从C中的二进制文件调用函数(main()) [英] Invoking a function (main()) from a binary file in C

问题描述

我有简单的c程序，例如my_bin.c:

I have simple c program like, my_bin.c:

#include <stdio.h>

int main()
{
    printf("Success!\n");
    return 0;
}

我用gcc编译并获得可执行文件:my_bin.

I compile it with gcc and got executable: my_bin.

现在，我想使用另一个C程序调用main(或运行此my_bin).我对mmap和函数指针所做的操作如下:

Now I want to invoke main (or run this my_bin) using another C program. That I did with mmap and function pointer like this:

#include <stdio.h>
#include <fcntl.h>
#include <sys/mman.h>
int main()
{
   void (*fun)();
   int fd;
   int *map;
   fd = open("./my_bin", O_RDONLY);
   map = mmap(0, 8378, PROT_READ, MAP_SHARED, fd, 0);
   fun = map;
   fun();
   return 0;
}

添加了PROT_EXEC 从响应中使其更加清晰... 我想在第二个程序中调用一个外部二进制程序.

EDIT 1: added PROT_EXEC Making it more clear from responses ... I want to call an external binary program within second program.

我不知道如何使用main(其他程序)的地址初始化函数指针.有什么主意吗?

I don't know how to initialize function pointer with the address of main(other program's). any idea?

为什么谷歌搜索后发现段错误，这是因为我的mmap的大小和offset参数.它应该是页面大小的倍数. [参考:在C中使用mmap读取二进制文件时出现段错误

Why seg fault, after googling, figured out, its because of my size and offset argument of mmap. It should be multiple of pagesize. [Reference: Segfault while using mmap in C for reading binary files

现在代码如下:

#include <stdio.h>
#include <fcntl.h>
#include <sys/mman.h>
int main()
{
   void (*fun)();
   int fd;
   int *map;
   int offset = 8378;
   int pageoffset = offset % getpagesize();
   fd = open("./my_bin", O_RDONLY);
   if(fd == -1) {
        printf("Err opening file\n");
        return -1;
   }
   map = mmap(0, 8378 + pageoffset, PROT_READ|PROT_EXEC,
                        MAP_SHARED, fd, offset - pageoffset);
   perror("Err\n"); //This is printing err and Success!
   //fun = map; // If I uncomment this and 
   //fun();     // this line then, still it 
                // print err and Success! from perror
                // but later it says Illegal instruction. 
   return 0;
}

仍然带有fun()或没有它仍无法打印...不确定如何提供main()地址.

Still with fun() or without that its not printing ... not sure how to give main() address.

第一件事:我没有正确读取定义，我已经给出了应该从中读取二进制文件的地址. 第二个:mmap:大小和偏移量应为页面大小的倍数.

First thing: I didn't read definition properly, I have already given address from which I should read binary file. Second: mmap: size and offset should be multiple of pagesize.

推荐答案

main()通常不是C程序中的第一个函数.链接器将在此之前放置一些设置/初始化代码，其中包括设置环境，获取命令行参数，将其解析为字符串数组之类的东西.

main() usually isn't the first function in a C program. The linker will put some setup/init code before that which, among other things, will set up the environment, get the command line arguments, parse them into a string array, stuff like that.

新的main()函数开始设置内存分配例程时会出现问题-基本上，这将破坏主应用程序的所有重要数据结构.

It gets problematic when the new main() function starts to set up the memory allocation routines - basically, this will ruin all the important data structures of your main application.

如果要执行功能(即不使用main())，则将C代码编译到共享库中，并使用dlopen()或操作系统的等效库进行加载.

If you want to execute a function (i.e. without main()), then compile your C code into a shared library and load that with dlopen() or your OS's equivalent.

如果您确实需要main()，请使用fork()和exec().

If you really need main(), use fork() and exec().

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