__atrribute__ in C

Take a look at this code for example (in Cygwin master branch /winsup/cygwin/winsup.h)

#include<stdio.h> 


__attribute__((constructor)) void fun1()
{ 
    printf("Hello "); 
} 
__attribute__((destructor)) void fun2() 
{ 
    printf("Worldn"); 
} 
int main(void) 
{
    return 0; 
}

Then gcc xxx.c and then run the code it will print out “Hello World” ~

So why it works?

The code with constructor attribute run before main function and code with destructor attr run after main function

The __attribute__ can do more than above

Another usage of __attribute__ is to customize the section of a certain code , take a look at this example:

 __attributes__((section(".my_custom_section"))) int main(void) { return 0; }

Then compile the code and run readelf tool to check the section in the code:
$ readelf -S a.out

You can see a section called my_custom_section and with AX acess(alloc + executable)

This proves that we can use __attribute__ to customize the section