Call to undeclared library function 'memset', Pyodide support

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.

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?

Are you using the limited API?

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

Disabling the limited API fixes the issue with memset.

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>

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.

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

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

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Call to undeclared library function 'memset', Pyodide support #179

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

Call to undeclared library function 'memset', Pyodide support #179

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