Calling void function before main()

问题

I wanted to know if it was possible to call a void function without using a temp variable. E.g. in the following code block...

#include <iostream>

void earlyInit()
{
  std::cout << "The void before the world." << std::endl;
}

int g_foo = (earlyInit(), 0);

int main( int argc, char* argv[] )
{
  std::cout << "Hello, world!" << std::endl;
}

...I have no need for g_foo and would rather it not exist. Is there a way to call void functions without an intermediate temp variable?

回答1:

I wanted to know if it was possible to call a void function without using a temp variable. E.g. in the following code block.

The language does not provide any such mechanism. As other answers have pointed out, there maybe compiler-specific ways to do that.

However, I don't see anything wrong with your approach. I use the following pattern a lot.

#include <iostream>

namespace mainNS  // A file-specific namespace.
{
   void earlyInit()
   {
      std::cout << "The void before the world." << std::endl;
   }

   struct Initializer
   {
      Initializer();
   };
}

using namespace mainNS;
static Initializer initializer;

Initializer::Initializer()
{
   earlyInit();
   // Call any other functions that makes sense for your application.
}

int main( int argc, char* argv[] )
{
  std::cout << "Hello, world!" << std::endl;
}

回答2:

Check if your compiler supports #pragma startup (or equivalent), eg:

#include <iostream>

void earlyInit()
{
  std::cout << "The void before the world." << std::endl;
}
#pragma startup earlyInit

int main( int argc, char* argv[] )
{
  std::cout << "Hello, world!" << std::endl;
}

回答3:

An answer to this question brought __attribute__((constructor)) to my attention, however, for reasons I don't fully understand, I observed a SIGSEGV when using that if the void function used std::cout (printf did not cause the SIGSEGV).

I posted a version of this question earlier (but stupidly deleted it). An answerer at that time pointed me to this excellent article which discusses the situation and solution:

http://dbp-consulting.com/tutorials/debugging/linuxProgramStartup.html

Excerpt of the solution (modified slightly to pass compilation) is here:

#include <cstdio>
#include <cstdlib>

void preinit(int argc, char * * argv, char * * envp) {
  printf("%s\n", __FUNCTION__);
}
void init(int argc, char * * argv, char * * envp) {
  printf("%s\n", __FUNCTION__);
}

void fini() {
  printf("%s\n", __FUNCTION__);
}
__attribute__((section(".init_array"))) typeof (init) * __init = init;
__attribute__((section(".preinit_array"))) typeof (preinit) * __preinit = preinit;
__attribute__((section(".fini_array"))) typeof (fini) * __fini = fini;

void __attribute__((constructor)) constructor() {
  printf("%s\n", __FUNCTION__);
}

void __attribute__((destructor)) destructor() {
  printf("%s\n", __FUNCTION__);
}

void my_atexit() {
  printf("%s\n", __FUNCTION__);
}

void my_atexit2() {
  printf("%s\n", __FUNCTION__);
}

int main() {
  atexit(my_atexit);
  atexit(my_atexit2);
}

(Apologies to the original answerer that I deleted my original post and can't give due credit.)

来源：https://stackoverflow.com/questions/43150645/calling-void-function-before-main

标签

c++

static-initialization