In GCC C compiler, we have nice feature to execute the certain function before and after main( ). So some startup code can be executed before main( ) starts, and some cleanup code can be executed after main() ends. For example, in the following program, StartupFunction( ) is called before main( ) and CleanupFunction( ) is called after main( ).

 

#include<stdio.h>

/* Apply the constructor attribute to StartupFunction() so that it
    is executed before main() */
void StartupFunction(void) __attribute__ ((constructor));


/* Apply the destructor attribute to CleanupFunction() so that it
   is executed after main() */
void CleanupFunction(void) __attribute__ ((destructor));


/* implementation of StartupFunction*/
void StartupFunction(void)
{
    printf ("startup code before main()\n");
}

/* implementation of CleanupFunctiom */
void CleanupFunction(void)
{
    printf ("cleanup code after main()\n");
}

int main (void)
{
    printf ("hello\n");
    return 0;
}

 

Output

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