Julia calling C: A minimal example

This blog is a “Hello World” example of Julia calling C.

We start of by at bit of C code we want to call from Julia. We write the following in calc_mean.c

double mean(double a, double b) {
  return (a+b) / 2;

To build the library, we need to create a Makefile

CFLAGS=-c -Wall -fPIC
    $(CC) $(CFLAGS) $< -o $@ 
lib: $(OBJECTS)
    $(CC) -shared -fPIC -o libmean.so $(OBJECTS)
    rm *.o *.so

The option fPIC and -shared are essential for Julia to be able to resolve the function in our library. Now we are almost ready to build our library. From the bash terminal we invoke:

make lib

This will generate a libmean.so file.

In Julia we call the function in our c library by


For this to work,

  • Julia must be running either on the same path where libmean.so resides,
  • the path to libmean.so is in LD_LIBRARY_PATH, or
  • the path to the library is pushed to Libdl.DL_LOAD_PATH via


P.S. Thanks to Christopher Rackauckas for tips on Julia highlighting.

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