__attribute__ ((constructor)) decorated functions perform __ functions before the main function

Recent research on QEMU has similar code when initializing:

#define MODULE_INIT (function, type)                                         \
static void __attribute__ ((constructor)) Do_qemu_init_ # # function (void ) {  \
    register_module_init (function, type);                                   \
}

do_qemu_init_** must be the initialization of the module, but did not call the Do_qemu_init place, odd strange, why.

Take a closer look at this function and the modifier contains __attribute__ (constructor), which is probably what this guy did.

Write code test:

#include <stdio.h>
#include <stdlib.h>


void static __attribute__ ((constructor)) Before_main ()
{
    printf ("before main\n");
}

void Static __attribute__ ((destructor)) After_main ()
{
    printf ("after main\n");
}

int main (int argc, char** argv)
{
    printf ("Hello world!\n");
}

Compile execution:

root@mothership:/home/source/test# gcc a.c-o a
root@mothership:/home/source/test#./A
before main
Hello world!
After main
root@mothership:/home/source/test#

Sure enough,

__attribute__ ((constructor))-decorated functions are executed before the main function

__attribute__ ((destructor))-decorated functions are executed after the main function