Embedding PyPy

PyPy has a very minimal and a very strange embedding interface, based on the usage of cffi and the philosophy that Python is a better language than C. It was developed in collaboration with Roberto De Ioris from the uwsgi project. The PyPy uwsgi plugin is a good example of using the embedding API.

NOTE: As of 1st of December, PyPy comes with --shared by default on linux, linux64 and windows. We will make it the default on all platforms by the time of the next release.

The first thing that you need is to compile PyPy yourself with the option --shared. We plan to make --shared the default in the future. Consult the how to compile PyPy doc for details. This will result in libpypy.so or pypy.dll file or something similar, depending on your platform. Consult your platform specification for details.

The resulting shared library exports very few functions, however they are enough to accomplish everything you need, provided you follow a few principles. The API is:

void rpython_startup_code(void);

This is a function that you have to call (once) before calling anything else. It initializes the RPython/PyPy GC and does a bunch of necessary startup code. This function cannot fail.

void pypy_init_threads(void);

Initialize threads. Only need to be called if there are any threads involved

long pypy_setup_home(char* home, int verbose);

This function searches the PyPy standard library starting from the given “PyPy home directory”. It is not strictly necessary to execute it before running Python code, but without it you will not be able to import any non-builtin module from the standard library. The arguments are:

  • home: NULL terminated path to an executable inside the pypy directory (can be a .so name, can be made up)
  • verbose: if non-zero, it will print error messages to stderr

Function returns 0 on success or -1 on failure, can be called multiple times until the library is found.

int pypy_execute_source(char* source);

Execute the Python source code given in the source argument. In case of exceptions, it will print the Python traceback to stderr and return 1, otherwise return 0. You should really do your own error handling in the source. It’ll acquire the GIL.

int pypy_execute_source_ptr(char* source, void* ptr);

Note

Not available in PyPy <= 2.2.1

Just like the above, except it registers a magic argument in the source scope as c_argument, where void* is encoded as Python int.

void pypy_thread_attach(void);

In case your application uses threads that are initialized outside of PyPy, you need to call this function to tell the PyPy GC to track this thread. Note that this function is not thread-safe itself, so you need to guard it with a mutex.

Simple example

Note that this API is a lot more minimal than say CPython C API, so at first it’s obvious to think that you can’t do much. However, the trick is to do all the logic in Python and expose it via cffi callbacks. Let’s assume we’re on linux and pypy is installed in /opt/pypy with the library in /opt/pypy/bin/libpypy-c.so. (It doesn’t need to be installed; you can also replace this path with your local checkout.) We write a little C program:

#include "include/PyPy.h"
#include <stdio.h>

const char source[] = "print 'hello from pypy'";

int main()
{
  int res;

  rpython_startup_code();
  // pypy_setup_home() is not needed in this trivial example
  res = pypy_execute_source((char*)source);
  if (res) {
    printf("Error calling pypy_execute_source!\n");
  }
  return res;
}

If we save it as x.c now, compile it and run it with:

fijal@hermann:/opt/pypy$ gcc -o x x.c -lpypy-c -L.
fijal@hermann:/opt/pypy$ LD_LIBRARY_PATH=. ./x
hello from pypy

Worked!

Note

If the compilation fails because of missing PyPy.h header file, you are running PyPy <= 2.2.1, please see the section Missing PyPy.h.

Missing PyPy.h

Note

PyPy.h is in the nightly builds and goes to new PyPy releases (>2.2.1).

For PyPy <= 2.2.1, you can download PyPy.h from PyPy repository (it has been added in commit c4cd6ec):

cd /opt/pypy/include
wget https://bitbucket.org/pypy/pypy/raw/c4cd6eca9358066571500ac82aaacfdaa3889e8c/include/PyPy.h

More advanced example

Note

This example depends on pypy_execute_source_ptr which is not available in PyPy <= 2.2.1. You might want to see the alternative example below.

Typically we need something more to do than simply execute source. The following is a fully fledged example, please consult cffi documentation for details. It’s a bit longish, but it captures a gist what can be done with the PyPy embedding interface:

#include "include/PyPy.h"
#include <stdio.h>

char source[] = "from cffi import FFI\n\
ffi = FFI()\n\
@ffi.callback('int(int)')\n\
def func(a):\n\
    print 'Got from C %d' % a\n\
    return a * 2\n\
ffi.cdef('int callback(int (*func)(int));')\n\
c_func = ffi.cast('int(*)(int(*)(int))', c_argument)\n\
c_func(func)\n\
print 'finished the Python part'\n\
";

int callback(int (*func)(int))
{
    printf("Calling to Python, result: %d\n", func(3));
}

int main()
{
    int res;
    void *lib, *func;

    rpython_startup_code();
    res = pypy_setup_home("/opt/pypy/bin/libpypy-c.so", 1);
    if (res) {
        printf("Error setting pypy home!\n");
        return 1;
    }
    res = pypy_execute_source_ptr(source, (void*)callback);
    if (res) {
        printf("Error calling pypy_execute_source_ptr!\n");
    }
    return res;
}

you can compile and run it with:

fijal@hermann:/opt/pypy$ gcc -g -o x x.c -lpypy-c -L.
fijal@hermann:/opt/pypy$ LD_LIBRARY_PATH=. ./x
Got from C 3
Calling to Python, result: 6
finished the Python part

As you can see, we successfully managed to call Python from C and C from Python. Now having one callback might not be enough, so what typically happens is that we would pass a struct full of callbacks to pypy_execute_source_ptr and fill the structure from Python side for the future use.

Alternative example

As pypy_execute_source_ptr is not available in PyPy 2.2.1, you might want to try an alternative approach which relies on -export-dynamic flag to the GNU linker. The downside to this approach is that it is platform dependent.

#include "include/PyPy.h"
#include <stdio.h>

char source[] = "from cffi import FFI\n\
ffi = FFI()\n\
@ffi.callback('int(int)')\n\
def func(a):\n\
    print 'Got from C %d' % a\n\
    return a * 2\n\
ffi.cdef('int callback(int (*func)(int));')\n\
lib = ffi.verify('int callback(int (*func)(int));')\n\
lib.callback(func)\n\
print 'finished the Python part'\n\
";

int callback(int (*func)(int))
{
    printf("Calling to Python, result: %d\n", func(3));
}

int main()
{
    int res;
    void *lib, *func;

    rpython_startup_code();
    res = pypy_setup_home("/opt/pypy/bin/libpypy-c.so", 1);
    if (res) {
        printf("Error setting pypy home!\n");
        return 1;
    }
    res = pypy_execute_source(source);
    if (res) {
        printf("Error calling pypy_execute_source!\n");
    }
    return res;
}

Make sure to pass -export-dynamic flag when compiling:

$ gcc -g -o x x.c -lpypy-c -L. -export-dynamic
$ LD_LIBRARY_PATH=. ./x
Got from C 3
Calling to Python, result: 6
finished the Python part

Finding pypy_home

Function pypy_setup_home takes one parameter - the path to libpypy. There’s currently no “clean” way (pkg-config comes to mind) how to find this path. You can try the following (GNU-specific) hack (don’t forget to link against dl):

#if !(_GNU_SOURCE)
#define _GNU_SOURCE
#endif

#include <dlfcn.h>
#include <limits.h>
#include <stdlib.h>

// caller should free returned pointer to avoid memleaks
// returns NULL on error
char* guess_pypyhome() {
    // glibc-only (dladdr is why we #define _GNU_SOURCE)
    Dl_info info;
    void *_rpython_startup_code = dlsym(0,"rpython_startup_code");
    if (_rpython_startup_code == 0) {
        return 0;
    }
    if (dladdr(_rpython_startup_code, &info) != 0) {
        const char* lib_path = info.dli_fname;
        char* lib_realpath = realpath(lib_path, 0);
        return lib_realpath;
    }
    return 0;
}

Threading

In case you want to use pthreads, what you need to do is to call pypy_thread_attach from each of the threads that you created (but not from the main thread) and call pypy_init_threads from the main thread.