产生在运行时函数用C [英] Generating functions at runtime in C

问题描述

我想生成在运行时C.一个函数，我的意思是我想基本上是想在它分配一些内存，点，通过函数指针执行它。我意识到这是一个非常复杂的话题，我的问题是幼稚的。我也意识到有一些非常强大的图书馆，在那里，做到这一点（如 nanojit ）。

不过，我想学的技术，从最基本的开始。可能有人懂行的给我一个很简单的例子用C？

编辑： 下面的答案是伟大的，但这里是Windows的同一个例子：的

 的#include＆LT;＆WINDOWS.H GT;MEMSIZE的#define 100 * 1024 * 1024
无效的typedef（* func_t）（无效）;诠释主（）{    HANDLE PROC = GetCurrentProcess（）;
    LPVOID P =的VirtualAlloc（
        空值，
        MEMSIZE，
        MEM_RESERVE | MEM_COMMIT，
        PAGE_EXECUTE_READWRITE）;    func_t FUNC =（func_t）P;
    PDWORD code =（PDWORD）P;
    code [0] = 0xC3; // RET    如果（FlushInstructionCache（
        PROC，
        空值，
        0））
    {
        FUNC（）;
    }    CloseHandle的（PROC）;
    VirtualFree（对，0，MEM_RELEASE）;
    返回0;
}
 

解决方案

正如所说previously其他的海报，你需要知道你的平台pretty好。

忽略铸造对象指向一个函数指针存在的问题，在技术上，UB，这里是对于在x86 / x64 OS X（以及可能的Linux太）工作的例子。所有生成code不被返回给调用者。

 的#include＆LT;＆unistd.h中GT;
＃包括LT＆; SYS / mman.h＆GT;无效的typedef（* func_t）（无效）;诠释主（）{
    / *
     *获取的内存RWX位。
     *我们不能只用malloc因为它返回的存储可能不
     *是可执行的。
     * /
    无符号字符* code = MMAP（NULL，为getpagesize（）
            PROT_READ | PROT_EXEC | PROT_WRITE，
            MAP_SHARED | MAP_ANON，0，0）;    / *技术上未定义的行为* /
    func_t FUNC =（func_t）code;    code [0] = 0xC3; / * 86RET指令* /    FUNC（）;    返回0;
}
 

显然，这将是在不同的平台不同，但它概述所需的基本知识：获取可执行内存部分，写指令，执行指令

I would like to generate a function at runtime in C. And by this I mean I would essentially like to allocate some memory, point at it and execute it via function pointer. I realize this is a very complex topic and my question is naïve. I also realize there are some very robust libraries out there that do this (e.g. nanojit).

But I would like to learn the technique, starting with the basics. Could someone knowledgeable give me a very simple example in C?

EDIT: The answer below is great but here is the same example for Windows:

#include <Windows.h>

#define MEMSIZE 100*1024*1024
typedef void (*func_t)(void);

int main() {

    HANDLE proc = GetCurrentProcess();
    LPVOID p = VirtualAlloc(
        NULL,
        MEMSIZE,
        MEM_RESERVE|MEM_COMMIT,
        PAGE_EXECUTE_READWRITE);

    func_t func = (func_t)p;
    PDWORD code = (PDWORD)p;
    code[0] = 0xC3; // ret

    if(FlushInstructionCache(
        proc,
        NULL,
        0))
    {
        func();
    }

    CloseHandle(proc);
    VirtualFree(p, 0, MEM_RELEASE);
    return 0;
}

解决方案

As said previously by other posters, you'll need to know your platform pretty well.

Ignoring the issue of casting a object pointer to a function pointer being, technically, UB, here's an example that works for x86/x64 OS X (and possibly Linux too). All the generated code does is return to the caller.

#include <unistd.h>
#include <sys/mman.h>

typedef void (*func_t)(void);

int main() {
    /*
     * Get a RWX bit of memory.
     * We can't just use malloc because the memory it returns might not
     * be executable.
     */
    unsigned char *code = mmap(NULL, getpagesize(),
            PROT_READ|PROT_EXEC|PROT_WRITE,
            MAP_SHARED|MAP_ANON, 0, 0);

    /* Technically undefined behaviour */
    func_t func = (func_t) code;

    code[0] = 0xC3; /* x86 'ret' instruction */

    func();

    return 0;
}

Obviously, this will be different across different platforms but it outlines the basics needed: get executable section of memory, write instructions, execute instructions.

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