如何使用cffi嵌入在C语言中返回字符串的Python函数? [英] How can I embed a Python function that returns a string in C using cffi?

问题描述

我正在尝试使用PyPy和cffi将Python函数嵌入C中.我正在遵循PyPy文档中的本指南.

I'm trying to embed a Python function in C using PyPy and cffi. I'm following this guide from the PyPy documentation.

问题是，我发现的所有示例都在ints上运行，并且我的函数接收一个字符串并返回一个字符串.我似乎无法弄清楚如何将此函数嵌入C中，因为C似乎并没有真正的字符串，而是使用char数组.

The problem is, all the examples I've found operate on ints, and my function takes a string and returns a string. I can't seem to figure out how to embed this function in C, as C doesn't seem to really have strings, rather making do with arrays of chars.

这是我尝试过的:

# interface.py

import cffi

ffi = cffi.FFI()
ffi.cdef('''
struct API {
    char (*generate_cool_page)(char url[]);
};
''')

...


@ffi.callback("char[] (char[])")
def generate_cool_page(url):
    # do some processing with BS4
    return str(soup)

def fill_api(ptr):
    global api 
    api = ffi.cast("struct API*", ptr)
    api.generate_cool_page = generate_cool_page

-

// c_tests.c

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

struct API {
    char (*generate_cool_page)(char url[]);
};

struct API api;   /* global var */

int initialize_api(void)
{
    static char source[] =
        "import sys; sys.path.insert(0, '.'); "
        "import interface; interface.fill_api(c_argument)";
    int res;

    rpython_startup_code();
    res = pypy_setup_home(NULL, 1);
    if (res) {
        fprintf(stderr, "Error setting pypy home!\n");
        return -1;
    }
    res = pypy_execute_source_ptr(source, &api);
    if (res) {
        fprintf(stderr, "Error calling pypy_execute_source_ptr!\n");
        return -1;
    }
    return 0;
}

int main(void)
{
    if (initialize_api() < 0)
        return 1;

    printf(api.generate_cool_page("https://example.com"));

    return 0;
}

当我运行gcc -I/opt/pypy3/include -Wno-write-strings c_tests.c -L/opt/pypy3/bin -lpypy3-c -g -o c_tests然后运行./c_tests时，出现此错误:

When I run gcc -I/opt/pypy3/include -Wno-write-strings c_tests.c -L/opt/pypy3/bin -lpypy3-c -g -o c_tests and then run ./c_tests, I get this error:

debug: OperationError:
debug:  operror-type: CDefError
debug:  operror-value: cannot render the type <char()(char *)>: it is a function type, not a pointer-to-function type
Error calling pypy_execute_source_ptr!

我对C没有太多的经验，我觉得我在错误地表示字符串参数/返回值.如何正确执行此操作?

I don't have a ton of experience with C and I feel like I'm misrepresenting the string argument/return value. How do I do this properly?

感谢您的帮助！

推荐答案

请注意，您不应使用pypy弃用的界面进行嵌入；相反，请参见 http://cffi.readthedocs.io/en/latest/embedding.html .

Note that you should not be using pypy's deprecated interface to embedding; instead, see http://cffi.readthedocs.io/en/latest/embedding.html.

C语言没有字符串"，而只有字符数组.在C语言中，通常要编写一个要返回字符串"的函数 不同的是:它接受指向预先存在的缓冲区(类型为char[])的指针作为第一个参数，而接受该缓冲区的长度作为第二个参数；并在调用时填充缓冲区.这可能很杂乱，因为理想情况下，您需要在调用方中处理缓冲区太小的情况，例如分配更大的数组，然后再次调用该函数.

The C language doesn't have "strings", but only arrays of chars. In C, a function that wants to return a "string" is usually written differently: it accepts as first argument a pointer to a pre-existing buffer (of type char[]), and as a second argument the length of that buffer; and when called, it fills the buffer. This can be messy because you ideally need to handle buffer-too-small situations in the caller, e.g. allocate a bigger array and call the function again.

或者，某些函数放弃并返回新的malloc() -ed char *.然后，呼叫者必须记住要free()它，否则会发生泄漏.在这种情况下，我建议使用这种方法，因为在调用之前猜测字符串的最大长度可能很困难.

Alternatively, some functions give up and return a freshly malloc()-ed char *. Then the caller must remember to free() it, otherwise a leak occurs. I would recommend that approach in this case because guessing the maximum length of the string before the call might be difficult.

所以，类似的东西.假设你开始 http://cffi.readthedocs.io/en/latest/embedding.html ， 改变 plugin.h包含::

So, something like that. Assuming you start with http://cffi.readthedocs.io/en/latest/embedding.html, change plugin.h to contain::

// return type is "char *"
extern char *generate_cool_page(char url[]);

并更改plugin_build.py ::

ffibuilder.embedding_init_code("""
    from my_plugin import ffi, lib

    @ffi.def_extern()
    def generate_cool_page(url):
        url = ffi.string(url)
        # do some processing
        return lib.strdup(str(soup))   # calls malloc()
""")
ffibuilder.cdef("""
    #include <string.h>
    char *strdup(const char *);
""")

从C代码中，您根本不需要initialize_api() 新的嵌入模式；相反，您只说#include "plugin.h" 并直接调用该函数:

From the C code, you don't need initialize_api() at all in the new embedding mode; instead, you just say #include "plugin.h" and call the function directly::

char *data = generate_cool_page("https://example.com");
if (data == NULL) { handle_errors... }
printf("Got this: '%s'\n", data);
free(data);   // important!

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