Call to undeclared library function 'memset', Pyodide support

[HexDecimal]

When trying to see if my library can be built using Pyodide I encountered a relatively minor issue: memset is from <string.h> which is not included in the source file. There were no other warnings or errors caused by cffi. I tested the latest stable version and main with no differences between them.

https://github.com/libtcod/python-tcod/actions/runs/15768029104/job/44447935001

  build/temp.emscripten_3_1_58_wasm32-cpython-312/tcod._libtcod.c:520:5: error: call to undeclared library function 'memset' with type 'void *(void *, int, unsigned long)'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    520 |     memset((void *)p, 0, (size_t)datasize);
        |     ^
  build/temp.emscripten_3_1_58_wasm32-cpython-312/tcod._libtcod.c:520:5: note: include the header <string.h> or explicitly provide a declaration for 'memset'

I would've added #include <string.h> into ffi_builder.set_source but the error happens before the inserted code. I'm also unable to suppress or ignore this error.

I may be able to make a PR for this. My goal is to have a CFFI out-of-line API mode project successfully build for Pyodide in cibuildwheel. I can test PR's and branches.

[mattip]

I think this happens from

• memset is used in the generated code before code included via set_source
• The generated code does include "Python.h", which has this (at least for Python3.11):

  // stdlib.h, stdio.h, errno.h and string.h headers are not used by Python
  // headers, but kept for backward compatibility. They are excluded from the
  // limited C API of Python 3.11.
  #if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 < 0x030b0000
  #  include <stdlib.h>
  #  include <stdio.h>              // FILE*
  #  include <errno.h>              // errno
  #  include <string.h>             // memcpy()
  #endif
  

Are you using the limited API?

[HexDecimal]

Are you using the limited API?

Yes, I am using the Limited API. I'll see what happens when I disable it.

[HexDecimal]

Disabling the limited API fixes the issue with memset.

[mattip]

Right, so probably need to review the places we have #include <Python.h> and add a #include <string.h>, like here

cffi/src/cffi/_cffi_include.h

Lines 57 to 61 in 92645ec

	#include <Python.h>
	#ifdef __cplusplus
	extern "C" {
	#endif
	#include <stddef.h>

and here

cffi/src/cffi/vengine_cpy.py

Lines 819 to 820 in 92645ec

	#include <Python.h>
	#include <stddef.h>

[HexDecimal]

Specifically the Py_LIMITED_API definition was directly from my own build scripts to ensure that the limited API was enabled. I can use "PYODIDE" in os.environ to conditionally skip this definition without affecting other platforms.

I now have working Pyodide wheels, or at least they compile. It's easy enough for me to reproduce and check the issue with any new cffi revision.

[mattip]

Hmm. Maybe PYODIDE or some other marker should be defined by default when using CFFI on that platform?

[HexDecimal]

The PYODIDE environment marker is set by Pyodide during package setup/build to indicate that the package is being built for Emscripten (because sys.platform can not be "emscripten" unless the script is actually running on the platform, not during cross-compilation).
https://pyodide.org/en/stable/usage/faq.html#how-to-detect-that-code-is-run-with-pyodide

To be extra clear, this is that my build code looks like:

Py_LIMITED_API = 0x03100000
define_macros: list[tuple[str, Any]] = []  # For ffi_builder.set_source

if "PYODIDE" not in os.environ:  # Current workaround
    define_macros.append(("Py_LIMITED_API", Py_LIMITED_API))  # Explicitly setting Py_LIMITED_API on regular builds