__attribute__((constructor)) and __attribute__((destructor)) syntaxes in C

Write two functions in C using GCC compiler, one of which executes before main function and other executes after the main function.

GCC specific syntaxes :

1. __attribute__((constructor)) syntax : This particular GCC syntax, when used with a function, executes the same function at the startup of the program, i.e before main() function.

2. __attribute__((destructor)) syntax : This particular GCC syntax, when used with a function, executes the same function just before the program terminates through _exit, i.e after main() function.

Explanation :
The way constructors and destructors work is that the shared object file contains special sections (.ctors and .dtors on ELF) which contain references to the functions marked with the constructor and destructor attributes, respectively. When the library is loaded/unloaded, the dynamic loader program checks whether such sections exist, and if so, calls the functions referenced therein.

Few points regarding these are worth noting :
1. __attribute__((constructor)) runs when a shared library is loaded, typically during program startup.
2. __attribute__((destructor)) runs when the shared library is unloaded, typically at program exit.
3. The two parentheses are presumably to distinguish them from function calls.
4. __attribute__ is a GCC specific syntax;not a function or a macro.

Driver code :

// C program to demonstrate working of
// __attribute__((constructor)) and
// __attribute__((destructor))
#include<stdio.h>

// Assigning functions to be executed before and
// after main()
void __attribute__((constructor)) calledFirst();
void __attribute__((destructor)) calledLast();

void main()
{
    printf("\nI am in main");
}

// This function is assigned to execute before
// main using __attribute__((constructor))
void calledFirst()
{
    printf("\nI am called first");
}

// This function is assigned to execute after
// main using __attribute__((destructor))
void calledLast()
{
    printf("\nI am called last");
}

Output:

I am called first
I am in main
I am called last

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