问题描述
我从我的同事那里了解到，无需编写 main() 函数即可编写和执行 C 程序.可以这样做:
I learned from my colleague that one can write and execute a C program without writing a main() function. It can be done like this:
my_main.c
/* Compile this with gcc -nostartfiles */
#include <stdlib.h>
void _start() {
int ret = my_main();
exit(ret);
}
int my_main() {
puts("This is a program without a main() function!");
return 0;
}
用这个命令编译它:
gcc -o my_main my_main.c –nostartfiles
使用以下命令运行它:
./my_main
什么时候需要做这种事情?是否有任何真实世界的场景可以使用这种方法?
When would one need to do this kind of thing? Is there any real world scenario where this would be useful?
推荐答案
符号 _start 是程序的入口点.也就是说，那个符号的地址就是程序开始时跳转到的地址.通常，名为 _start 的函数由名为 crt0.o 的文件提供，该文件包含 C 运行时环境的启动代码.它设置一些东西，填充参数数组argv，计算有多少参数，然后调用main.main 返回后，exit 被调用.
The symbol _start is the entry point of your program. That is, the address of that symbol is the address jumped to on program start. Normally, the function with the name _start is supplied by a file called crt0.o which contains the startup code for the C runtime environment. It sets up some stuff, populates the argument array argv, counts how many arguments are there, and then calls main. After main returns, exit is called.
如果一个程序不想使用C运行时环境，它需要为_start提供自己的代码.例如，Go 编程语言的参考实现这样做是因为它们需要一个非标准的线程模型，这需要对堆栈进行一些魔术.当您想编写非常小的程序或执行非常规操作的程序时，提供您自己的 _start 也很有用.
If a program does not want to use the C runtime environment, it needs to supply its own code for _start. For instance, the reference implementation of the Go programming language does so because they need a non-standard threading model which requires some magic with the stack. It's also useful to supply your own _start when you want to write really tiny programs or programs that do unconventional things.
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