codon/docs/interop/cpp.md

Calling C/C++ from Codon is quite easy with from C import, but Codon can also be called from C/C++ code. To make a Codon function externally visible, simply annotate it with @export:

@export
def foo(n: int):
    for i in range(n):
        print(i * i)
    return n * n

Note that only top-level, non-generic functions can be exported. Now we can create a shared library containing foo (assuming source file foo.codon):

codon build --relocation-model=pic --lib -o libfoo.so foo.codon

Now we can call foo from a C program (if you're using C++, mark the Codon function as extern "C"):

#include <stdint.h>
#include <stdio.h>

int64_t foo(int64_t);
// In C++, it would be:
// extern "C" int64_t foo(int64_t);

int main() {
  printf("%llu\n", foo(10));
}

Compile:

gcc -o foo -L. -lfoo foo.c  # or g++ if using C++

Now running ./foo should invoke foo() as defined in Codon, with an argument of 10.

Note that if the generated shared library is in a non-standard path, you can either:

• Add the rpath to the gcc command: -Wl,-rpath=/path/to/lib/dir
• Add the library path to LD_LIBRARY_PATH (or DYLD_LIBRARY_PATH if using macOS): export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/path/to/lib/dir.

Converting types

The following table shows the conversions between Codon and C/C++ types:

Codon	C/C++
int	long or int64_t
float	double
bool	bool
byte	char or int8_t
str	{int64_t, char*} (length and data)
tuple	Struct of fields
class	Pointer to corresponding tuple