Why isn't #pragma once automatically assumed?

Question

What\'s the point of telling the compiler specifically to include the file only once? Wouldn\'t it make sense by default? Is there even any reason to include a single file m

Answer 1

In the firmware for the product I mainly work on, we need to be able to specify where functions and global/static variables should be allocated in memory. Real-time processing needs to live in L1 memory on chip so the processor can access it directly, fast. Less important processing can go in L2 memory on chip. And anything that doesn't need to be handled particularly promptly can live in the external DDR and go through caching, because it doesn't matter if it's a little slower.

The #pragma to allocate where things go is a long, non-trivial line. It'd be easy to get it wrong. The effect of getting it wrong would be that the code/data would be silently put into default (DDR) memory, and the effect of that might be closed-loop control stopping working for no reason that's easy to see.

So I use include files, which contain just that pragma. My code now looks like this.

Header file...

#ifndef HEADERFILE_H
#define HEADERFILE_H

#include "set_fast_storage.h"

/* Declare variables */

#include "set_slow_storage.h"

/* Declare functions for initialisation on startup */

#include "set_fast_storage.h"

/* Declare functions for real-time processing */

#include "set_storage_default.h"

#endif

And source...

#include "headerfile.h"

#include "set_fast_storage.h"

/* Define variables */

#include "set_slow_storage.h"

/* Define functions for initialisation on startup */

#include "set_fast_storage.h"

/* Define functions for real-time processing */

You'll notice multiple inclusions of the same file there, even just in the header. If I mistype something now, the compiler will tell me it can't find the include file "set_fat_storage.h" and I can easily fix it.

So in answer to your question, this is a real, practical example of where multiple inclusion is required.