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用C語言模仿Python函數的一種簡單實現方法

投稿：mrr

首先得說明一點，C 語言不是函數式編程語言，要想進行完全的函數式編程，還得先寫個虛擬機，然後再寫個解釋器才行（相當於 CPython ）。

下面我們提供一個例子，說明 C 語言函數可以“適度地模仿” Python 函數。

我們有如下的 Python 程序：

def line_conf(a, b):
  def line(x):
    return a*x + b
  return line
line1 = line_conf(1, 1)
line2 = line_conf(4, 5)
print(line1(5), line2(5))

我們在C程序中適度地模擬其中的line_conf函數：

/* MIT License
Copyright (c) 2017 Yuandong-Chen
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE. */
///////////////////////////////////////////////////////////////////////////////
// Note: The C program is almost equivalent to the Python program as follows:
// def line_conf(a, b):
//   def line(x):
//     return a*x + b
//   return line
//
// line1 = line_conf(1, 1)
// line2 = line_conf(4, 5)
// print(line1(5), line2(5))
#include <stdio.h> 
#include <stdlib.h> 
#include <unistd.h> 
#include <stdarg.h>
typedef int Func();
Func *line_conf(int x, int y,...)
{ 
  va_list ap; 
  va_start(ap, y);
  asm volatile(
    "push %%eax\n\t"
    "subl $40, %%esp\n\t"
    "movl 8(%%ebp), %%eax\n\t"
    "movl %%eax, -36(%%ebp)\n\t"
    "movl 12(%%ebp), %%eax\n\t"
    "movl %%eax, -40(%%ebp)\n\t"
    "addl $40, %%esp\n\t"
    "pop %%eax\n\t"
    :::"memory"
    );
if(va_arg(ap,int) == 1){
LINE:
  asm volatile(
    "push %%ebp\n\t"
    "movl %%esp, %%ebp\n\t"
    "movl 8(%%ebp), %%eax\n\t"
    "imul -36(%%ebp), %%eax\n\t"
    "addl -40(%%ebp), %%eax\n\t"
    "movl %%ebp, %%esp\n\t"
    "pop %%ebp\n\t"
    "ret\n\t"
    :::"memory","%eax"
    );
}  
__END: 
  va_end(ap);
  return (Func *)(&&LINE);
}
int main(int argc, const char *argv[]){ 
  printf("====TEST START====\n");
  printf("34*234+6 ?= %d\n",line_conf(34,6)(234));
  printf("1*3+2 ?= %d; 324*65+3 ?= %d; 13*66+2 ?= %d\n",line_conf(1,2)(3),line_conf(324,3)(65),line_conf(13,2)(66));
  int fd = line_conf(1,6)(4);
  Func *fun = line_conf(3,3);
  int a = 1; // Limited point
  printf("3*3+3 ?= %d; 1*4+6 ?= %d\n",fun(3),fd);
  printf("====TEST END====\n");
  return 0; 
}
// Compile it by the following command:
// gcc -m32 -O0 -fno-stack-protector CFunctional.c; ./a.out
// The terminal output should looks like:
// ====TEST START====
// 34*234+6 ?= 7962
// 1*3+2 ?= 5; 324*65+3 ?= 21063; 13*66+2 ?= 860
// 3*3+3 ?= 12; 1*4+6 ?= 10
// ====TEST END====
//Note: The limitation happens between line 86 and line 88, we cannot insert any function here
// whose stack is larger than 40 bytes.(Why is 40? check the inline assembler language)

結果在MacOSX和Ubuntu上（i386）都能通過簡單的測試。但是可以看到，僅僅是簡單的模擬，我們也得用到大量（按比例）的匯編，可讀性很差，而且模擬程度非常有限，代碼長度也更長。相反，對於這類一般功能的函數，Python可以很容易地模擬C語言的函數，而且模擬程度很高。

以上所述是小編給大家介紹的用C語言模仿Python函數的一種簡單實現方法，希望對大家有所幫助，如果大家有任何疑問歡迎給我留言。