be done for ‘all’

    There is no error but none dll file is created.
    The Cygwin warning looks just like Lisa got (see post above). So it should not be a problem for creating dll file.
    Any idea of what could be the problem?
    Thanks a lot

  26. By: dmcglinn on April 16, 2014 at 1:43 pm      

    Hey Jayalal,

    see bullet #4 in the tutorial which covers the loading and calling of the C code. In the examples in the tutorial the returned values (e.g., ‘sumVect’) are accessed as such:
    .C(“sumSeq”, start = as.integer(10), size = as.integer(5), sumVect = as.integer(rep(0, 5)))$sumVect

    So we input rep(0, 5) for the sumVect object to the C-code and it will return a 5 integer long vector that has the vector sums we want.

  27. By: dmcglinn on April 16, 2014 at 1:47 pm      

    Hmm, could you please try to do this without using cygwin. Instead use the windows dos prompt. To get that to pull up just type cmd in the Windows start search box. For other tasks I’ve always found that Cygwin makes a task too complicated to the point where I have to abandon it. As a side note, my favorite program for using the terminal on windows is the Git Bash terminal – http://git-scm.com/downloads

  28. By: Mauricio on May 28, 2014 at 7:00 pm      

    Hello, thank you for your detailed tutorial. I was careful to follow all of your instructions, but for some reason I get this message when compiling:

    C:\CLearn>R CMD SHLIB sequence_examples.c
    Warning message:
    running command ‘make -f “c:/R/R-3.1.0/etc/x64/Makeconf” -f “c:/R/R-3.1.0/share/
    make/winshlib.mk” SHLIB=”sequence_examples.dll” WIN=64 TCLBIN=64 OBJECTS=”sequen
    ce_examples.o”‘ had status 127

    None of the .o or .dll files appear in the folder. I haven’t been able to find help online. Do you have any idea of why this could be?, I’m working on Windows 8.1 and downloaded the most recent Rtools.

    Thank you, I appreciate any help!


  29. By: Dan D on July 3, 2014 at 5:27 pm      


    It looks like everyone is having so much fun calling C functions from R, but all I get is Lynh’s error:
    “unable to load shared object ‘c:/program files/r/r-3.0.2/bin/sequence.dll’:
    LoadLibrary failure: %1 is not a valid Win32 application.”


  30. By: John on August 7, 2014 at 1:26 am      

    This is a very helpful post. However, you only treat standalone functions. Have you tried functions that call other functions? That would probably be necessary in any reasonable complex real world application.

  31. By: dmcglinn on September 2, 2014 at 7:26 pm      

    Hey John,

    Good question. It is not too much more difficult to use R to interact with a C function that calls other C functions. The biggest complication is knowing which variables in your C functions as pointer variables. If I have a chance I’ll dig up an example to post here.


  32. By: jonas on December 9, 2014 at 7:40 am      

    Two issues to watch out for:


    does not work (at least on a WIN8.1 machine).

    the device letters have to be capital:


    #2 if you are running R on x64, make sure to selct the appropriate folder (x64 instead of i386)

  33. By: ISEStudent on February 10, 2015 at 7:14 pm      

    Thanks a lot for the helpful post!

  34. By: Perikles on March 4, 2015 at 3:04 pm      

    Thanks Daniel for the very useful tutorial!

    I’m working on a 64-bit machine, and in R console (R i386 3.1.2) it’s working fine, but not in the more useful RStudio, where I’m getting “LoadLibrary failure: %1 is not a valid Win32 application”

    Probably a no-brainer, but how can I get it to work in RStudio?

  35. By: dmcglinn on March 9, 2015 at 9:42 am      

    In general there are not differences between standard R and the R that R studio uses; however, you have configured your Rstudio to use the 64 bit version of R where as you used the 32 bit version of R to compile the .C code. The thing to check is which version of R you added to the path. If you want to run 64 bit R then you’ll need the 64 bit version of R on your path when you compile. I mention this above. Good luck!

  36. By: Collins on April 1, 2015 at 1:56 am      

    Dan, you are amazing. It works for me.



  37. By: Soheil on April 11, 2015 at 6:43 am      

    Many thanks. Such a step-by-step guide was really useful for me as a beginner, plus the fact that you’ve mentioned almost every single detail that might be needed.
    Thank you so much.

Leave a response

Your response: