通过函数指针定义“main”

Question

In C++, is it possible for main to be defined by a function pointer? For example:

int f(int, char**) {
    return 0;
}

int (*main)(int, char**) = &f;

This code compiles and links correctly, but triggers a segmentation fault when it's run. I believe that may be because it's trying to execute the value of the function pointer as code.

Additionally, if it's not possible in plain C++, then can it be achieved by non-standard features of gcc (maybe somehow changing the type of the exported symbol).

Finally, if it can't be achieved with gcc directives, can it be done with a custom linker script?

Answer 1

Paragraph 3.6.1/1 of the C++ Standard says:

"A program shall contain a global function called main, which is the designated start of the program"

This makes the pointer declaration illegal.

Answer 2

I believe what you try to do is not possible in c++. Easiest solution I can think of is to simply call f in your main:

int f(int, char**) {
    return 0;
}

int main(int argc, char** argv) {
  return f(argc, argv);
}

I also can't imagine a scenario where the above solution will not do.

Answer 3

I can't imagine why you would want this, but no, it's definitely not possible with plain C++.

If you really need something like this, just use a function pointer that you call as the first command in main.

Answer 4

With GCC you can pass options to the linker with the -Wl option. Then you can use the --entry option of the linker to specify which function should be called first.

WARNING: This will not do what you expect! The entry function of a program is actually not the main function, but another function which sets up the run-time environment and then call your main function.

Answer 5

If you are on linux, & using gcc, you can declare main as an alias to any other function.

eg, below code works (with gcc at least, not tested with g++):

int f(int argc, char* argv[]) {
    printf("hello world\n");
    return 0;
}

int main(int argc, char* argv[]) __attribute__ ((alias("f")));

Answer 6

main is a keyword in C as well as in C++ and gets overwritten as the function pointer name in your program. main defines the entry point of execution. Hence your program does not have that function and overwrites main it segfaults.

Answer 7

Another "dirty" way is to create a fake function using assembly & as the first instruction of that function, jump to f .

Below is a gcc specific code for intel target:

asm(".globl main");           // main is not static
asm(".type main, @function"); // main is a 'function'
asm("main: jmp f");           // main starts here. jump to `f`

int g(){ //Some random function may be present in between "main" & "f"...
    puts("hello universe");
}

int f(int argc, char* argv[]) {
    int i;
    puts("hello world");
    printf("%d arguments given\n",argc - 1);
    for(i=0;i<argc;i++) puts (argv[i]);
    g();
    return 0;
}

Answer 8

int (*main)(int, char**) = &f;

doesn't look correct.

What about

int (*main)(int, char**) = f;

?