Julia ccall: could not find function in library

Question

I am trying to use ccall to call functions from a shared library I created. Currently, when I try to run ccall, I get an error:

ERROR: ccall: could not find function add in library libbar
 in anonymous at no file
 in include at boot.jl:244
 in include_from_node1 at loading.jl:128
 in process_options at client.jl:282
 in _start at client.jl:351
while loading /somedir/juliatest.jl, in expression starting on line 2

However, when I view the contents of libbar.so, the function is listed:

... 000000000000057c T add(float, float) ...

Here is my setup:

// bar.hpp
#ifndef __cplusplus
extern "C" {
#endif

extern float add(float a, float b);

#ifndef __cplusplus
}
#endif

// bar.cpp
#include "bar.hpp"
float add(float a, float b)
{
return a+b;
}

Here is how I compile it:

g++ -Wall -fPIC -c bar.cpp
gcc -shared -o libbar.so -Wl,-soname,libbar.so.1 -o libbar.so.1.0 bar.o
sudo mv libbar.so.1.0 /opt/lib
sudo ln -sf /opt/lib/libbar.so.1.0 /opt/lib/libbar.so.1
sudo ln -sf /opt/lib/libbar.so.1.0 /opt/lib/libbar.so

Here is my julia script:

println("Running Test Function")
shouldBeThree = ccall( (:add, "libbar"), Float32, (Float32, Float32), 1.0, 2.0)
println("Should be Three: ", shouldBeThree)

Answer 1

You are compiling with g++. Therefore __cplusplus is defined, and your extern "C" is excluded by the preprocessor.

Answer 2

The problem is that you use #ifndef instead of #ifdef in the header file.

// bar.hpp
#ifdef __cplusplus
extern "C" {
#endif

float add(float a, float b);

#ifdef __cplusplus
}
#endif

With that change the file should compile with both C and C++ compilers.