I have been trying to figure out how to deal with any situation when the result is bigger than what a integer declared can hold. For example:

/*
 * Old_guy.c
 *
 */

 #include <stdio.h>
 
int main()
{
     unsigned long int  hartbits;
     
 /* years * days * hours * minutes * seconds */    
     hartbits = 90 * 365 * 24 * 60 * 60;  /* approx. bits a 90 years old hart has done */
     
     printf("%d", hartbits); /* integer overflow */
     
     getchar();
     return(0);
}

In this case a double makes not difference neither. How does programmers if they need to compute something like 1*2*3*4*5*6...*45?
No knowing this, is bugging me.

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When you know that you need values bigger than any type can hold, you use something like GMP to handle arbitrary precision.

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If you want to test that you might need ultimate zip to unzip it.

I have been trying to figure out how to deal with any situation when the result is bigger than what a integer declared can hold.

I believe your actual question was addressed with GMP.

But with the code you posted, a good compiler or linter ought to tell you something like this (since it is a compile-time constant):

../main.c:13: warning: integer overflow in expression

For run-time checking of standard types, you can check for overflow something like this:
Checking for Integer Overflow

/FWIW

Thank you guys to all of you. I got to read the documentation about GMP so I can understand better.

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